Objective: To describe the association between breastfeeding and ovarian cancer risk in two prospective cohorts.
Materials & methods: We pooled data from the Nurses' Health Study and Nurses' Health Study II. There were 391 cases of epithelial ovarian cancer diagnosed among 149,693 parous women with up to 16 years of follow-up. Data were analyzed using multivariate Cox proportional hazards models, controlling for age, parity, duration of oral contraceptive use, tubal ligation, and age at menarche.
Results: Ever breastfeeding was associated with a non-significant reduction in ovarian cancer risk compared with never breastfeeding (RR=0.86, 95% CI 0.70-1.06); the median duration of breastfeeding among women who breastfed was nine months. Breastfeeding of 18 or more months was associated with a significant decrease in ovarian cancer risk compared to never breastfeeding (RR=0.66, 95% CI 0.46-0.96). For each month of breastfeeding the relative risk decreased by 2% (RR=0.98 per month, 95% CI 0.97-1.00).
Conclusions: These data support a linear inverse association between breastfeeding and risk of epithelial ovarian cancer.