Dual Effect of Parity on Breast Cancer Risk in African-American Women

higher among African-American women than among white women before age 45 but lower at older ages. To explore whether differences in childbearing patterns can explain this observation, we assessed the relation of several childbearing variables to breast cancer risk in a large prospective cohort study of U.S. African-American women. Methods: Black Women’s Health Study participants were enrolled in 1995 and were followed by mailed questionnaires every 2 years (in 1997 and 1999). Of the 64 500 women enrolled, 56 725 (88%) completed at least one of the follow-up questionnaires. Dur-ing 214 862 person-years of follow-up, participants reported 349 breast cancers, of which 128 were among women younger than 45 years and 221 were among women aged 45–70 years. Incidence rate ratios (IRRs) with 95 % confi-dence intervals (CIs) were derived from age-stratified Co