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.

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