Breastfeeding and breast cancer risk by age 50 among women in Germany

Background: Epidemiological evidence which suggests that prolonged breastfe eding protects against breast cancer has accumulated in recent years. Issue s with regard to the timing of breastfeeding and effect modification by cor relates of breastfeeding and other risk factors of breast cancer remain unr esolved. Methods: A population-based case-control family study of breast cancer amon g women diagnosed by the age of 50, conducted in two geographic areas in Ge rmany, was used to evaluate the effect of breastfeeding on risk of breast c ancer. Results: Among parous women in this study (553 cases, 1094 age-matched popu lation controls), having ever breastfed a child for at least 1 month did no t confer protection (odds ratio of 0.9 and 95% confidence interval (CI) 0.8 -1.2). However, risk of breast cancer significantly decreased with increasi ng duration of breastfeeding (p for trend = 0.01) and the estimated relativ e risk was 0.6 (95% CI 0.4-0.9) for 13-24 months of cumulative breastfeedin g and 0.5 (95% CI 0.3-1.1) for 25 months or more. Risk was less related to number of children breastfed than to increasing average length of breastfee ding per child (p for trend = 0.03). Conclusions: The reduction in risk associated with duration of breastfeedin g was not primarily due to breastfeeding the firstborn and more evident in women who were older ( > 25 years) when they first breastfed and among wome n who experienced a recent full-term pregnancy. Risks were modified somewha t by a first-degree family history of breast cancer whereby a greater reduc tion in risk per additional month of breastfeeding was observed among women with a family history than those without (0.9 vs. 1.0). The study results support a protective role of prolonged breastfeeding against the developmen t of breast cancer in predominantly premenopausal women in Germany.