centage epithelial area or epithelial ER and reproductive score (continuous) were analysed by linear regression. Odds ratios (ORs) were evaluated using multinomial logistic regression comparing quartile groups of epithelial area or epithelial ER (cut points based on the overall distribution and the lowest 25% as the reference group in all analyses) with reproductive score for all women and stratified by menopausal status.

In secondary analyses, we stratified by menopausal status at biopsy. In total, 55 women were premenopausal at biopsy collection, and 95 were postmenopausal. Menopausal status was missing for three participants.

Two-tailed *p* values were used for all analyses with a *p* value of less than 0.05 considered to be statistically significant. Statistical analyses were performed using SPSS IBM, version 23.

Results {#Sec7}
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Association of risk factors with epithelial area {#Sec8}
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Distributions of demographics, menstrual and reproductive history, and selected risk factors for breast cancer are described in Table [1](#Tab1){ref-type="table"}. Histological sections from a total of 153 individuals were examined for association of risk factors with the epithelial tissue compartment (Table [2](#Tab2){ref-type="table"}). Table 2Linear regression analysis of log-transformed epithelial area expressed as percentage of the total area of the section by risk factors, for all women and stratified by menopausal status, adjusted for age and BMIVariablesEpithelial area (%)^a^Epithelial area (%)^a^Epithelial area (%)^b^All women (*N *= 153)Premenopausal women (*N *= 55)Postmenopausal women (*N *= 95)*N*Estimates β (SE)*pN*Estimates β (SE)*pN*Estimates β (SE)*p*Age at biopsy (y)153− 0.022 (0.011)0.049^b^550.010 (0.042)0.820^a^95− 0.011 (0.018)0.531^b^BMI (Kg/m^2^)151− 0.053 (0.023)0.026^c^54− 0.056 (0.045)0.214^c^94− 0.057 (0.027)0.038^c^Age at menarche (y)1440.008 (0.075)0.911530.112 (0.185)0.42191− 0.076 (0.088)0.390Age at f