ratio, *CI* confidence interval^a^1st, 2nd, 3rd and 4th quartile respectively: ≤ 0.0, 0.1--1.0, 1.1--4.6, \> 4.7%^b^Determined using log-transformed epithelial area as continuous variable^c^Variables included in the reproductive score: parous (no = 0, yes = 1), number of pregnancies without birth (0--4), number of births (0--5), age at first birth (nulliparous = 0, \< 20 = 1, 20--25 = 2, \> 25 = 3), and minimum duration of breastfeeding (0, \< group mean = 1, \> group mean = 2)

Among all women, a higher reproductive score was positively associated with a 11.1% greater epithelial content per unit increase (*p* for trend = 0.001) (Table [3](#Tab3){ref-type="table"}), with the OR in the highest quartile compared with the lowest quartile being 1.35 (95% CI 1.13--1.62) when adjusting for age and BMI. The findings remained significant for postmenopausal women (11.0% increase per unit reproductive score) (*p* for trend = 0.002) (Table [3](#Tab3){ref-type="table"}, Fig. [1](#Fig1){ref-type="fig"}), with the OR in the highest quartile compared with the lowest quartile being 1.28 (95% CI 1.04--1.57).

Association of risk factors with epithelial protein expression {#Sec9}
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In the total data set, higher age was significantly associated with a 1.0% higher expression of ER per year increase at biopsy (*p *= 0.033) (Table [4](#Tab4){ref-type="table"}) and a 2.2% lower expression of PR per year (*p *= 0.049) (Supplemental Table S2). Furthermore, higher BMI was associated with a significantly higher expression of PR (6.4%; *p *= 0.041). Increasing number of births was significantly associated with greater expression of ER in the total data set (10.0% per birth; *p *= 0.017), which remained significant among postmenopausal women (13.2% per birth; *p *= 0.013) (Table [4](#Tab4){ref-type="table"}, Fig. [2](#Fig2){ref-type="fig"}). Parous postmenopausal women also had 40.8% higher