Lactation in relation to postmenopausal breast cancer

A modest inverse association between lactation and breast cancer risk has m ost consistently been observed in premenopausal women, and certain breastfe eding patterns, such as prolonged duration and early age at first lactation , may be important determinants of risk. However, these associations have n ot generally been observed in relation to postmenopausal breast cancer. As part of a multicenter population-based case-control study, the authors exam ined postmenopausal breast cancer risk according to breastfeeding character istics. Breast cancer patients aged 50-79 years were identified from statew ide tumor registries in Massachusetts, New Hampshire, and Wisconsin from Ju ly 1992 through July 1995. Similarly aged control women were randomly selec ted from population lists. Information regarding lactation history and brea st cancer risk factors was obtained through telephone interviews. This anal ysis included only data on parous postmenopausal women (3,633 cases and 3,7 90 controls). After adjustment for age, parity, age at first birth, and oth er breast cancer risk factors, breastfeeding for at least 2 weeks was assoc iated with a slightly reduced risk of breast cancer in comparison with wome n who had never lactated (relative risk = 0.87, 95% confidence interval 0.7 8-3.96). There was only a modest suggestion that increasing cumulative dura tion of lactation was inversely associated with breast cancer risk; the rel ative risk for women who had breastfed for greater than or equal to 24 mont hs was 0.73 (95% confidence interval 0.56-0.94) (p-trend for duration = 0.1 0). Age at first lactation was not consistently associated with risk. Modes t inverse associations appeared to persist even up to 50 years since first lactation. Use of hormones to suppress lactation was not associated with po stmenopausal breast cancer, nor was inability to breastfeed related to risk . These results suggest that lactation may have a slight and perhaps long-l asting protective effect on postmenopausal breast cancer risk.