Lactation and cancer of the breast. A summary of an international study.

An international collaborative study has been carried out to test the hypothesis that prolonged lactation protects women against cancer of the breast. While pregnancy itself seemed to confer some protection against breast cancer in all areas studied, no consistent differences in duration of lactation were found between breast cancer patients and unaffected women, once the fact that breast cancer patients have fewer pregnancies had been allowed for. Even in areas where some women had lactated for a total of 5 years or more, such women occurred proportionately no less frequently among breast cancer patients than among unaffected women. In the light of this and other recent evidence, it is unlikely that lactation has any protective effect against breast cancer in women, and other explanations must be sought for the remarkable international differences in the frequency of this disease.