In  Nigeria , the  Tiv  ethnic group  cut four lines  into the abdomen of their girls during menarche. The lines are supposed to represent fertility. [ 44 ]


The  Navajo  have a celebration called  kinaalda  (kinn-all-duh). Girls are expected to demonstrate their strength through footraces. The girls make a cornmeal pudding for the tribe to taste. The girls who experience menarche wear special clothes and style their hair like the Navajo goddess " Changing Woman ". [ 44 ]


The  Nuu-chah-nulth  (also known as the Nootka) believe that physical endurance is the most important quality in young women. At menarche the girl is taken out to sea and left there to swim back. [ 44 ]


In the horror movie  Carrie  (1976), an adaptation of the  Stephen King   novel of the same name , protagonist  Carrie White  experiences her first period as she showers after the school gym class. Unaware of what is happening to her, she panics and pleads for help, but the other girls respond by bullying her. Carrie's first period unleashes her violent powers and is central to her dangerous and out of control transformation. [ 49 ]  This theme is common to horror movies, another notable example being the Canadian horror movie  Ginger Snaps  (2000), where the protagonist's first period is central to her gradual transformation into a  werewolf . The theme of transformation around the menarche is similarly present in  Turning Red  (2022), although the film also explores other aspects of puberty as a whole and the protagonist does not actually start her first period. Girls experiencing their first period is part of many movies, particularly ones that include  coming-of-age  plot lines, such as  The Blue Lagoon  (1980),  The Company of Wolves  (1984),  An Angel at My Table  (1990),  My Girl  (1991),  Return to the Blue Lagoon  (1991),  Eve’s Bayou  (1997), and  A Walk on the Moon  (1999). Menarche is also discussed in an episode of the animated series  Baymax!  (2022) in which the eponymous healthcare robot helps a girl deal with her first period.


The  menstrual cycle  is a series of natural changes in  hormone  production and the structures of the  uterus  and  ovaries  of the  female reproductive system  that makes  pregnancy  possible. The ovarian cycle controls the production and release of  eggs  and the cyclic release of  estrogen  and  progesterone . The uterine cycle governs the preparation and maintenance of the lining of the uterus (womb) to receive an  embryo . These cycles are concurrent and coordinated, normally last between 21 and 35 days, with a  median  length of 28 days.  Menarche  (the onset of the first period) usually occurs around the age of 12 years; menstrual cycles continue for about 30–45 years.


Naturally occurring hormones drive the cycles; the cyclical rise and fall of the  follicle stimulating hormone  prompts the production and growth of  oocytes  (immature egg cells). The hormone estrogen stimulates the uterus lining ( endometrium ) to thicken to accommodate an embryo should  fertilization  occur. The blood supply of the thickened lining provides  nutrients  to a successfully  implanted  embryo. If implantation does not occur, the lining breaks down and blood is released. Triggered by falling progesterone levels,  menstruation  (a "period", in common parlance) is the cyclical shedding of the lining, and is a sign that pregnancy has not occurred.


Each cycle occurs in phases based on events either in the ovary (ovarian cycle) or in the uterus (uterine cycle). The ovarian cycle consists of the  follicular phase ,  ovulation , and the  luteal phase ; the uterine cycle consists of the menstrual, proliferative and secretory phases. Day one of the menstrual cycle is the first day of the period, which lasts for about five days. Around day fourteen, an egg is usually released from the ovary.


The menstrual cycle can cause some women to experience  premenstrual syndrome  with symptoms that may include   tender breasts , and  tiredness . More severe symptoms that affect daily living are classed as  premenstrual dysphoric disorder , and are experienced by 3–8% of women. During the first few days of menstruation some women experience  period pain  that can spread from the abdomen to the back and upper thighs. The menstrual cycle can be modified by  hormonal birth control .


The menstrual cycle encompasses the ovarian and uterine cycles. The ovarian cycle describes changes that occur in the  follicles  of the  ovary , [ 1 ]  whereas the uterine cycle describes changes in the  endometrial lining  of the uterus.  Both cycles can be divided into phases.  The ovarian cycle consists of alternating  follicular  and  luteal phases , and the uterine cycle consists of the  menstrual phase , the proliferative phase, and the secretory phase. [ 2 ]  The menstrual cycle is controlled by the  hypothalamus  in the brain, and the  anterior pituitary gland  at the base of the brain. The hypothalamus releases  gonadotropin-releasing hormone  (GnRH), which causes the nearby anterior pituitary to release  follicle-stimulating hormone  (FSH) and  luteinizing hormone  (LH). Before  puberty , GnRH is released in low steady quantities and at a steady rate. After puberty, GnRH is released in large pulses, and the frequency and magnitude of these determine how much FSH and LH are produced by the pituitary. [ 3 ]


Measured from the first day of one menstruation to the first day of the next, the length of a menstrual cycle varies but has a  median  length of 28 days. [ 4 ]  The cycle is often less regular at the beginning and end of a woman's reproductive life. [ 4 ]  At puberty, a child's body begins to mature into an adult body capable of  sexual reproduction ; the first period (called  menarche ) occurs at around 12 years of age and continues for about 30–45 years. [ 5 ] [ 6 ]  Menstrual cycles end at  menopause , which is usually between 45 and 55 years of age. [ 7 ] [ 8 ]


Between menarche and menopause the ovaries regularly alternate between luteal and follicular phases during the monthly menstrual cycle. [ 9 ]  Stimulated by gradually increasing amounts of  estrogen  in the follicular phase, discharges of blood flow stop and the uterine lining thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one, or occasionally two, become dominant, while non-dominant follicles shrink and die. About mid-cycle, some 10–12 hours after the increase in luteinizing hormone, known as the LH surge, [ 4 ]  the dominant follicle releases an  oocyte , in an event called  ovulation . [ 10 ]


After ovulation, the oocyte lives for 24 hours or less without  fertilization , [ 11 ]  while the remains of the dominant follicle in the ovary become a  corpus luteum  –  a body with the primary function of producing large amounts of the hormone  progesterone . [ 12 ] [ a ]  Under the influence of progesterone, the uterine lining changes to prepare for potential  implantation  of an  embryo  to establish a pregnancy. The thickness of the endometrium continues to increase in response to mounting levels of estrogen, which is released by the  antral follicle  (a mature ovarian follicle) into the blood circulation. Peak levels of estrogen are reached at around day thirteen of the cycle and coincide with ovulation. If implantation does not occur within about two weeks, the corpus luteum degenerates into the  corpus albicans , which does not produce hormones, causing a sharp drop in levels of both progesterone and estrogen. This drop causes the uterus to lose its lining in menstruation; it is around this time that the lowest levels of estrogen are reached. [ 14 ]


In an ovulatory menstrual cycle, the ovarian and uterine cycles are concurrent and coordinated and last between 21 and 35 days, with a population average of 27–29 days. [ 15 ]  Although the average length of the human menstrual cycle is similar to that of the  lunar cycle , there is  no causal relation  between the two. [ 16 ]


The ovaries contain a finite number of  egg stem cells ,  granulosa cells  and  theca cells , which together form primordial follicles. [ 12 ]  At around 20 weeks into  gestation  some 7 million immature eggs have already formed in an ovary. This decreases to around 2 million by the time a girl is born, and 300,000 by the time she has her first period. On average, one egg matures and is released during ovulation each month after menarche. [ 17 ]  Beginning at puberty, these mature to primary follicles independently of the menstrual cycle. [ 18 ]   The development of the egg is called  oogenesis  and only one cell survives the  divisions  to await fertilization. The other cells are discarded as  polar bodies , which cannot be fertilized. [ 19 ]  The follicular phase is the first part of the ovarian cycle and it ends with the completion of the  antral follicles . [ 9 ]   Meiosis  (cell division) remains incomplete in the egg cells until the antral follicle is formed. During this phase usually only one ovarian follicle fully matures and gets ready to release an egg. [ 20 ]  The follicular phase shortens significantly with age, lasting around 14 days in women aged 18–24 compared with 10 days in women aged 40–44. [ 14 ]


Through the influence of a rise in  follicle stimulating hormone  (FSH) during the first days of the cycle, a few ovarian follicles are stimulated. These follicles, which have been developing for the better part of a year in a process known as  folliculogenesis , compete with each other for dominance. All but one of these follicles will stop growing, while one dominant follicle – the one that has the most FSH receptors –  will continue to maturity. The remaining follicles die in a process called  follicular atresia . [ 21 ]   Luteinizing hormone  (LH) stimulates further development of the ovarian follicle. The follicle that reaches maturity is called an antral follicle, and it contains the  ovum  (egg cell). [ 22 ]


The theca cells develop receptors that bind LH, and in response secrete large amounts of  androstenedione . At the same time the granulosa cells surrounding the maturing follicle develop receptors that bind FSH, and in response start secreting androstenedione, which is converted to estrogen by the enzyme  aromatase . The estrogen inhibits further production of FSH and LH by the pituitary gland. This  negative feedback  regulates levels of FSH and LH. The dominant follicle continues to secrete estrogen, and the rising estrogen levels make the pituitary more responsive to GnRH from the hypothalamus. As estrogen increases this becomes a  positive feedback  signal, which makes the pituitary secrete more FSH and LH. This surge of FSH and LH usually occurs one to two days before ovulation and is responsible for stimulating the rupture of the antral follicle and release of the oocyte. [ 18 ] [ 23 ]


Around day fourteen, the egg is released from the ovary. [ 24 ]   Called  ovulation , this occurs when a mature egg is released from the ovarian follicles into the pelvic cavity and enters the  fallopian tube , about 10–12 hours after the peak in LH surge. [ 4 ]  Typically only one of the 15–20 stimulated follicles reaches full maturity, and just one egg is released. [ 25 ]  Ovulation only occurs in around 10% of cycles during the first two years following menarche, and by the age of 40–50, the number of ovarian follicles is depleted. [ 26 ]  LH initiates ovulation at around day 14 and stimulates the formation of the corpus luteum. [ 2 ]  Following further stimulation by LH, the corpus luteum produces and releases estrogen, progesterone,  relaxin  (which relaxes the uterus by inhibiting contractions of the  myometrium ), and  inhibin  (which inhibits further secretion of FSH). [ 27 ]


The release of LH matures the egg and weakens the follicle wall in the ovary, causing the fully developed follicle to release its oocyte. [ 28 ]  If it is fertilized by a sperm, the oocyte promptly matures into an  ootid , which blocks the other  sperm cells  and becomes a mature egg. If it is not fertilized by a sperm, the oocyte degenerates. The mature egg has a diameter of about 0.1 mm (0.0039 in), [ 29 ]  and is the largest human cell. [ 30 ]


