Infertility and risk of postmenopausal breast cancer in the women's health initiative.

Purpose: Although infertility (i.e., failure to conceive after ≥ 12 months of trying) is strongly correlated with established breast cancer risk factors (e.g., nulliparity, number of pregnancies, and age at first pregnancy), its association with breast cancer incidence is not fully understood. Previous studies were primarily small clinic-based or registry studies with short follow-up and predominantly focused on premenopausal breast cancer. The objective of this study was to assess the relationship between infertility and postmenopausal breast cancer risk among participants in the Women's Health Initiative (analytic sample = 131,784; > 25 years of follow-up).

Methods: At study entry, participants were asked about their pregnancy history, infertility history, and diagnosed reasons for infertility. Incident breast cancers were self-reported with adjudication by trained physicians reviewing medical records. Cox proportional hazards models were used to estimate risk of incident postmenopausal breast cancer for women with infertility (overall and specific infertility diagnoses) compared to parous women with no history of infertility. We examined mediation of these associations by parity, age at first term pregnancy, postmenopausal hormone therapy use at baseline, age at menopause, breastfeeding, and oophorectomy.

Results: We observed a modest association between infertility (n = 23,406) and risk of postmenopausal breast cancer (HR = 1.07; 95% CI 1.02-1.13). The association was largely mediated by age at first term pregnancy (natural indirect effect: 46.4% mediated, CI 12.2-84.3%).

Conclusion: These findings suggest that infertility may be modestly associated with future risk of postmenopausal breast cancer due to age at first pregnancy and highlight the importance of incorporating reproductive history across the life course into breast cancer analyses.