Breast cancer and breastfeeding: results from an Australian case-control study.

A case-control study of breast cancer, diet, and lactational history was carried out in Brisbane, Australia, between 1981 and 1985. Cases were recruited from both the private and the public health care sectors. The breastfeeding histories of 459 cases and 1,091 controls selected from the electoral roll were analyzed. An odds ratio of 0.85 (95% confidence interval 0.55-1.30) for any lactation versus no lactation was observed, with no suggestion of a monotonic trend with increasing duration. This estimate was only slightly altered by multiple logistic regression analysis. A statistically significant nonlinear association was noted for breastfeeding of the first liveborn child (chi 2 = 14.8, df = 6; p less than 0.05), with a slightly elevated odds ratio for lactations of less than one month, a lowered odds ratio for lactations of one month to three months, and a weakly elevated odds ratio in women breastfeeding for more than nine months. Results for premenopausal women were very similar to those for postmenopausal women. Taken together with other recent studies, these results suggest that lactation may play a modest direct or indirect part in reducing the risk of breast cancer.