Occupational Medicine, Shandong Academy of Medical Science,

Results from studies of western populations investigating lactation and breast cancer risk have been inconsistent. To examine this issue, the authors conducted a hospital-based case-control study in Shandong Province, China, in 1997–1999. A total of 404 cases and an equal number of controls were included. Detailed information regarding lactation, menstruation, and reproduction was collected through in-person interview. The authors found a significant inverse association between duration of lactation and breast cancer risk. For women who breastfed for more than 24 months per child, the odds ratio was 0.46 (95 % confidence interval (CI): 0.27, 0.78) when compared with those who breastfed for 1–6 months per child. A significantly reduced risk of breast cancer was also found for those whose lifetime duration of lactation totaled 73–108 months (odds ratio = 0.47, 95 % CI: 0.23, 0.95) and for those who breastfed for ≥109 months (odds ratio = 0.24, 95 % CI: 0.11, 0.53). The test for trend was statistically significant for both mean duration of lactation per child (p = 0.02) and lifetime duration of lactation (p = 0.00). Further stratification by menopausal status resulted in the same conclusion. These data suggest that prolonged lactation reduces breast cancer risk. Am J Epidemiol 2000;152:1129–35. breast feeding; breast neoplasms; case-control studies; lactation; reproductive history Received for publication November 23, 1999, and accepted for publication March 31, 2000