Which of the two ovaries – left or right – ovulates appears random; [ 31 ]  no left and right coordinating process is known. [ 32 ]  Occasionally both ovaries release an egg; if both eggs are fertilized, the result is  fraternal twins . [ 33 ]  After release from the ovary into the pelvic cavity, the egg is swept into the fallopian tube by the  fimbria  –  a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg disintegrates or dissolves in the fallopian tube, and a fertilized egg reaches the uterus in three to five days. [ 34 ]


Fertilization usually takes place in the  ampulla , the widest section of the fallopian tubes. A  fertilized egg  immediately starts the process of  embryonic development . The developing embryo takes about three days to reach the uterus, and another three days to implant into the endometrium. It has reached the  blastocyst  stage at the time of implantation: this is when pregnancy begins. [ 35 ]  The loss of the corpus luteum is prevented by fertilization of the egg.  The  syncytiotrophoblast  (the outer layer of the resulting embryo-containing blastocyst that later becomes the outer layer of the placenta) produces  human chorionic gonadotropin  (hCG), which is very similar to LH and preserves the corpus luteum. During the first few months of pregnancy, the corpus luteum continues to secrete progesterone and estrogens at slightly higher levels than those at ovulation. After this and for the rest of the pregnancy, the  placenta  secretes high levels of these hormones – along with hCG, which stimulates the corpus luteum to secrete more progesterone and estrogens, blocking the menstrual cycle. [ 36 ]  These hormones also prepare the mammary glands for milk [ b ]  production. [ 36 ]


Lasting about 14 days, [ 4 ]  the luteal phase is the final phase of the ovarian cycle and it corresponds to the secretory phase of the uterine cycle. During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone. [ 38 ] [ c ]  The increased progesterone starts to induce the production of estrogen. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. The level of FSH and LH fall quickly, and the corpus luteum atrophies. [ 40 ]  Falling levels of progesterone trigger menstruation and the beginning of the next cycle. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle at 10 to 16 days (average 14 days). [ 14 ]


The uterine cycle has three phases: menses, proliferative and secretory. [ 41 ]


Menstruation (also called menstrual bleeding, menses or a period) is the first and most evident phase of the uterine cycle and first occurs at puberty. Called menarche, the first period occurs at the age of around twelve or thirteen years. [ 8 ]  The average age is generally later in the  developing world  and earlier in the  developed world . [ 42 ]   In  precocious puberty , it can occur as early as age eight years, [ 43 ]  and this can still be normal. [ 44 ] [ 45 ]


Menstruation is initiated each month by falling levels of estrogen and progesterone and the release of  prostaglandins , [ 20 ]  which constrict the  spiral arteries . This causes them to  spasm , contract and break up. [ 46 ]  The blood supply to the endometrium is cut off and the cells of the top layer of the endometrium (the stratum functionalis) become deprived of oxygen and die. Later the whole layer is lost and only the bottom layer, the stratum basalis, is left in place. [ 20 ]  An  enzyme  called  plasmin  breaks up the  blood clots  in the menstrual fluid, which eases the flow of blood and broken down lining from the uterus. [ 47 ]  The flow of blood continues for 2–6 days and around 30–60  milliliters  of blood is lost, [ 15 ]  and is a sign that pregnancy has not occurred. [ 48 ]


The flow of blood normally serves as a sign that a woman has not become pregnant, but this cannot be taken as certainty, as several factors can cause  bleeding during pregnancy . [ 49 ]  Menstruation occurs on average once a month from menarche to menopause, which corresponds with a woman's fertile years. The average age of menopause in women is 52 years, and it typically occurs between 45 and 55 years of age. [ 50 ]  Menopause is preceded by a stage of hormonal changes called  perimenopause . [ 7 ]


Eumenorrhea  denotes normal, regular menstruation that lasts for around the first 5 days of the cycle. [ 24 ]  Women who experience  menorrhagia  (heavy menstrual bleeding) are more susceptible to  iron deficiency  than the average person. [ 51 ]


The proliferative phase is the second phase of the uterine cycle when estrogen causes the lining of the uterus to grow and proliferate. [ 40 ]  The latter part of the follicular phase overlaps with the proliferative phase of the uterine cycle. [ 31 ]  As they mature, the ovarian follicles secrete increasing amounts of  estradiol , an estrogen. The estrogens initiate the formation of a new layer of endometrium in the uterus with the spiral arterioles. [ 2 ]


As estrogen levels increase, cells in the cervix produce a type of  cervical mucus [ 53 ]  that has a higher  pH  and is less  viscous  than usual, rendering it more friendly to sperm. [ 54 ]  This increases the chances of fertilization, which occurs around day 11 to day 14. [ 11 ]  This cervical mucus can be detected as a vaginal discharge that is copious and resembles raw egg whites. [ 55 ]  For women who are practicing  fertility awareness , it is a sign that ovulation may be about to take place, [ 55 ]  but it does not mean ovulation will definitely occur. [ 15 ]


The secretory phase is the final phase of the uterine cycle and it corresponds to the luteal phase of the ovarian cycle. During the secretory phase, the corpus luteum produces progesterone, which plays a vital role in making the endometrium  receptive  to the  implantation  of a  blastocyst  (a fertilized egg, which has begun to grow). [ 56 ]   Glycogen ,  lipids , and  proteins  are secreted into the uterus [ 57 ]  and the cervical mucus thickens. [ 58 ]  In early pregnancy, progesterone also increases blood flow and reduces the  contractility  of the  smooth muscle  in the uterus [ 22 ]  and raises  basal body temperature . [ 59 ]


If pregnancy does not occur the ovarian and uterine cycles start over again. [ 47 ]


Only two-thirds of overtly normal menstrual cycles are ovulatory, that is, cycles in which ovulation occurs. [ 15 ]  The other third lack ovulation or have a short luteal phase (less than ten days [ 60 ] ) in which progesterone production is insufficient for normal physiology and fertility. [ 61 ]  Cycles in which ovulation does not occur ( anovulation ) are common in girls who have just begun menstruating and in women around menopause. During the first two years following menarche, ovulation is absent in around half of cycles. Five years after menarche, ovulation occurs in around 75% of cycles and this reaches 80% in the following years. [ 62 ]  Anovulatory cycles are often overtly identical to normally ovulatory cycles. [ 63 ]  Any alteration to balance of hormones can lead to anovulation. Stress, anxiety and  eating disorders  can cause a fall in GnRH, and a disruption of the menstrual cycle. Chronic anovulation occurs in 6–15% of women during their reproductive years. Around menopause, hormone feedback dysregulation leads to anovulatory cycles. Although anovulation is not considered a disease, it can be a sign of an underlying condition such as  polycystic ovary syndrome . [ 64 ]  Anovulatory cycles or short luteal phases are normal when women are under stress or athletes increasing the intensity of training. These changes are reversible as the stressors decrease or, in the case of the athlete, as she adapts to the training. [ 60 ]


Although a normal and natural process, [ 65 ]  some women experience  premenstrual syndrome  with symptoms that may include  acne ,  tender breasts , and  tiredness . [ 66 ]  More  severe symptoms that affect daily living are classed as  premenstrual dysphoric disorder  and are experienced by 3 to 8% of women. [ 4 ] [ 67 ] [ 66 ] [ 68 ]   Dysmenorrhea  (menstrual cramps or period pain) is felt as painful cramps in the abdomen that can spread to the back and upper thighs during the first few days of menstruation. [ 69 ] [ 70 ] [ 71 ]  Debilitating period pain is not normal and can be a sign of something severe such as  endometriosis . [ 72 ]  These issues can significantly affect a  woman's health  and quality of life and timely interventions can improve the lives of these women. [ 73 ]


There are common culturally communicated misbeliefs that the menstrual cycle affects women's moods, causes depression or irritability, or that menstruation is a painful, shameful or unclean experience. Often a woman's normal mood variation is falsely attributed to the menstrual cycle. Much of the research is weak, but there appears to be a very small increase in mood fluctuations during the luteal and menstrual phases, and a corresponding decrease during the rest of the cycle. [ 74 ]  Changing levels of estrogen and progesterone across the menstrual cycle exert systemic effects on aspects of physiology including the brain, metabolism, and musculoskeletal system. The result can be subtle physiological and observable changes to women's athletic performance including strength, aerobic, and anaerobic performance. [ 75 ]


Changes to the brain have also been observed throughout the menstrual cycle [ 76 ]  but do not translate into measurable changes in intellectual achievement – including academic performance, problem-solving, and memory. [ 77 ]  Improvements in  spatial reasoning ability  during the menstruation phase of the cycle are probably caused by decreases in levels of estrogen and progesterone. [ 74 ]


In some women, ovulation features a characteristic pain [ d ]  called  mittelschmerz  (a German term meaning  middle pain ). The cause of the pain is associated with the ruptured follicle, causing a small amount of blood loss. [ 20 ]


Even when normal, the changes in hormone levels during the menstrual cycle can increase the incidence of disorders such as  autoimmune diseases , [ 81 ]  which might be caused by estrogen enhancement of the  immune system . [ 4 ]


Around 40% of women with  epilepsy  find that their  seizures  occur more frequently at certain phases of their menstrual cycle. This  catamenial epilepsy  may be due to a drop in progesterone if it occurs during the luteal phase or around menstruation, or a surge in estrogen if it occurs at ovulation. Women who have regular periods can take medication just before and during menstruation. Options include progesterone supplements, increasing the dose of their regular  anticonvulsant  drug, or temporarily adding an anticonvulsant such as  clobazam  or  acetazolamide . If this is ineffective, or when a woman's menstrual cycle is irregular, then treatment is to stop the menstrual cycle occurring. This may be achieved using  medroxyprogesterone ,  triptorelin  or  goserelin , or by sustained use of oral contraceptives. [ 82 ] [ 83 ]


Hormonal contraceptives prevent pregnancy by inhibiting the secretion of the hormones, FSH, LH and GnRH. Hormonal contraception that contains estrogen, such as  combined oral contraceptive pills  (COCPs), stop the development of the dominant follicle and the mid-cycle LH surge and thus ovulation. [ 84 ]   Sequential dosing and discontinuation of the COCP can mimic the uterine cycle and produce bleeding that resembles a period. In some cases, this bleeding is lighter. [ 85 ]


