Invasive breast cancer incidence by estrogen plus progestin use at baseline. Analyses were adjusted for age, race or ethnic group, body mass index, education, smoking status, alcohol use, self-reported health, level of physical activity, presence or absence of a family history of breast cancer, estimated breast-cancer risk based on the Gail model, and bilateral oophorectomy and stratified by baseline age group. All statistical tests were two-sided. CI = confidence interval; HR = hazard ratio.