Lactation and the risk of epithelial ovarian cancer. The WHO Collaborative Study of Neoplasia and Steroid Contraceptives.

The relationship between lactation and the development of epithelial ovarian cancer was assessed in data from seven countries that were collected for a multinational hospital-based case-control study conducted between 1979 and 1988. Three hundred and ninety-three cases of ovarian cancer were compared to 2565 controls matched on age, hospital, and year of interview. A non-significant reduction in risk with short-term lactation was observed but no further reduction in risk was seen with long-term lactation. The reduction in risk associated with months of lactation was not as great as the reduction with months of pregnancy, which may be a result of lactation being a less effective form of ovulation suppression than pregnancy. The short-term lactation that takes place in developed countries, may provide as great a reduction in risk as the long-term lactation practised in the developing countries included in this study.