Progestin-only methods of hormonal contraception do not always prevent ovulation but instead work by stopping the cervical mucus from becoming sperm-friendly. Hormonal contraception is available in a variety of forms such as pills,  patches ,  skin implants  and  hormonal intrauterine devices  (IUDs). [ 86 ]


Most  female mammals  have an  estrous cycle , but only ten primate species, four  bat  species, the  elephant shrews  and the  Cairo spiny mouse  ( Acomys cahirinus ) have a  menstrual cycle . [ 87 ] [ 88 ]  The cycles are the same as in humans apart from the length, which ranges from 9 to 37 days. [ 89 ] [ 87 ]  The lack of immediate relationship between these groups suggests that four distinct evolutionary events have caused menstruation to arise. [ 90 ]  There are four theories on the evolutionary significance of menstruation: [ 90 ]


Media related to  Menstrual cycle  at Wikimedia Commons


[ citation needed ]


Menstrual Hygiene Day  ( MHD,   MH Day  in short) is an annual  awareness day  on May 28 to highlight the importance of good  menstrual hygiene management  (MHM) at a global level. It was initiated by the German-based  NGO   WASH United  in 2013 and observed for the first time in 2014. [ 1 ]


In  developing countries , women's choices of menstrual hygiene materials are often limited by the costs, availability and  social norms . [ 2 ] [ 3 ]  Adequate  sanitation  facilities and access to  feminine hygiene  products are important but opening discussion making adequate  education for women and girls  is of equal importance. Research has found that not having access to menstrual hygiene management products can keep girls home from school during their period each month. [ 4 ]


Menstrual Hygiene Day is an occasion for publicizing information in the media, including  social media , and to engage decision-makers in policy dialogue. The day aims to  advocate  for the integration of menstrual hygiene management  into  global, national and local policies and programs.


Menstrual hygiene management  can be particularly challenging for females in  developing countries , where  clean water and toilet facilities  are often inadequate. In addition, traditional cultures make it difficult to discuss menstruation openly. This limits access to relevant and important information about the normal functions of the female body. This directly affects their health, education and  dignity .  Access to information can be considered a  human right . [ 5 ] [ 6 ]


In 2012, several organizations involved in public health began to break the silence on MHM and turn their attention to the issue globally, including grassroots organizers, social entrepreneurs and United Nations agencies. [ 5 ]


In May 2013, WASH United used a 28-day  social media  campaign, for example on  Twitter , called "May #MENSTRAVAGANZA" to generate awareness about menstruation and MHM as important considerations within water, sanitation and hygiene (WASH) development initiatives. [ 7 ] [ 8 ]  Those involved with the social media campaign, including  WASH  Advocates, Girls' Globe and Ruby Cup, were encouraged by the positive feedback for the "May #MENSTRAVAGANZA" and they decided to create a global awareness day for menstruation. [ 8 ]


On 28 May 2014, many people around the world celebrated Menstrual Hygiene Day for the first time with rallies,  exhibitions , movie screenings, workshops and speeches. [ 9 ]  There were 145 partners involved with the first MHD. [ 8 ] [ 10 ]


For 2015, a  hashtag  campaign on social media lent a light-hearted look at challenging societal norms with the tag #IfMenHadPeriods. [ 11 ]  The campaign by  WaterAid , released in time for Menstrual Hygiene Awareness Day, created videos "spoof ads" where men are proud of having their periods and used "Manpons" instead of tampons. [ 11 ] [ 12 ]  The campaign helped "raise awareness about women who don't have access to 'safe water, hygiene and sanitation,' when their monthly visitor comes along." [ 12 ]  Another aspect of the campaign is that it helped bring men into the conversation so that they could "help tackle the stigma in largely patriarchal societies and encourage women and girls to embrace their cycle with pride instead of shame." [ 13 ]  In  Uganda , 2015 celebrations kicked off with a march to  Parliament  where a charter on MHM was signed and then the march continued to the National theatre for presentations by primary and secondary schools. [ 4 ]


Menstrual hygiene day is meant to serve as a platform to bring together individuals, organizations,  social businesses  and the media to create a united and strong voice for women and girls. It is designed to break the silence about menstrual hygiene management. [ 14 ] [ 15 ]


The objectives of MHD include: [ 16 ] [ 8 ]


Menstrual Hygiene Day makes audible and visible a growing movement that promotes body literacy and autonomy, as well as  gender equality . [ 16 ]


May 28 has symbolic meaning: May is the 5th month of the year, and the average length of menstruation is 5 days every month. Also, the menstrual cycle averages 28 days. [ 7 ]


For partners working in  developing countries , the day is not only an opportunity to raise awareness, but also to strengthen government accountability related to MHM issues.  For example, in 2015 the Ministry of Health in Kenya launched a national MHM strategy. Kenya, jointly with UNICEF, held a virtual conference on Menstrual Hygiene Management in Schools that same year. [ 17 ]


Menstrual Man  is a 2013  documentary film  by Amit Virmani. The film tells the story of  Arunachalam Muruganantham , an Indian social entrepreneur and inventor whose machines enable rural women to manufacture low-cost  sanitary pads  for their communities. It premiered at the 2013  Full Frame Documentary Film Festival , and was voted a Top Ten Audience Favourite at both  Hot Docs  and  IDFA  the same year. [ 1 ] [ 2 ]  The film was nominated for  Best Feature Documentary  at the  Asia Pacific Screen Awards . [ 3 ]


Virmani first read about Muruganantham in December 2011 and was instantly drawn to his story because "it played like a  Bollywood  movie". [ 4 ]  He contacted Muruganantham and began filming in January 2012 as he accompanied the inventor around India for the next five months. In June 2012, Virmani and his co-producer, Seah Kui Luan, secured funding from the  Media Development Authority of Singapore  to shoot reenactments and complete post-production on the film. Virmani also licensed footage from Bollywood films for use in the film.


Menstrual Man  received positive reviews. Writing in  NOW , Canadian feminist author and activist  Susan G. Cole  noted the smart filmmaking behind the film, adding it "makes perfect sense by the end, when you've come to realize Muruganantham and the male educators he worked with are honorary feminists". [ 5 ]   John Lui  from  The Straits Times  gave the film four-and-a-half stars, noting that the film avoids being "all po-faced liberal-guilt seriousness" with its use of vintage Bollywood clips and thanks to its charismatic protagonist. [ 6 ]  PBS  POV  columnist Tom Roston called the film his greatest discovery at the 2013 Hot Docs festival, describing the film as "an engaging, funny, heart-breaking story of the power of what one ordinary man can achieve". [ 7 ]  However, he felt that the title would discourage audiences from watching the film and that Virmani's refusal to change its title could inhibit the film's success. [ 8 ]  Virmani defended his decision, stating that people being ashamed to talk about menstruation was a big part of the problem of women's hygiene in rural India. "If I changed the title because educated people might not watch the film, then I'm a hypocrite. I become part of the problem." [ 9 ]


Menstrual suppression  refers to the practice of using  hormonal management  to stop or reduce  menstrual bleeding . In contrast to  surgical  options for this purpose, such as  hysterectomy  or  endometrial ablation , hormonal methods to manipulate menstruation are reversible.


There are a number of  medical conditions  for which fewer  menstrual periods  and less  blood loss  may be beneficial. [ 1 ] [ 2 ]  In addition, suppression of hormonal cyclicity may benefit  menstrual-related mood swings  or other conditions which increase in frequency with  menses . [ 3 ]  Management of menstruation may be a challenge for those with  developmental delay  or  intellectual disability , and menstrual suppression can benefit individuals with specific job- or activity-related needs. [ 1 ] [ 4 ]  There is increasing attention being given to menstrual suppression for  transgender men  and  non-binary   transmasculine  people who may experience  dysphoria  with menstruation. [ 5 ]  Menstrual suppression is also being used by individuals with a variety of personal reasons to have less frequent or no menses, including honeymoon, vacations, travel, or other specific reasons.


Options for menstrual suppression include  hormonal medications  like  extended cycle combined hormonal contraceptive pills ,  progestogen-only contraceptives  (including  progestogen-only pills ,  progestogen-containing implants ,  progestogen-containing intrauterine devices , and  progestogen-only injectable contraceptives ),  gonadotropin-releasing hormone modulators , and  testosterone , as well as the surgical options of  hysterectomy  (removal of the  uterus ) and  endometrial ablation  (removal of the  endometrium ). [ 6 ] [ 7 ]  Most options for the suppression of menstrual bleeding are not immediately 100% effective, and with many options, unscheduled bleeding (termed " breakthrough bleeding ") can occur; for many options for menstrual suppression, breakthrough bleeding becomes less frequent with time. [ 8 ]


Hormonal therapies to reduce or stop  menstrual bleeding  have long been used to manage a number of  gynecologic conditions  including  menstrual cramps  ( dysmenorrhea ),  heavy menstrual bleeding , irregular or other  abnormal uterine bleeding , menstrual-related  mood  changes ( premenstrual syndrome  or  premenstrual dysphoric disorder ), and  pelvic pain  due to  endometriosis  or  uterine fibroids . [ 1 ] [ 9 ] [ 10 ] [ 2 ] [ 11 ]  Medical conditions that are associated with  anemia  or excessive  blood loss , including  sickle cell disease ,  iron deficiency anemia ,  Fanconi anemia ,  von Willebrand disease , low  platelets  ( thrombocytopenia ) from  immune thrombocytopenia , or other blood/ hematologic disorders  such as  clotting factor deficiencies  could all benefit from menstrual suppression. [ 1 ] [ 2 ]  In patients with  malignancies  who will receive  chemotherapy  that could result in low  blood counts  or anemia, or individuals with recurrent malignancies who will receive a  stem cell transplant , excessive menstrual bleeding during this treatment could be medically serious, and thus menstrual suppression might be indicated. [ 1 ] [ 12 ]  In addition, there are a number of other medical conditions with menstrual exacerbation that may benefit from menstrual suppression, including  catamenial seizures , menstrual  migraine headaches ,  irritable bowel syndrome , and  asthma . [ 1 ]


Menstrual hygiene  issues, as in those individuals with  developmental delay  or  intellectual disability  or other manual dexterity or mobility/wheelchair challenges such as  spina bifida  or  cerebral palsy  may prompt an individual or caregiver to request menstrual suppression. [ 13 ] [ 14 ]  Job- or activity-related indications for menstrual suppression may include deployed military as occurred during  Operation Desert Storm , travel, wilderness camping, astronauts, or athletes with concerns about menses occurring during competition or training. [ 1 ]  There is also a growing recognition that  transgender men  and  non-binary   transmasculine  people may experience  dysphoria  with menses, and thus may request medical therapy for menstrual suppression. [ 15 ] [ 16 ]


