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Does Breastfeeding Prevent Breast Cancer?
Is it true that the longer your breastfeed your babies, the lower your risk of breast cancer?
Answer (Published 10/28/2004)

Yes, breastfeeding does appear to protect against breast cancer, probably by affecting levels of estrogen in a woman's body. Results of a study published in the July 20, 2002, issue of The Lancet showed that the more babies a woman has and the longer she nurses them, the lower her risk of breast cancer. The researchers reviewed 47 reports of studies in 30 countries that contained information about 50,000 women with breast cancer and almost 97,000 women who didn’t develop breast cancer.

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The study reported that women in developed countries who had never had breast cancer breastfed their babies for an average of three months per child. In developing countries in Africa and Asia, women breastfed each of their babies for an average of 30 months.

We’ve known for a long time that the more often a woman gives birth, the lower her risk of breast cancer. This study showed that each birth reduced a woman’s risk by seven percent. Among those who breastfed their children (compared to those who had children but didn’t breastfeed) the breast cancer risk declined by about four percent for each year of breastfeeding.

The researchers also looked at how breastfeeding and pregnancy would affect the risk of breast cancer if women in developed countries had the same number of babies and breastfed them as long as women in developing countries. They found that the risk would be cut by more than half and that almost two thirds of the reduced risk was due to breastfeeding alone.

The Collaborative Group on Hormonal Factors in Breast Cancer, which conducted the study, concluded that the short duration of breastfeeding among women in developed countries makes a major contribution to the higher incidence of breast cancer in these areas. The researchers said their findings suggest that if women had an average of two-and-one-half children and breastfed each one an extra six months, five percent of breast cancers would be prevented each year; if they breastfed each child for an additional 12 months, 11 percent of cancers would be prevented each year.

That’s a powerful argument for breastfeeding. We know that breastfeeding is good for babies. This study proved that it can be very good for mothers, too.

Andrew Weil, M.D.

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