Women can reduce their risk of breast cancer by breast-feeding, even if they have their first child later in life, according to a study presented at the annual meeting of the American Association for Cancer Research in Los Angeles.

Researchers analyzed data on women who had participated in the Women's Contraceptive and Reproductive Experiences (CARE) Study, the results of which were published in 2003. In the current study, researchers looked at the data for women over the age of 54, 995 of whom had breast cancer and 1,498 of whom did not. They compared the women's risk of acquiring breast cancer with their history of breastfeeding and their age at first birth.

There are 200,000 new cases of breast cancer in the United States every year, and it is the third most common cause of cancer death in the country. There are two main types of breast tumors: those that contain hormone receptors, and those that do not.

Researchers have previously found that having a first child after the age of 25 increases a woman's chance of acquiring hormone-receptor-positive breast cancer. Among women who have their first child before the age of 25, those who have many children have an even lower breast cancer risk.

In the current study, researchers found that breastfeeding decreased a woman's risk for both receptor-positive and receptor-negative cancers, regardless of the age at which she had her first child.

"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," said Giske Ursin, an associate professor of preventive medicine at the Keck School of Medicine of the University of Southern California."

"Breastfeeding may have a protective effect that negates the increased risk of breast cancer associated with late pregnancies," Ursin said.