The use of hormonal methods containing estrogen ( combined oral contraceptives , the  contraceptive patch  or  contraceptive ring ), may be associated with risks that outweigh benefits for individuals with specific medical problems, such as  migraine headaches  with aura, a history of  breast cancer , or a history of  deep vein thrombosis . [ 17 ]  Thus these options would be contraindicated for menstrual suppression with such conditions. Progestin-only options (depot medroxyprogesterone acetate, oral progestins) are appropriate for these individuals. Drug-drug interactions are also important to consider, particularly with combined hormonal options. [ citation needed ]


As extended cycle regimens of combined hormonal contraceptives provide a greater cumulative dose of steroid hormones, questions have been raised about their safety. Data currently provide reassurance that these options are safe. [ 9 ] [ 10 ] [ 18 ]


While some forms of  birth control  do not affect the menstrual cycle, hormonal contraceptives work by disrupting it.  Progestogen  negative feedback decreases the pulse frequency of  gonadotropin-releasing hormone  (GnRH) release by the  hypothalamus , which decreases the release of  follicle-stimulating hormone  (FSH) and  luteinizing hormone  (LH) by the  anterior pituitary . Decreased levels of FSH inhibit follicular development, preventing an increase in  estradiol  levels. Progestogen negative feedback and the lack of  estrogen   positive feedback  on LH release prevent a mid-cycle  LH surge . Inhibition of follicular development and the absence of an LH surge prevent ovulation. [ 19 ] [ 20 ] [ 21 ]


The use of  combined hormonal contraceptives  such as  the pill , the  patch , and the  vaginal ring  are methods of contraception that contain both an  estrogen  and  progestogen . These methods have traditionally been used in a cyclic fashion, with three weeks (21   days) of hormones, followed by a 7-day hormone-free interval (with combined oral contraceptives, often with a week of  placebo  pills) during which time withdrawal bleeding or a hormonally-induced menstrual period occurs, mimicking an idealized spontaneous menstrual cycle. [ 22 ]  When these methods are taken without the hormone-free week, the withdrawal bleeding is reduced or eliminated. Thus  extended cycle combined hormonal contraceptive pills  are commonly used for menstrual suppression, although breakthrough bleeding is common in the initial months of use. The rate of  amenorrhea  (no bleeding) is in the range of 60% for users who are continuing to use combined hormonal contraceptive pills at the end of a year.


Combined hormonal contraceptives include both an estrogen and a progestogen. Estrogen negative feedback on the  anterior pituitary  greatly decreases the release of FSH, which makes combined hormonal contraceptives more effective at inhibiting follicular development and preventing ovulation. Estrogen also reduces the incidence of irregular  breakthrough bleeding . [ 19 ] [ 20 ] [ 21 ]  Several combined hormonal contraceptives—the pill, vaginal ring, and contraceptive patch—are usually used in a way that causes regular  withdrawal bleeding . In a normal cycle, menstruation occurs when estrogen and progesterone levels drop rapidly. [ 23 ]  Temporarily discontinuing use of combined hormonal contraceptives (a placebo week, not using patch or ring for a week) has a similar effect of causing the uterine lining to shed. If withdrawal bleeding is not desired, combined hormonal contraceptives may be  taken continuously , although this increases the risk of breakthrough bleeding. [ medical citation needed ]


Progestogen -only medications, including  progestogen-only pills  and a  slow-release (depot) injectable  medication,  depot medroxyprogesterone acetate  (DMPA; Depo-Provera) do not contain an  estrogen . DMPA is given as an injection every 90   days, and is typically associated with amenorrhea in about 50 to 60% of users at the end of one year. Progestogens that are not typically used for birth control, such as  norethisterone acetate , may be used to induce amenorrhea. [ 24 ]


The degree of  ovulation suppression  in progestogen-only contraceptives depends on the  progestogenic  activity and dose of the formulation used. Low-dose progestogen-only contraceptives, including traditional progestogen-only pills (e.g.,  norethisterone  (Micronor, Nor-QD, Noriday)),  levonorgestrel-releasing implants  (Norplant, Jadelle), and  hormonal intrauterine devices  (IUDs) (e.g.,  levonorgestrel  (Mirena)), inhibit ovulation in about 50% of cycles and rely mainly on other effects, such as thickening of cervical mucus, for their contraceptive effectiveness. [ 25 ]  Intermediate-dose progestogen-only contraceptives, including the progestogen-only pill  desogestrel  (Cerazette) and the subdermal implant  etonogestrel  (Nexplanon, Implanon), allow some follicular development but more consistently inhibit ovulation in 97 to 99% of cycles. The same cervical mucus changes occur as with very low-dose progestogens. High-dose progestogen-only contraceptives—the injectables DMPA (Depo-Provera) and  norethisterone enanthate  (Noristerat)—completely inhibit follicular development and ovulation. [ 25 ]


Injections such as DMPA became available in the 1960s and later became used to also achieve amenorrhea. A majority of patients will achieve amenorrhea within 1 year of initiating DMPA therapy. DMPA therapy is typically successful in achieving amenorrhea but also has side effects of decreased  bone mineral density  that must be considered before beginning therapy. [ 12 ]


When using the subdermal progestogen-only implants, unpredictable bleeding continues and amenorrhea is not commonly achieved amongst patients. [ 12 ]   Progestogen-only contraceptive pills  (sometimes called the "mini pill") are taken continuously without a 7-day span of using placebo pills, and therefore menstrual periods are less likely to occur than with the combined pill with placebo pills. However, disturbance of the menstrual cycle is common with the mini-pill; one-third to one-half of women taking it will experience prolonged periods, and up to 70% experience break-through bleeding ( metrorrhagia ). Irregular and prolonged bleeding is the most common reason that women discontinue using the mini pill. [ 26 ]


Hormonal IUDs containing the progestogen levonorgestrel have the side effect of inducing amenorrhea, and some types of IUDs have been shown to markedly decrease menstrual blood loss, and thus are efficacious in treating heavy and abnormal menstrual bleeding. [ 27 ]  The rate of amenorrhea after one year of use is in the range of 20 to 50%, although most users of the hormonal IUDs Mirena and Liletta experience a marked decrease in menstrual bleeding, which is beneficial and has led to reported high rates of user satisfaction.


Levonorgestrel IUDs have also been used been shown to induce amenorrhea. The lower dose device has a lower rate of achieving amenorrhea compared to the higher dose device where 50% of users have been found to achieve amenorrhea within 1   year of use. A concern for usage of these devices is the invasive administration and initial breakthrough bleeding while utilizing these devices however they have the advantage of the most infrequent dosing schedule of every 5   years. Use of IUDs have also shown to reduce menorrhagia and dysmenorrhea. [ 12 ] [ 28 ]


Gonadotropin-releasing hormone  (GnRH)  modulators , including both  GnRH agonists  and  GnRH antagonists , are associated with amenorrhea, and have been used to induce therapeutic amenorrhea. Among oncologists caring for adolescents with cancer, GnRH modulators were the most commonly recommended treatment for menstrual suppression to prevent or treat heavy bleeding during therapy. [ 12 ]


The  hormonal agent   danazol  (Danocrine) was once used for the treatment of endometriosis, and was associated with amenorrhea, but its use was limited by  androgenic  side effects such as the potential for permanent  lowering of the voice  or  hair growth . As these side effects may be desired in  transgender men  and  non-binary   transmasculine  people, there has been some consideration of this option for menstrual population in this group of individuals.


Testosterone  and its  esters  are effective as a form of menstrual suppression and help to suppress menstruation in transgender men and non-binary transmasculine people. [ 6 ]  Testosterone is not used in cisgender women due to its  masculinizing  effects at required doses. Other  anabolic–androgenic steroids , such as  nandrolone  and  oxandrolone , may also produce menstrual suppression at sufficiently high doses.


Historically, women and girls had far fewer menstrual periods throughout their lifetimes, a result of shorter life expectancies, as well as a greater length of time spent pregnant or breast-feeding, which reduced the number of periods they experienced. [ 29 ]


When the first birth control pill was being developed, the researchers were aware that they could use the contraceptive to space menstrual periods up to 90   days apart, but they settled on a 28-day cycle that would mimic a natural menstrual cycle and produce monthly periods. The intention behind this decision was the hope of the inventor, John Rock, to win approval for his invention from the Roman Catholic Church. That attempt failed, but the 28-day cycle remained the standard when the pill became available to the public. [ 30 ]


Historically, the concept that menstruation did not have beneficial effects, and that menstruation could be controlled was raised in the 1990s, by Dr.  Elsimar Coutinho . [ 31 ]  The English language version, title, "Is Menstruation Obsolete: How suppressing menstruation can help women who suffer from anemia, endometriosis, or PMS?" was published in 1999.


