Prolonged lactation and endometrial cancer. WHO Collaborative Study of Neoplasia and Steroid Contraceptives.

BACKGROUND: The risk of endometrial cancer is related to oestrogen levels, showing an increased risk with increasing endogenous or exogenous oestrogen stimulation and a reduced risk when oestrogen is opposed by progesterone. During breastfeeding, the reduction of endogenous oestrogen exposure is larger than that of progesterone, suggesting that breastfeeding may possibly reduce the risk of endometrial cancer.
METHODS: The relationship between lactation and endometrial cancer was assessed in data from six countries, including four developing countries, that were collected for a multinational hospital-based case-control study conducted between 1979 and 1988. In all, 136 cases were compared with 933 controls matched on age, hospital, and year of interview. Standardized questionnaires, administered in the local language, ascertained information on the length of time breastfed, age started and stopped breastfeeding, reproductive and contraceptive practices, and other risk factors for endometrial cancer. Conditional logistic regression was used to control for the confounding effects of gravidity and age at menarche.
RESULTS: Significant decreasing trends in risk were observed with increasing duration of lactation, and with months of breastfeeding per pregnancy. Risk was lowest in women who had most recently lactated, and the apparent protective effect declined with time since cessation of breastfeeding, so that there was no evidence for a protective effect after age 55 even in women who had breastfed for over 5 years.
CONCLUSIONS: The long-term lactation that takes place in developing countries probably reduces the risk of endometrial cancer, but this effect may not persist into the ages at which this disease is most common.