ls through exhausted ovaries.

Duration of breastfeeding was associated with increased epithelial area later in life in our overall study population and among postmenopausal women. These findings are in agreement with that of Figueroa et al. \[[@CR30]\] who found a positive association between ever having breastfed and the number of TDLUs in postmenopausal women, and others who found positive associations between fibroglandular tissue and duration of lactation \[[@CR24], [@CR37]\]. On the other hand, our results conflict with those studies that found no association \[[@CR38], [@CR39]\]. We did not observe any significant changes in stromal compartment associated with duration of breastfeeding, however; amount of adipose tissue was inversely associated with breastfeeding (Supplemental information). Duration of breastfeeding was also positively associated with epithelial expression of PR in our overall study population and among postmenopausal women. Breastfeeding is protective for the luminal A, B and basal-like subtypes of breast cancer, with the largest protective effect of breastfeeding on the risk of the latter \[[@CR40]\]. Basal-like cancers are thought to originate from undifferentiated luminal progenitor cells \[[@CR41]\]. Fully differentiated type 4 lobules do not form until the end of pregnancy and during lactation when the breast attains its maximum development and increase in lobule numbers and size \[[@CR42]\]. Progesterone is necessary for the lobuloalveolar mammary gland development \[[@CR43], [@CR44]\], and the luminal population of epithelial cells express more PR compared to the stem cell-enriched fraction \[[@CR45], [@CR46]\]. After menopause, the type 4 lobules regress to predominantly type 1-lobules, although more differentiated \[[@CR47]\]. Data also suggest that breastfeeding is associated with less involution in postmenopausal women \[[@CR30]\]. Although we did not determine lobule types and differentiation in this study, we hypothesise that epithelial expression of PR would increase with mo