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Breastfeeding reduces risk of breast cancer for women who delay childbirth

April 17, 2007

Breastfeeding can offset the increased risk of invasive breast cancer for women who had their first full-term pregnancy after the age of 25, a study led by researchers at the University of Southern California (USC) suggests.

The findings of the study were presented at a news conference on Monday, April 16 at the 2007 annual meeting of the American Association for Cancer Research held in Los Angeles.

"Breastfeeding may have a protective effect that negates the increased risk of breast cancer associated with late pregnancies," says Giske Ursin, M.D., Ph.D, associate professor of preventive medicine at the Keck School of Medicine of the University of Southern California. "As more women may choose to delay pregnancy until after 25, it is important to note that breastfeeding provides protection against both estrogen and progesterone receptor positive and negative tumors."

While having a first full-term pregnancy before the age of 25 and having many children protect against the type of breast cancers that express estrogen and progesterone receptors, these factors do not protect against the rarer tumors that do not have these receptors. Breastfeeding, however, appears to protect against both types of breast tumors, Ursin says.

Researchers analyzed data for women aged 55 and older—including 995 invasive breast cancer patients-who participated in the Women's Contraceptive and Reproductive Experiences (CARE) Study. The women varied by their age at first birth, their breastfeeding history and hormone receptor status.

Previous results from the Women's CARE Study have shown that early age at first pregnancy (younger than 25) and having many children (defined as four or more) are associated with a lower risk of breast cancer, Ursin says. Researchers sought to gain a better understanding of the associations between reproductive factors and breast cancer risk in women with a late age at first birth, she says.

Breastfeeding appears to have a protective effect regardless of when women started giving birth, Ursin says. This is important since having many children was only protective among women who gave birth early, she says. Giving birth after age 25—the average age that women in the U.S. first give birth, according to Census data—was associated with increased risk of hormone receptor negative breast cancer.

"Evidence suggests that women who have children after age 25 can reduce their risk of breast cancer by choosing to breastfeed," Ursin says.

University of Southern California