History of infertility and risk of breast cancer: a prospective cohort study.

PURPOSE: Research on infertility and risk of breast cancer has been conflicting, potentially because many well-established breast cancer risk factors, such as pregnancy history, are strongly correlated with infertility.

METHODS: We followed participants in the Nurses' Health Study II from 1989 to 2015 (n = 103,080) for the development of invasive breast cancer and calculated Hazard Ratios (HR) and 95% confidence intervals (CI) using Cox regression. Participants with a self-reported history of infertility (12 months of trying without conception) were compared to gravid women with no history of infertility. We classified breast cancer by menopausal status and investigated mediation by reproductive factors.

RESULTS: Over 26 years of follow-up, 26,208 (25.4%) women reported a history of infertility, and 3,201 women were newly diagnosed with invasive breast cancer. We observed no association between infertility history and risk of overall breast cancer (HR: 1.05, 95% CI: 0.97-1.14) or premenopausal breast cancer (RR: 0.93, 95% CI: 0.83-1.03). However, we observed a modest association between history of infertility and risk of postmenopausal breast cancer (HR: 1.13, 95% CI: 1.00-1.28), approximately 50% of which could be attributed to lower total parity and later age at first birth (95% CI: 8.2%-91.0%).

CONCLUSIONS: Women with a history of infertility were at increased risk of postmenopausal breast cancer. Older age at first birth and lower total parity explained approximately half of the association between infertility and risk of postmenopausal breast cancer.