Menstrual synchrony , also called the  McClintock effect , [ 1 ]  or the  Wellesley effect , [ 2 ]  is a contested process whereby women who begin living together in close proximity would experience their  menstrual cycle  onsets (the onset of  menstruation  or menses) becoming more synchronized together in time than when previously living apart. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period." [ 3 ]


Martha McClintock 's 1971 paper, published in  Nature , says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier. [ 3 ]


After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony, including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. Reviews in 2006 and 2013 concluded that menstrual synchrony likely does not exist. [ 4 ] [ 5 ] [ 6 ] [ 7 ]


Martha McClintock  published the first study on menstrual synchrony among women living together in  dormitories  at  Wellesley College , a women's liberal arts college in  Massachusetts , US. [ citation needed ]


McClintock hypothesized that  pheromones  could cause menstrual cycle synchronization. [ 3 ] [ 8 ]  However, other mechanisms have been proposed, most prominently synchronization with  lunar phases . [ 7 ]


No scientific evidence supports the lunar hypothesis, and doubt has been cast on pheromone mechanisms. [ 7 ] [ 9 ]


After the initial studies reporting menstrual synchrony began to appear in the  scientific literature , other researchers began reporting the failure to find menstrual synchrony. [ 10 ] [ 11 ]


These studies were followed by critiques of the methods used in early studies, which argued that biases in the methods used produced menstrual synchrony as an artifact. [ 12 ] [ 13 ] [ 14 ] [ 15 ]


More recent studies, which took into account some of these methodological criticisms, failed to find menstrual synchrony. [ 12 ] [ 5 ] [ 4 ]


The term  synchrony  has been argued to be misleading because no study has ever found that menstrual cycles become strictly concordant, nevertheless  menstrual synchrony  is used to refer the phenomenon of menstrual cycle onsets becoming closer to each other over time. [ 12 ] [ clarification needed ]


In a 2013 systematic review of menstrual synchrony, Harris and Vitzthum concluded, "In light of the lack of empirical evidence for MS [menstrual synchrony]  sensu stricto , it seems there should be more widespread doubt than acceptance of this hypothesis" (pp. 238–239). [ 7 ]


The experience of synchrony may be the result of the mathematical fact that menstrual cycles of different  frequencies  repeatedly converge and diverge over time and not due to a process of synchronization, [ 5 ]  and the probability of encountering such overlaps by chance is high. [ 12 ]


Researchers are divided on whether menstrual synchrony would be adaptive. [ 7 ] [ 8 ] [ 12 ]  McClintock has suggested that menstrual synchrony may not be adaptive but rather  epiphenomenal , lacking any  biological function . [ 8 ]  Among those who postulate an adaptive function, one argument is that menstrual synchrony is only a particular aspect of the much more general phenomenon of  reproductive synchrony , an occurrence familiar to ecologists studying animal populations in the wild. Whether seasonal, tidal, or lunar, reproductive synchrony is a relatively common mechanism through which co-cycling females can increase the number of males included in the local breeding system.


Conversely, it has been argued that if there are too many females cycling together, they would be competing for the highest quality males, forcing female–female competition for high quality mates and thereby lowering fitness. In such cases, selection should favor avoiding synchrony. [ 16 ]  Divergent climate regimes differentiating  Neanderthal  reproductive strategies from those of modern  Homo sapiens  have recently been analysed in these terms. [ 17 ]


Turning to the evolutionary past, a possible adaptive basis for the biological capacity would be reproductive levelling: among primates, synchronising to any natural clock makes it difficult for an alpha male to monopolise fertile sex with multiple females. [ 18 ] [ 19 ] [ 20 ] [ 21 ] [ 22 ]  This would be consistent with the striking gender egalitarianism of extant non-storage hunter-gatherer societies. [ 23 ]  When early Pleistocene hominids in Africa were attempting to survive by robbing big cats of their kills, [ 24 ] [ 25 ] [ 26 ]  according to some evolutionary scientists, it may have been adaptive to restrict overnight journeys—including sexual liaisons—to times when there was a moon in the sky. [ 27 ] [ 28 ]


The question of whether those who live together do in fact synchronize their menstrual cycles has also received attention in the popular media. [ 1 ] [ 29 ] [ 30 ] [ 31 ] [ 32 ]


The idea that menstruation is – or ideally ought to be – in harmony with wider cosmic rhythms is one of the most tenacious ideas central to the myths and rituals of traditional communities across the world.


String was first made by the two Wawalik Sisters at Mudawa, near  Buckingham Bay .... The sisters sat down, looking at each other, with their feet out and legs apart, and both menstruated... Each one made a loop of the other one's menstrual blood, after which they put the string loops around their necks. [ 34 ]


The  !Kung  (or Ju|'hoansi) hunter-gatherers of the  Kalahari  "believe ... that if a woman sees traces of menstrual blood on another woman's leg or even is told that another woman has started her period, she will begin menstruating as well". [ 35 ]  Among the  Yurok people  of northwestern California, according to one ethnographic study, "all of a household's fertile women who were not pregnant menstruated at the same time...". [ 36 ]


She said that she had been instructed in the menstrual laws by her maternal aunts and grandmother, who in their times were well-known, conservative Yurok women.... the young woman said that in old-time village life all of a household's fertile women who were not pregnant menstruated at the same time, a time dictated by the moon; that these women practised bathing rituals together at this time... If a woman got out of synchronization with the moon and with the other women of the household, she could 'get back in by sitting in the moonlight and talking to the moon, asking it to balance her'. [ 37 ]


The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. [ 3 ]  The phenomenon is not synchronization in the strict sense of concordance of menstrual cycle onsets but the term  menstrual synchrony  is still used perhaps misleadingly. [ 12 ]  As an undergraduate, Martha McClintock published the first study on menstrual synchrony; her report detailed the menstrual synchrony of undergraduate women living in a  dormitory  in  Wellesley College . [ 3 ]  Since then, there have been attempts to  replicate  her findings and to determine the conditions under which synchrony occurs, if it exists. Her work was followed up by studies reporting menstrual synchrony [ 38 ] [ 39 ] [ 40 ] [ 41 ] [ 42 ] [ 43 ] [ 44 ] [ 45 ]  and by other studies that failed to find synchrony. [ 12 ] [ 10 ] [ 11 ] [ 5 ] [ 4 ] [ 46 ] [ 47 ] [ 48 ] [ 49 ] [ 50 ]


Thus, a number of studies were published from the 1980s to the mid 2000s, which attempted to replicate menstrual synchrony in college women, determine the conditions under which menstrual synchrony occurred, and to address methodological issues that were raised as these studies were published. The rest of this section discusses these studies in chronological order, briefly presenting their findings and main conclusions grouped by decade followed by general methodological issues in menstrual synchrony research.


McClintock's study consisted of 135 female college students who were 17 to 22 years old at the time of the study. They were all residents of a single dormitory, which had four main corridors. The women were asked when their last and second to last menstrual period had started three times during the academic year (which ranged from September to April). They also were asked who (other women in the dormitory) they associated with most and how often each week they associated with males.  From these data, McClintock placed women into pairs of close friends and roommates and she also placed them into groups of friends ranging in size from 5 to 10 women.  She reported statistically significant synchrony for both her pairwise sorting of women and her group sorting of women. That is, whether women were placed into pairs of close friends and roommates or whether they were placed into larger groups of friends, she reported that they synchronized their menstrual cycles. She also reported that the more often women associated with males, the shorter their menstrual cycles were. She speculated that this may be a pheromone effect paralleling the  Whitten effect  in mice but that it could not explain menstrual synchrony among women.  Finally, she speculated that there could be a pheromone mechanism of menstrual synchrony similar to the  Lee-Boot effect  in mice. [ 3 ]


Graham and McGrew were the first researchers to attempt to replicate McClintock's study. There were 79 women living in  halls of residence  or apartments on the campus of a college in  Scotland . The women were 17 to 21 years old at the time of the study and the procedures followed were similar to those used in McClintock's study. She partially replicated McClintock's study reporting that close friends but not neighbors synchronized their cycles. Unlike in McClintock's study, close friends did not synchronize in groups. They considered a pheromone mechanism a possible explanation of synchrony, but noted that if pheromones were the cause, neighbors should have synchronized as well. They concluded that the mechanism of synchrony remains unknown, but emotional attachment may play a role. [ 38 ]


Quadagno et al. conducted the second replication of McClintock's study. There were 85 women living in dormitories,  sorority houses , and apartments who attended a large midwestern university in the  United States . Their study used methods similar to McClintock's study except in addition to two women living together, there were also groups of three and four women living together. They reported that the women synchronized their menstrual cycles and concluded that pheromones may have played a role in synchronization. [ 39 ]


Jarett's study was the third to attempt to replicate McClintock's original study using college roommates. There were 144 women who attended two colleges. The women were 17 to 22 years old and the procedures followed were similar to McClintock's study except only pairs of roommates were used. She reported that the women did not synchronize. Jarett concluded that whether menstrual synchrony occurs in a group of women may depend on the variability of their menstrual cycles. She conjectured that the reason the women in her study did not synchronize their menstrual cycles was because they happened to have longer and more irregular menstrual cycles than in McClintock's original study. [ 46 ]


Wilson, Kiefhabe, and Gravel conducted two studies with college women. The first study consisted of 132 women who were members of a sorority or roommates of members at the  University of Missouri . The women were 18 to 22 years of age and the study aimed to replicate McClintock's original study.  However, instead of asking women to recall when their last and next to last menstrual onsets occurred, one of the researchers visited the sorority daily to record the occurrence of menstrual onsets and to collect other biographical data.  The second study consisted of 24 women living in a cooperative house near the University of Missouri. The women were 18 to 31 years of age. One of the researchers visited the house three times a week recording menstrual onset and collecting more extensive biographical and psychological test data than in the first study. They found no menstrual synchrony in either study. They considered the possibility that women with irregular cycles may reduce the likelihood of detecting synchrony, so they re-analyzed their data after they removed women with irregular cycles, but again there was no statistically significant effect of synchrony. They concluded that "It is clear no meaningful process of selection or exclusion of pairs can produce a significant level of menstrual synchrony in our samples... Therefore, whether or not menstrual synchrony occurs among women who spend time together must remain a hypothesis requiring further investigation" (p. 358). [ 10 ]


Weller and Weller conducted a study with 20  lesbian  couples. They hypothesized that contact within couples should be maximal and contact with men minimal compared to previous studies, which should maximize the likelihood of detecting synchrony. The women ranged in age between 19 and 34 years of age. This was the first study that did not explicitly use college women, but instead the women were recruited at a  bar  by a research assistant who was a  proprietor of a bar . Unlike previous studies, they only asked the women for the date of their last menstrual onset. They then assumed that all the women had menstrual cycles that were exactly 28 days long. Based on this assumption and one menstrual onset for each woman in a couple, they calculated the degree of synchrony. They reported that more than half of the couples had synchronized within a two-day timespan of each other. [ 40 ]


Trevathan, Burleson, and Gregory also conducted a study with 29 lesbian couples (22 to 48 years of age), but they incorporated the methodological critique of Wilson into the design of their study. [ 13 ]  In particular, Wilson emphasized the importance of using actual menstrual cycle lengths with their inherent variability. The lesbian couples were drawn from a larger sample of women who had kept daily records of their menstrual cycles for three months and who had participated in a previous study. They found no evidence of synchrony. They discussed several factors that could have prevented synchrony in their study but they strongly suggested that menstrual synchrony may not be a real phenomenon because of the methodological issues Wilson raised [ 13 ]  and because menstrual synchrony appears to lack adaptive significance. [ 11 ]


