By design, the Sister Study enrolled women with a family history of breast cancer in order to enrich the cohort for environmental and genetic risk factors for breast cancer. To the extent that early menarche–a breast cancer risk factor–was more common among our study participants than among women in the general population, the statistical power for estimating associations with early-life exposures would be enhanced.39 In particular, the frequency of early menarche (20%) in our cohort of predominantly white women was lower than in white girls from the Bogalusa Heart Study cohort (30%)40 but higher than in white girls from a large clinical study conducted in the 1990s.41 There is also concern that women with a family history of breast cancer may report age at menarche either more or less accurately than women without such family history. However, our estimates of mean age at menarche by birth decade for whites (data not shown) were similar to results from the National Health and Nutrition Examination Survey (NHANES).42 Furthermore, given that the entire cohort has a family history of breast cancer and all
