Yes, according to an analysis published in January by the American Institute for Cancer Research, a nonprofit group based in Washington, D.C., that fosters research on nutrition and cancer.

After analyzing data from 98 studies on lactation and breast cancer risk, the group said the evidence is now "convincing" that breastfeeding lowers the risk of both pre-menopausal and post-menopausal breast cancer. There is a 2 percent drop in breast cancer risk for each five months of breastfeeding. The research on breastfeeding to lower the risk of ovarian cancer is suggestive, but not convincing, the group added.

The most likely explanation for the reduced risk is that lactation works through hormones to delay ovulation and the return of a woman's monthly cycles, Karen Collins, a registered dietitian and nutrition adviser to the group, said in an e-mail. It has long been known that the fewer menstrual cycles a woman has over the course of her life, the lower the risk of breast cancer.

Lactation, like a full-term pregnancy, may nudge breast cells to become more differentiated and mature; mature cells are less sensitive to the DNA damage that can lead to cancer.

It may not make sense to breastfeed if the only goal is to reduce cancer risk, said Dr. Eric Winer, director of the Breast Oncology Center at the Dana-Farber Cancer Institute, "because you have to breastfeed for a fairly long time to have an impact on risk." But there are plenty of other reasons to breastfeed, he noted, including the fact that breast-fed infants are less likely to become obese.