In addition to the study they conducted with lesbian couples, Weller and Weller conducted a number of other studies on menstrual synchrony during the 1990s. In most studies they reported finding menstrual synchrony, [ 41 ] [ 42 ] [ 43 ] [ 44 ] [ 45 ]  but in some studies they did not find synchrony. [ 48 ] [ 49 ] [ 50 ]  In a methodological review article in 1997, [ 51 ]  they refined their approach to measuring to better handle the problem of cycle variability. Specifically, they concluded that several menstrual cycles should be measured from each woman and that the longest average cycle length in a pair or group of women should be the basis for calculating the expected cycle onset difference. [ 51 ]  Thus, their research falls into the pre-1997 methodology [ 40 ] [ 41 ] [ 42 ] [ 48 ] [ 49 ]  and post-1997 methodology. [ 43 ] [ 44 ] [ 45 ] [ 50 ]


In 1997, Weller and Weller published one of the first studies to investigate when menstrual synchrony occurs in complete families. Their study was conducted in  Bedouin  villages in northern  Israel . Twenty seven families, which had from two to seven sisters 13 years or older and collected data on menstrual cycle onsets over a three-month period. Using the methods of, [ 51 ]  they reported menstrual synchrony occurred for the first two months, but not for the third month for roommate sisters, close friend roommates, and for families as a whole. [ 43 ]


Strassmann investigated whether menstrual synchrony occurred in a  natural fertility  population of Dogon village women. Her study consisted of 122 Dogon women with an average lifetime fertility rate of 8.6 ± .3 live births per woman. Their median cycle length was 30 days, which is indistinguishable from western women. [ 52 ]  In analyzing whether menstrual synchrony occurs among Dogon women, she was aware of Wilson's [ 13 ]  methodological criticisms of previous studies and aware that menstrual synchrony isn't synchrony  per se , but rather the closeness of menstruation among women. She used  Cox regression  to determine whether the likelihood of menstruating was influenced by other women. She considered the levels of all the women in the village, all the women in the same lineage, and all the women in the same economic unit (i.e., they worked together). She found no significant relationship at any level, which means that there was no evidence of synchronization. She concluded that this result undermined the view that menstrual synchrony is adaptive and the view held by many anthropologists at the time that menstrual synchrony occurred in preindustrial societies. [ 12 ]


Menstrual synchrony research declined after the published critiques in the 1990s and around the turn of the century. [ 12 ] [ 13 ] [ 14 ] [ 15 ]  The two studies published during this decade incorporated the methodological critiques into their designs and used more appropriate statistical methods. [ 5 ] [ 4 ]


Yang and Schank conducted the largest study to date with 186 Chinese college women. Ninety three of the women lived in 13 dorm rooms, 5 to 8 women per room. The other ninety three women lived in 16 dorm rooms, 4 to 8 women per room for a total of 29 rooms. The women were given notebooks to record the onset of each of their cycles and they collected data for over a year for most of the women. [ 5 ]


Following the statistical critiques of Schank, [ 14 ] [ 15 ]  they argued that  circular statistics  were required to analyze periodic data for the existence of synchrony. However, menstrual cycles are variable in frequency (e.g., 28 or 31 day cycles) and in length. They pointed out that there are no  statistical  methods for analyzing messy data like this, so they developed  Monte Carlo methods  for detecting synchrony. [ 5 ]


They found that in 9 of the 29 groups, women's cycles converged for one cycle closer than expected by chance, but then they diverged again. Upon further analysis, they found that for women with the cycle variability reported in this study, on average 10 out of 29 groups of women would show this pattern of convergence followed by divergence. They concluded that finding 9 out of 29 groups with convergence and then divergence is about what would be expected by chance and concluded that there was no evidence the women in this study synchronized their menstrual cycles. [ 5 ]


Ziomkiewicz conducted a study with 99 Polish college women living in two dormitories. Thirty six of the women lived in 18 double rooms and sixty three lived in 21 triple rooms. Women recorded their menstrual cycle onsets on menstrual calendars provided to them and 181 days' worth of menstrual cycle data were collected. The mean menstrual cycle length was 30.5 days ( SD  = 4.56). [ 4 ]


Based on the mean cycle length of the women in this study, the  expected  difference by chance in menstrual cycle onset was approximately 7.5 days. The mean difference in cycle onset was calculated for the beginning, middle, and end of the study for the pairs and triples of women. Ziomkiewicz found no statistically significant difference from the 7.5 day expected difference at either the beginning, middle, or end of the study. She concluded that there was no evidence of menstrual synchrony. [ 4 ]


H. Clyde Wilson  argued that at the start of any menstrual synchrony study, the minimum cycle onset difference must be calculated by using two onset differences from each woman in a group.  For example, suppose two women have exactly 28-day cycles. The greatest distance that one cycle onset can be from another is 14 days. Suppose the first two onsets for woman  A  are July 1 and July 29 and for woman  B , they are July 24 and August 21. If only the first two recorded onsets of  A  and  B  are compared, the difference between onsets is 23 days, which is greater than the 14 days that can actually occur. Wilson argued that McClintock [ 3 ]  did not correctly calculate the initial onset difference among women and concluded that the initial onset difference among women in a group was biased towards asynchrony. [ 13 ]


Yang and Schank followed up on this point by using computer simulations to estimate the average onset difference that would occur by among women with variable cycle lengths and a mean cycle length of 29.5 days reported by McClintock. [ 3 ]  They reported that the average onset difference by chance among women with cycle characteristics reported by McClintock was about 5 days. They also calculated the expected onset difference at the beginning of the study using McClintock's method for calculating initial cycle onset differences. They reported that the initial cycle onset difference for the groups of women using McClintock's method was about 6.5 days. McClintock reported that groups of women had an initial cycle onset difference at the beginning of her study of about 6.5 days and then subsequently synchronized to an average difference of a little less than 5 days. Yang and Schank point out that since the expected cycle onset differences they calculated were so close to the differences reported by McClintock, that there may be no statistical difference. They concluded that If their analysis is correct, it implies that synchrony did not occur in McClintock's original study. [ 5 ]


Cutler [ 53 ]  and Law [ 54 ]  hypothesized that menstrual synchrony is caused by menstrual cycles synchronizing with  lunar phases . However, neither of them agree on what phase of the lunar cycle menstrual cycles synchronize with. Cutler hypothesizes the synchronization with the full moon [ 53 ]  and Law with the new moon. [ 54 ]   Neither offer hypotheses regarding how lunar phases cause menstrual synchrony and neither are consistent with previous studies that found no relationship between menstrual cycles and lunar cycles. [ 55 ] [ 56 ]  More recently, Strassmann investigated menstrual synchrony among  Dogon  village women. The women were outdoors most nights and did not have electrical lighting. She hypothesized that Dogon women would be ideal for detecting a lunar influence on menstrual cycles, but she found no relationship. [ 12 ]


Jarett hypothesized that women who were more affiliative and concerned with how others viewed them would synchronize more. In her study, however, women with low affiliation scores were associated with greater synchrony.  She found that women with a need for social recognition and approval from others were associated with synchrony, which is partially consistent with her hypothesis. Nevertheless, the group of women she studied did not synchronize their menstrual cycles. [ 46 ]


When McClintock published her study on menstrual synchrony, she speculated that pheromones may cause menstrual synchrony. [ 3 ]   In a study on Norway rats, McClintock proposed and tested a coupled oscillator hypothesis (see section on rats below). The coupled-oscillator hypothesis proposed estrous cycles in rats were cause by two, estrous phase dependent pheromones that mutually modulated the length of cycles in a group and thereby causing synchrony. [ 57 ]


This idea was extended to humans in a study by Stern and McClintock. [ 58 ]  They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from  axillae  (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), and applying the compounds daily under the noses of recipient women.  In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were cut into smaller squares, frozen to preserve the scent, and readied for distribution to the recipients. The recipients were split into two groups, and were exposed to the compounds via application of the thawed axillary pad under their noses daily. [ 58 ]


The researchers concluded that odorless compounds collected from women during the late follicular phase of their menstrual cycles triggered hormonal events that shortened the menstrual cycles of the recipient women, and that odorless compounds collected from women during the time of ovulation triggered a hormonal event in the recipient women that lengthened their menstrual cycles.  Stern and McClintock concluded that these findings "proved the existence of human pheromones" as well as illustrated manipulation of the human menstrual cycle. [ 58 ]


Researchers pointed out several flaws in their study. [ 9 ] [ 59 ] [ 60 ]  Whitten's main critiques was with their using only their first cycles as a control for the subsequent conditions. He argued that this eliminate all within-subject variance. Control conditions should have been run between each experimental condition and not just at the beginning of the study. He was also skeptical about whether the coupled-oscillator model from rat research [ 61 ]  could be applied to humans. [ 59 ]


Arden and Dye investigated women's awareness and perception of menstrual synchrony. [ 62 ]  Their study consisted of 122 women (students and staff) at  Leeds University . A four-page questionnaire was sent to each participant. After providing personal details, they were given a description of menstrual synchrony: "Menstrual synchrony occurs when two or more women, who spend time with each other, have their periods at approximately the same time" (p. 257). [ 62 ]  After reading the description they were asked whether they were aware of menstrual synchrony and whether they had experienced it. They were then asked details about their experience of synchrony such as how many times they experienced and how long it lasted. [ 62 ]


They found that 84% of the women were aware of the phenomenon of menstrual synchrony and 70% reported the personal experience of synchrony. The experience of synchrony occurred most commonly with close friends followed by roommates. There was considerable variation in the reported time spent together before synchrony occurred ranging from zero to four weeks to 12 months or more. The most common time was 12 months or more. The duration of menstrual synchrony also was highly variable with responses ranging from one to two months to 12 months or more. They conclude that "Whether or not future research concludes that menstrual synchrony is an objective phenomenon, subjective experiences, which are apparently widespread, need to be given careful consideration." (p. 265) [ 62 ]


Both Wilson [ 13 ]  and Arden and Dye [ 63 ]  pointed out that menstrual synchrony can occur by chance when there is menstrual cycle variability. Yang and Schank [ 5 ]  argued that when there is cycle variability (i.e., either women have irregular cycles, have cycles of different frequencies, or both), most women will have the opportunity to experience synchrony even though it is a result of cycle variability and not a result of a mechanism such as the exchange of pheromones. For example, consider two women  A  and  B .  Suppose  A  has menstrual cycles that are 28 days long and  B  has cycles that are 30 days long.  Suppose further that when  A  and  B  become close friends,  B  has a cycle onset 14 days before  A' s next onset. The next time both of them have menstrual cycle onsets,  B  will have a cycle onset 12 days before  A .  B  will continue to gain two days on  A  until their onsets coincide, then their cycles will begin to diverge again. The cycles of  A  and  B  will repeatedly converge and diverge creating the appearance of synchrony during convergence. This is a mathematical property of cycles of different frequencies and not due to the interaction of  A  and  B . [ 5 ]  If, in addition, the duration of menstruation is considered (typically 3 to 5 days with a range of 2 to 7 days [ 64 ] ), then the experience of synchrony may last a number of months. [ 5 ]


Strassmann [ 12 ]  argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age [ 65 ]  —is 7 days. Considering that the mean duration of menses is 5 days [ 12 ]  and the range is 2 to 7 days, [ 64 ]  the probability of menstruation overlap by chance should be high. [ 12 ]


In order to work out why menstrual synchrony might have evolved, it is necessary to investigate why individuals who synchronized their cycles might have had increased survival and reproduction in the evolutionary past. The relevant field in this case is  behavioral ecology .


