Evidence for a protective effect of lactation on risk of breast cancer in young women. Results from a case-control study.

Female residents of King County, Washington state, aged 25-54 years, in whom breast cancer was diagnosed in 1981 and 1982 were interviewed regarding their reproductive histories. Their responses were compared with the responses of a sample of women from the same population, who were selected by random digit-dialing from the same county, so that their age distribution resembled that of the cases. Premenopausal women who had ever lactated had 0.49 times the risk of developing breast cancer, as compared to premenopausal women who had never lactated (95% confidence interval, 0.30-0.82). Among both premenopausal and postmenopausal women, risk of breast cancer decreased with increasing duration of lifetime lactation experience, although the effect was consistently stronger for premenopausal women. This protective effect persisted after adjustment for age, parity, and age at first full-term pregnancy. Other reproductive factors, and demographic factors such as income and education, did not confound the relationship. This study confirms other recent findings of a protective effect of lactation against the development of breast cancer in young women.