Lactation history and breast cancer risk.

Lifetime lactation in relation to breast cancer risk was examined in a case-control study in two counties in western New York. Cases were women age 40 years and over with incident, primary, histologically confirmed breast cancer. Controls were age- and county-frequency matched, selected from New York State driver's license records (for those less than age 65 years) and from Health Care Finance Administration Records (for those age 65 or more). Included were women with at least one livebirth (253 premenopausal and 367 postmenopausal cases and 266 premenopausal and 427 postmenopausal controls). Breast cancer risk was very weakly associated with long duration of lactation among premenopausal women; the odds ratio for at least 20 months lifetime lactation was 0.50 (95% confidence interval 0.21-1.12). Among postmenopausal women, the protective effect of lactation was restricted to women with first lactation before age 25 years (odds ratio = 0.67, 95% confidence interval 0.46-0.95). However, age at first birth was highly correlated with age at first lactation. Neither insufficient milk as a reason for not breastfeeding nor having received medication to stop milk flow was associated with increased risk. These findings are in accordance with accumulating evidence that lactation may have a weak protective effect on breast cancer risk.