In mammalian mating systems generally, and among primates in particular, female spatio-temporal distribution – how clumped females are in the environment and how much they overlap their fertile periods – affects the ability of any single male to monopolize matings. [ 66 ] [ 67 ]  The basic principle is that the more females are fertile at any one time, the harder it is for any single male to monopolize access to them, impregnating all simultaneously at the expense of rival males. In the case of nonhuman primates, once the number of co-cycling females rises above a critical threshold, [ 68 ]  a harem-holder may be unable to prevent other males from invading and mating with his females. A dominant male can maintain his monopoly only if his females stagger their fertile periods, so that he can impregnate them one at a time (see figure  a , right). Suppose a group of female baboons need between them just one dominant male, desirable in view of his high-quality genes. Then, logically, they should avoid synchronizing their cycles. By the same token, if males during the course of human evolution became valued by females for additional purposes – hunting and bringing home food, for example – then females should resist being controlled by dominant male harem-holders. If males are useful partners to have and keep around, then ideally each female should have at least one for herself. Under those circumstances, according to this argument, the logical strategy would be for females to synchronize as tightly as they can (see figure  b , right). [ 69 ] [ 70 ]


One implication is that there may be a link between the degree of synchrony in a population (whether seasonal, lunar or both), and the degree of reproductive egalitarianism among males. Foley and Fitzgerald [ 71 ]  objected to the idea that synchrony could have been a factor in human evolution on the grounds that for hominins with inter-birth intervals of 3–5 years, achieving synchrony was unrealistic. Infant mortality would disrupt synchrony since it would be too costly for a mother who had miscarried or lost her baby to wait until everyone else had weaned their babies and resumed cycling before having sex and getting pregnant herself. On the other hand, while conceding that it would be impossible to get clockwork synchrony throughout an inter-birth interval, Power et al. [ 72 ]  argued that once we take account of birth seasonality – enhancing the effects of menstrual synchrony by clumping fertile cycles within a relatively brief time-window – it emerges that reproductive synchrony can be effective as a female strategy to undermine primate-style sexual monopolization by dominant males. The controversy remains unresolved.


Adopting a compromise position, one school of Darwinian thought sets out from the fact that the mean length of the human menstrual cycle is 29.3 days, [ 73 ]  which is strikingly close to the 29.5 day  periodicity of the moon . It is suggested that the human female may once have had adaptive reasons for evolving such a cycle length  – implying some theoretical potential for synchrony to a lunar clock – but did so in an African setting under prehistoric conditions which today no longer exist. [ 74 ] [ 75 ]  Not all archaeologists accept that lunar periodicity was ever relevant to human evolution. On the other hand, according to Curtis Marean (head of excavations at the important Middle Stone Age site of  Pinnacle Point , South Africa), anatomically modern humans around 165,000 years ago – when inland regions of the continent were dry, arid and uninhabitable – became restricted to small populations clustered around coastal refugia, reliant on marine resources including shellfish whose safe harvesting at spring low tides presupposed careful tracking of lunar phase. [ 76 ]


With gradual offshore platforms during spring low tides, substantial areas of the intertidal zone are revealed, and these are the most productive and safest shellfish collecting times… Foragers should schedule visits to coastal residential sites at times during the lunar month when spring tides are present and then move slightly inland during neaps to broaden the size of the exploitable terrestrial area. [ 76 ]


College students' menstrual periods can become synchronised when they live together as roommates, according to research by McClintock (McClintock, 1971). Since then, numerous investigations have supported the existence of menstrual synchronisation among women, including close friends, mothers and daughters, and coworkers [reviewed by Weller and Weller in 1993]. Women who spent the most time together were more likely to exhibit menstrual synchrony in each of these investigations. The axillary region's scents have been shown to be capable of mediating these effects (Preti et al., 1987; Russell et al., 1980; Stern and McClintock, 1998), but their active ingredients have not yet been discovered.


The main olfactory system, which receives sensory inputs from the olfactory mucosa and connects to the rest of the central nervous system via the main olfactory bulbs, and the accessory system, which receives inputs from the vomeronasal organ and connects to other brain centres via the accessory olfactory bulbs, are the two olfactory systems that are present in the majority of mammals (Scalia and Winans, 1976). There are connections from the olfactory bulbs to the hypothalamus, the brain region in charge of regulating the release of luteinizing hormone, in both systems.


In rats, the accessory system mediates the pheromonal action, [reviewed by Marchewska-Koj (Marchewska-Koj, in 1984)]. However, it appears that the pheromonal action in ewes and pigs is largely mediated via the primary olfactory system (Martin et al., 1986). (Dorries et al., 1997). If pheromones that mediate menstrual synchrony use the main olfactory system, a comparison of synchronised and non-synchronized women's ability to smell a particular pheromone can be used to infer a causal relationship between the ability to smell a pheromone and a potential role for the pheromone in mediating synchrony. In the current work, we looked at how menstrual synchrony and the sense of smell for the putative pheromones 3 androstenol and 5 androstenone related. [ 77 ]


Estrous synchrony, a phenomenon similar to menstrual synchrony, [ 78 ]
has been reported in several other mammalian species. [ 79 ] [ 80 ]


Menstrual or  estrous  synchrony has been reported in other species including  Norway rats , [ 78 ]   hamsters , [ 79 ]   chimpanzees , [ 81 ]  and  golden lion tamarins . [ 80 ]  In non-human primates, the term may also refer to the degree of overlap of menstrual or estrous cycles, which is the overlap of estrous or  menses  of two or more females in a group due, for example, to  seasonal breeding . [ 82 ]


However, as with early human studies on menstrual synchrony, non-human estrous synchrony studies also were criticized for methodological problems. [ 83 ] [ 84 ] [ 85 ]


Subsequent studies failed to find estrous synchrony in rats, [ 86 ]  hamsters, [ 87 ]  chimpanzees, [ 88 ] [ 89 ]  and golden lion tamarins. [ 90 ]


McClintock also conducted a 1978 study of estrous synchrony in Norway rats ( Rattus norvegicus ).  She reported that the estrous cycles of female rats living in groups of five were more regular than those of rats housed singly.  She also reported that social interaction, and more importantly a shared air supply that allowed for  olfactory communication  enhanced the regularity of the rats' cycles and synchronized their estrous phases after two or three cycles. McClintock hypothesized that estrous synchrony was caused by pheromones and that a coupled oscillator mechanism produced estrous synchrony in rats [ 78 ] [ 61 ]  This observation of menstrual synchrony in Norway rats is not the same as the  Whitten effect  because it was the result of the continuous interactions of ongoing cycles within a female group, rather than the result of an exposure to a single external stimulus such as male odor, which in the Whitten effect releases all exposed females simultaneously from an acyclic condition. [ 91 ]


The coupled-oscillator hypothesis asserted that females rats release two pheromone signals.  One signal is released during the  follicular phase  of the estrous cycle and it shortens estrous cycles. The second signal is released during the  ovulatory phase  of the estrous cycle and it lengthens estrous cycles. When rats live together or share the same air supply, the pheromones released by each female in a group as a function of the phase of her estrous cycle causes other females in the group to either lengthen or shorten their estrous cycles.  This mutual lengthening and shortening of estrous cycles was theorized to produce synchronization of estrous cycles over time. [ 78 ] [ 57 ] [ 61 ]


McClintock investigated the coupled oscillator hypothesis experimentally. She provided three groups of rats with airborne odors from female rats in three different phases of the estrous cycle: ovulatory phase, follicular phase, and  luteal phase .  She hypothesized that ovulatory phase odors would lengthen cycles, follicular phase odors would shorten cycles, and luteal phase odors would have no effect.  Her results showed a lengthening of estrous cycles for females who received ovulatory odors, shortening of cycles for females who received follicular odors, and no effect for females who received luteal phase odors. [ 57 ]


The coupled-ocillator hypothesis was also investigated using a computer simulation model, [ 61 ]  which was compared with data from McClintock's 1978 study. [ 78 ]   They found that a coupled oscillator mechanism could produce estrous synchrony in female rats, but the effect was very weak. [ 61 ]   The proposed mechanisms of this model were more precisely tested by controlling the airborne odors received by individual females. [ 92 ]  They found support for the hypothesis that follicular phase odors short the length of estrous cycles, but they did not find that ovulatory phase odors lengthened cycles [ 92 ]  as the earlier study by McClintock had found. [ 57 ]


Schank conducted another experiment to test whether female rats could synchronize their cycles. [ 83 ]  He found that female rats did not synchronize their cycles and he argued that in the original McClintock study, [ 78 ]  the random control group was more asynchronous than expected by chance.  When the experimental group was compared to the control group in McClintock's 1978 study, [ 78 ]  the experimental group was more synchronous than the control group but only because the control group was too asynchronous and not because the experimental group had synchronized their cycles.  In a follow-up study, Schank again found no effect of estrous synchrony in rats. [ 86 ]


In 1980, estrous synchrony was reported in female hamsters. In their study, hamsters were housed in four colony phase of the estrous cycle. They monitored and females in each room and removed the females that did not stay in phase. They placed a wire metal cage (i.e., condo consisting of four equally sized rectangular compartments) in the corner of each room. For each room, three animals were randomly selected and placed in three of the condo compartments. A fourth female was randomly selected from another room and placed in the remaining condo compartment. In the control condition, all four females placed in the condos came from the same room. Females were kept in the condos until all four animals exhibited 4 consecutive days of synchrony. They were then removed and a new group was formed until all combinations were tested. They found that the fourth female in the experimental condition always synchronized with the remaining three [ 79 ]


