Being breastfed in infancy and breast cancer incidence in adult life: Results from the two nurses' health studies

Events during perinatal and early life may influence the incidence of breas t cancer in adult life, and some case-control studies suggest that having b een breastfed may reduce breast cancer risk. The authors studied this assoc iation among premenopausal and postmenopausal women by using data from the two Nurses' Health Studies, the Nurses' Health Study (using data from 1992 to 1996) and the Nurses' Health Study ii (using data from 1991 to 1997). A history of being breastfed was self-reported by the study participants. Dur ing a total of 695,655 person-years, 1,073 cases of invasive breast cancer were diagnosed. The authors did not observe any important overall associati on between having been breastfed and the development of breast cancer later in life among premenopausal women (covariate-adjusted relative risk = 0.97 , 95% confidence interval (CI): 0.78, 1.20) or postmenopausal women (covari ate-adjusted relative risk = 1.12, 95% CI: 0.92, 1.37). No significant tren d was observed with increasing duration of breastfeeding. The authors also used data on breastfeeding retrospectively collected from 2,103 mothers of participants of the two Nurses' Health Studies. With the mothers' reports, the covariate-adjusted odds ratio of breast cancer was 1.11 (95% CI: 0.88, 1.39) for women who were breastfed compared with those who were not. Data f rom these two large cohorts do not support the hypothesis that being breast fed confers protection against subsequent breast cancer.