MENSTRUAL-CYCLE CHARACTERISTICS AND HISTORY OF OVULATORY INFERTILITY IN RELATION TO BREAST-CANCER RISK IN A LARGE COHORT OF US WOMEN

Menstrual cycle characteristics and ovulatory infertility were evaluat ed in relation to breast cancer risk among 116,678 women in the Nurses ' Health Study II, a prospective cohort study of female registered nur ses who were aged 25-42 years and living in 14 US states at enrollment in 1989. During 396,299 person-years of follow-up between return of t he baseline questionnaire and June 1993, 251 cases of breast cancer we re identified in this cohort. The multivariate relative risk (RR) asso ciated with age at menarche >13 years compared with age less than or e qual to 12 years was 0.66 (95% confidence interval (CI) 0.44-0.99). Sh ort and long menstrual cycle lengths at ages 18-22 years were associat ed with reduced risk. Compared with menstrual cycle length 26-31 days, the multivariate relative risks (95% Cls) for more extreme cycle leng ths were: <26 days, 0.50 (0.25-0.98); 32-39 days, 0.81 (0.51-1.28); an d >39 days or too irregular for estimation of a usual cycle length, 0. 41 (0.18-0.94). The multivariate relative risk associated with a histo ry of ovulatory infertility, compared with no such history, was 0.41 ( 95% CI 0.18-0.93). These results are consistent with the hypothesis th at reduced exposure to ovulatory menstrual cycles provides a protectiv e effect against breast cancer.