Breast cancer and lactation history in Mexican women.

The authors conducted a case-control study in Mexico City between September 1990 and December 1992 to determine whether a dose-response relation could be observed between duration of lactation and the risk of breast cancer. Cases, women aged 20-75 years, were identified through six hospitals in Mexico City (n = 349) and were interviewed to obtain data on risk factors for breast cancer, including a detailed history of lactation. Controls (n = 1,005) were selected from the general population using the Mexican national sampling frame. Parous women who had ever lactated had a reduction in breast cancer risk (age-adjusted odds ratio (OR) = 0.39, 95% confidence interval (CI) 0.25-0.62). A small decreasing trend of breast cancer risk in relation to duration of lactation (p < 0.001) was observed. Compared with parous women who had never breast-fed, women who had breast-fed for 12-24 months had an age-adjusted odds ratio of 0.47 (95% CI 0.27-0.83). A stronger protective effect was observed with lactation duration for the first live birth among pre- and postmenopausal women (for 4-12 months of lactation, OR = 0.56 (95 percent CI 0.32-0.96) and OR = 0.48 (95 percent CI 0.29-0.81) in pre- and postmenopausal women, respectively). Adjusting for potentially confounding factors modified these results only slightly. The declining trend in fertility and lactation among Mexican women could lead to a major epidemic of breast cancer such as that observed in Western countries.