Medical researchers and epidemiologists have long noted a relationship between increased risk for breast cancer and women who give birth for the first time later in life. They have also found that women who have many children and women who have their first child young seem to be protected from breast cancer. But the strongest protection seems to come from breastfeeding.

Not all breast cancers are the same. Some tumors have receptors on them that react to hormones and there are now several medications that treat these so-called "receptor positive tumors" quite effectively. Receptor negative tumors do not react to hormones and there are fewer treatments for those kinds of cancers.

According to breast cancer research er Giske Ursin, a University of Southern California epidemiologist, the receptor negative tumors seem to appear more often in older first-time mothers. "We found more than a two-fold increased risk of receptive negative breast cancers in those who had their first birth after age 25. And so this increased risk of the rarer types of breast cancer that in general have poorer prognosis is, of course, of concern.

Ursin was interested in seeing if women who had delayed the birth of their first child until after age 25 derived any protection against breast cancer by breastfeeding. So she looked at data collected in a large study of women's reproductive experiences. Ursin compared information from about 9,000 women, and found that no matter how old a woman was when she started giving birth, if she breastfed, she had a decreased risk for all kinds of breast cancers.

Ursin says the finding is definitely another argument for breastfeeding. "The bottom line from our study is that breastfeeding offers broad protection against breast cancer in all women, also in women with a late first term pregnancy." She adds that it's an important consideration because those women do not have the protective benefit of early child-bearing.

Researchers don't know exactly why breastfeeding seems to offer cancer protection. "It would be very nice if we could understand how breastfeeding works and how it protects against these hormone negative receptor tumors," Ursin says, pointing out that the information could help researchers develop preventive regimens against receptor positive and hormone receptor negative breast cancers.

Ursin presented her research recently at the annual meeting of the American Association for Cancer Research.