Their study was criticized as methodologically flawed because females were left together until the fourth female synchronized with the others.  When female hamsters are subjected to the stress of stranger hamsters, their cycles become irregular.  If only the female from another room's cycles change, then by chance, the longer the female is left with the other three, the more likely it is that she will synchronize by chance with the other three. [ 84 ]  In a follow-up experimental study motivated by this methodological critique, no evidence for estrous synchrony was found for female hamsters. [ 87 ]


In 1985, estrous synchrony was reported in female chimpanzees. In her study, 10 female chimpanzees were caged, at different times, in two groups of four and six in the same building. The anogenital swelling of each female was recorded daily. Synchrony was measured by calculating the absolute differences in days between (1) the day of swelling onset and (2) the day of maximum swelling.  She reported a statistically significant average difference of 5.7 days for onset of swelling and 8.0 days for maximum swelling. [ 81 ]  Schank, however, noted that due to females who became pregnant and who stopped cycling, most of the data were based on only four animals. [ 85 ]  He performed a computer simulation study to calculate the expected swelling onset and maximal swelling onset difference for female chimpanzees with the reported mean estrous cycle lengths of 36.7 (with a standard deviation of 4.3) days.  He reported an expected difference of 7.7 days.  Thus, a maximum swelling difference of 8.0 days is about what would be expected by chance and given that only four animals contributed data to the study, a 5.7 day onset difference is not significantly less than 7.7 days. [ 85 ]


Since then Matsumoto and colleagues have reported estrous asynchrony in groups of free-living chimpanzees in  Mahale Mountains National Park ,  Tanzania . [ 88 ] [ 89 ]  They subsequently investigated whether estrous asynchrony was adaptive for female chimpanzees.  They tested three hypotheses about the adaptiveness of estrous asynchrony: (1) females become asynchronous to increase copulation frequency and opportunities for giving birth; (2) paternity confusion to reduce  infanticide ; and (3)  sperm competition . They found no support for hypothesis (1) and partial support for hypotheses (2) and (3). [ 93 ]


In 1987, estrous synchrony was reported in female golden lion tamarins by French and Stribley.  Their subjects consisted of five adult female golden lion tamarins that were housed in two groups. Two females were housed with adult males and three females (a mother and two daughters) were housed with an adult male and infant male. They reported a 2.11 day difference in peak cycle estrogen for the two groups, which was less than the 4.5 day difference that they calculated would be the difference based on golden lion tamarins having a 19-day estrous cycle. [ 80 ]  Schank reanalyzed their study with the help of computer simulation and reported that a 2.11 day difference was not likely statistically significant. [ 85 ]  Monfort and colleagues conducted a study with eight females housed in pairs and found no evidence of synchrony. [ 90 ]


Setchella, Kendala, and Tyniec investigated whether menstrual synchrony occurred in a semi-free-ranging population of  mandrills  of 10-group years.  They reported that mandrills do not synchronize their menstrual cycles and concluded that cycle synchrony does not occur in non-human primates. [ 94 ]


Oestrus synchrony has been reported of  lions  in the wild. [ 95 ]


Menstruation  (also known as a  period , among other colloquial terms) is the regular discharge of  blood  and  mucosal tissue  from the  inner lining of the uterus  through the  vagina . The  menstrual cycle  is characterized by the rise and fall of  hormones . Menstruation is triggered by falling  progesterone  levels, and is a sign that  pregnancy  has not occurred.


The first period, a point in time known as  menarche , usually begins between the ages of 12 and 15. [ 1 ]  Menstruation starting as young as 8 years would still be considered normal. [ 2 ]  The average age of the first period is generally later in the  developing world , and earlier in the  developed world . [ 3 ]  The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women. In adults, the range is between 21 and 35 days with the average being 28 days. [ 2 ] [ 3 ]  Bleeding usually lasts around 2 to 7 days. Periods stop during  pregnancy  and typically do not resume during the initial months of  breastfeeding . [ 2 ]   Lochia  occurs after childbirth. [ 4 ]  Menstruation, and with it the possibility of pregnancy, ceases after  menopause , which usually occurs between 45 and 55 years of age. [ 5 ]


Up to 80% of women do not experience problems sufficient to disrupt daily functioning either during menstruation or in the days leading up to menstruation. Symptoms in advance of menstruation that do interfere with normal life are called  premenstrual syndrome  (PMS). Some 20 to 30% of women experience PMS, with 3 to 8% experiencing severe symptoms. [ 6 ]  These include  acne , tender breasts, bloating, feeling tired, irritability, and mood changes. [ 7 ]  Other symptoms some women experience include  painful periods  (estimates are between 50 and 90%) and heavy bleeding during menstruation and  abnormal bleeding  at any time during the menstrual cycle. [ 2 ]  A lack of periods, known as  amenorrhea , is when periods do not occur by age 15 or have not re-occurred in 90 days. [ 2 ]


The first menstrual period occurs after the onset of pubertal growth, and is called  menarche . The average age of menarche is 12 to 15 years. [ 1 ] [ 8 ]  However, it may occur as early as eight. [ 2 ]  The average age of the first period is generally later in the  developing world , and earlier in the  developed world . [ 3 ] [ 9 ]  The average age of menarche has changed little in the United States since the 1950s. [ 3 ]


Menstruation is the most visible phase of the  menstrual cycle  and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 35 days in adults. [ 2 ] [ 3 ]  The average length is 28 days; one study estimated it at 29.3 days. [ 10 ]  The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39 years. [ 11 ]  The variability increases slightly for women aged 40 to 44 years. [ 11 ]


Perimenopause  is when a woman's fertility declines, and menstruation occurs less regularly in the years leading up to the final menstrual period, when a woman stops menstruating completely and is no longer fertile. The medical definition of  menopause  is one year without a period and typically occurs between 45 and 55 years in Western countries. [ 5 ] [ 12 ] : 381   Menopause before age 45 is considered  premature  in industrialized countries. [ 13 ]  Illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier than it might have otherwise. [ 14 ]


The average volume of menstrual fluid during a monthly menstrual period is 35 millilitres (2.4 US tbsp) with 10–80 millilitres (0.68–5.41 US tbsp) considered typical. Menstrual fluid is the correct name for the flow, although many people prefer to refer to it as menstrual blood. Menstrual fluid is reddish-brown, a slightly darker color than venous blood. [ 12 ] : 381


About half of menstrual fluid is blood. This blood contains sodium, calcium, phosphate, iron, and chloride, the extent of which depends on the woman. As well as blood, the fluid consists of cervical mucus, vaginal secretions, and  endometrial tissue . Vaginal fluids in menses mainly contribute water, common electrolytes, organ moieties, and at least 14 proteins, including glycoproteins. [ 15 ]


Many women and girls notice blood clots during menstruation. These appear as clumps of blood that may look like tissue. If there was a  miscarriage  or a  stillbirth , examination under a microscope can confirm if it was endometrial tissue or pregnancy tissue ( products of conception ) that was shed. [ 16 ]  Sometimes menstrual clots or shed endometrial tissue is incorrectly thought to indicate an early-term miscarriage of an embryo. An  enzyme  called  plasmin  – contained in the endometrium – tends to inhibit the blood from  clotting . [ 17 ]


The amount of iron lost in menstrual fluid is relatively small for most women. [ better source needed ] [ 18 ]  In one study, premenopausal women who exhibited symptoms of  iron deficiency  were given  endoscopies . 86% of them actually had  gastrointestinal disease  and were at risk of being misdiagnosed simply because they were menstruating. [ non-primary source needed ] [ 19 ]  Heavy menstrual bleeding, occurring monthly, can result in  anemia . [ 20 ]


Although a normal and natural process, [ 21 ]  some women experience  premenstrual syndrome  with symptoms that may include  acne ,  tender breasts , and  tiredness . [ 22 ]  More  severe symptoms that affect daily living are classed as  premenstrual dysphoric disorder  and are experienced by 3 to 8% of women. [ 23 ] [ 24 ] [ 22 ] [ 25 ]   Dysmenorrhea  (menstrual cramps or period pain) is felt as painful cramps in the abdomen that can spread to the back and upper thighs during the first few days of menstruation. [ 26 ] [ 27 ] [ 28 ]  Debilitating period pain is not normal and can be a sign of something severe such as  endometriosis . [ 29 ]  These issues can significantly affect a  woman's health  and quality of life and timely interventions can improve the lives of these women. [ 30 ]


Premenstrual syndrome  (PMS) is a disruptive set of  emotional  and physical  symptoms  that regularly occur in the one to two weeks before the start of each menstrual period. [ 33 ] [ 34 ]  Symptoms resolve around the time  menstrual bleeding  begins. [ 33 ]  Symptoms vary, [ 35 ]  though commonly include one or more physical, emotional, or behavioral symptoms, that resolve with menses. [ 36 ]  The range of symptoms is wide, and most commonly are  breast tenderness ,  bloating ,  headache ,  mood swings ,  depression ,  anxiety ,  anger , and  irritability . To be diagnosed as PMS, rather than a normal discomfort of the menstrual cycle, these symptoms must interfere with daily living, during two menstrual cycles of prospective recording. [ 36 ]  PMS-related symptoms are often present for about six days. [ 37 ]  An individual's pattern of symptoms may change over time. [ 37 ]  PMS does not produce symptoms during  pregnancy  or following  menopause . [ 38 ]


Diagnosis requires a consistent pattern of emotional and physical symptoms occurring after  ovulation  and before menstruation to a degree that interferes with normal life. [ 39 ]  Emotional symptoms must not be present during the initial part of the  menstrual cycle . [ 39 ]  A daily list of symptoms over a few months may help in diagnosis. [ 37 ]  Other disorders that cause similar symptoms need to be excluded before a diagnosis is made. [ 37 ]


The cause of PMS is unknown, but the underlying mechanism is believed to involve changes in hormone levels during the course of the whole menstrual cycle. [ 38 ]  Reducing salt, alcohol, caffeine, and  stress , along with increasing exercise is typically all that is recommended for the management of mild symptoms. [ 38 ]   Calcium  and  vitamin D  supplementation may be useful in some. [ 37 ]  Anti-inflammatory drugs such as ibuprofen or  naproxen  may help with physical symptoms. [ 38 ]  In those with more significant symptoms,  birth control pills  or the  diuretic   spironolactone  may be useful. [ 38 ] [ 37 ]