Does lactation prevent subsequent breast cancer?

Abstract

This is a common question which readily suggests itself and it is difficult to imagine that breast feeding has no effect on the subsequent risk of breast cancer. However, ever since the large international study of McMahon, published in 1970,’ pointed out that all the observed effect could be explained by the confounding of amount of breast feeding of women, at an age of high risk of breast cancer, with age of first pregnancy. An often replicated result in this epidemiology is a lower risk among women with an early age of first full term pregnancy, who in turn on average will have more opportunity to breast feed. The finding of no effect once age of first pregnancy is allowed for is most common among women who are post menopausal, but increasingly studies are emerging which seem to show a protective effect of lactation, even after this adjustment, for pre-menopausal breast cancer. These findings have not been replicated subsequently in a case control study in England’ but in the US the large Cancer and Steroid Hormone Group study show a highly significant protective effect.3 The adjusted relative