Cumulative menstrual months and breast cancer risk by hormone receptor status and ethnicity: The Breast Cancer Etiology in Minorities Study.

Reproductive and hormonal factors may influence breast cancer risk via endogenous estrogen exposure. Cumulative menstrual months (CMM) can be used as a surrogate measure of this exposure. Using harmonized data from four population-based breast cancer studies (7284 cases and 7242 controls), we examined ethnicity-specific associations between CMM and breast cancer risk using logistic regression, adjusting for menopausal status and other risk factors. Higher CMM was associated with increased breast cancer risk in non-Hispanic Whites, Hispanics and Asian Americans regardless of menopausal status (all FDR adjusted P trends = .0004), but not in African Americans. In premenopausal African Americans, there was a suggestive trend of lower risk with higher CMM. Stratification by body mass index (BMI) among premenopausal African American women showed a nonsignificant positive association with CMM in nonobese (BMI <30 kg/m2 ) women and a significant inverse association in obese women (OR per 50 CMM = 0.56, 95% CI 0.37-0.87, Ptrend  = .03). Risk patterns were similar for hormone receptor positive (HR+; ER+ or PR+) breast cancer; a positive association was found in all premenopausal and postmenopausal ethnic groups except in African Americans. HR- (ER- and PR-) breast cancer was not associated with CMM in all groups combined, except for a suggestive positive association among premenopausal Asian Americans (OR per 50 CMM = 1.33, P = .07). In summary, these results add to the accumulating evidence that established reproductive and hormonal factors impact breast cancer risk differently in African American women compared to other ethnic groups, and also differently for HR- breast cancer than HR+ breast cancer.