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updated 2/28/2011 2:45:23 PM ET 2011-02-28T19:45:23

The more times a woman gives birth, the higher her risk of developing a relatively uncommon but particularly aggressive type of breast cancer, according to a new study.

Women who had given birth three or more times were 1.5 times more likely to develop "triple-negative" breast cancer than women who'd given birth only once, the study showed. In triple-negative breast cancer, the tumor does not turn on the genes for any of the three types of hormone receptors found in most breast cancers.

"We were surprised by th

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ese findings, because researchers have known for quite some time that women who have children, especially those who have them at an early age and have multiple, full-term pregnancies, have a lower risk of breast cancer overall," study researcher Amanda Phipps, of the Fred Hutchinson Cancer Research Center in Seattle, said in a statement.

Prior to the study, the researchers had thought that having children — and therefore, undergoing the hormonal changes of pregnancy — would not affect a woman's risk of triple-negative breast cancer, because the tumors lack hormone receptors, Phipps said.

The study also showed that women who never gave birth have a 40 percent lower risk of developing triple-negative breast cancer.

While the reasons are not cl

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ear for why having more children might raise women's risk of triple-negative breast cancer, the study suggests that the breast tissue of some women responds differently to the hormones of pregnancy than the tissue of others, Phipps said.

In most, the hormone changes seem to make the breast less susceptible to cancer, but it could be that some women have an abnormal response that makes their tissues more susceptible, she said.

Another possible explanation for the link is that pregnancy makes the breast more susceptible to specific carcinogens, even while reducing breast cancer risk overall, she said.

The lack of hormone receptors means triple-negative cancers don't respond to treatments designed to block hormones, such as tamoxifen. The disease accounts for 10 percent to 20 percent of all breast cancers, and has a poorer prognosis than other types of breast cancer.

Having children has long been associated with a reduced risk of developing other types of breast cancer, the researchers said. In line with this, the study found that women without children had about a 40 percent higher risk of developing estrogen-receptor-positive breast cancer — the most common form of the disease — than those who had one or more children.

This link is thought to be related to the fact that pregnancy-related changes in the breast confer a lifelong protective effect.

The results were based on reproductive history data from more than 150,000 postmenopausal women in the ongoing Women's Health Initiative study. More than 300 of the women developed triple-negative breast cancer.

The study is one of the largest ever conducted to review the effects of reproductive history on triple-negative breast cancer, Phipps said.

The study will be published in the March 16 issue of the Journal of the National Cancer Institute.

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