Background: To evaluate the effect of the cumulative number of ovulatory cycles and its contributing components on the risk of breast cancer among BRCA mutation carriers. Methods: We conducted a matched case-control study on 2,854 pairs of women with a BRCA1 or BRCA2 mutation. Conditional logistic regression was used to estimate the association between the number of ovulatory cycles and various exposures and the risk of breast cancer. Information from a subset of these women enrolled in a prospective cohort study was used to calculate age-specific breast cancer rates. Results: The annual risk of breast cancer decreased with the number of ovulatory cycles experienced (ρ =-0.69; P = 0.03). Age at menarche and duration of breastfeeding were inversely related with risk of breast cancer among BRCA1 (Ptrend <0.0001) but not among BRCA2 (Ptrend ≥ 0.28) mutation carriers. The reduction in breast cancer risk associated with surgical menopause [OR, 0.52; 95% confidence interval (CI), 0.40-0.66; Ptrend <0.0001] was greater than that associated with natural menopause (OR, 0.81; 95% CI, 0.62-1.07; P trend = 0.14). There was a highly significant reduction in breast cancer risk among women who had an oophorectomy after natural menopause (OR, 0.13; 95% CI, 0.02-0.54; P = 0.006). Conclusions: These data challenge the hypothesis that breast cancer risk can be predicted by the lifetime number of ovulatory cycles in women with a BRCA mutation. Both pre- and postmenopausal oophorectomy protect against breast cancer. Impact: Understanding the basis for the protective effect of oophorectomy has important implications for chemoprevention.
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