Why would breastfeeding confer this protection? We are aware that the circumstances, which influence a woman's risk of breast cancer, such as number of pregnancies, and time of the first menstrual period, are related to ovarian function. In areas where breast cancer is practically nonexistent, women give birth in adolescence, and are pregnant or nursing until menopause. Oncologist, William C. Dooley, MD, director of the Johns Hopkins Breast Cancer Center stated that " there's a relationship between the number of menstrual cycles a woman has before her first pregnancy and breast cancer, and a relationship between her total lifetime dose of estrogen and breast cancer ... What has happened in this country is that the average age of the onset of menstruation has fallen and the age of menopause has risen. And with delayed childbearing, there's also a longer interval between the first period and the first pregnancy, and that's where the risk is. Breastfeeding decreases the risk, and having more children decreases the risk." Most likely, extended breastfeeding reduces the risk of ovarian and breast cancer because it reduces ovarian function (Intelihealth News, June 1997).
Breastfed children also have less risk of developing breast cancer. A study involving over 1,000 women found that women who were breastfed as children, even for a limited period of time, had a 25 percent lower risk of developing breast cancer (both in the pre and postmenopausal years) than women who were artificially fed as babies (Freudenheim, 1994).