NEW YORK (Reuters Health) - New research provides more evidence that childbearing factors, especially those related to a first pregnancy, influence a woman's risk of breast cancer.

The latest findings are from a study comparing some 2,500 women who completed a first pregnancy and were subsequently diagnosed with breast cancer at least a year later, and 10,000 cancer-free mothers of single children.

Extremely premature delivery was associated with a two-fold increased risk of developing breast cancer, investigators report in the International Journal of Cancer. "Extreme prematurity has been characterized by high maternal estrogen levels, which could increase breast cell proliferation," they suggest.

There was also a tendency toward increased breast cancer risk in women who had twins or triplets during their first pregnancies, and the association was stronger among women who delivered multiples after age 30.

In contrast, preeclampsia was associated with a marked reduction in breast cancer risk among women who delivered their first child after their 30th birthday.

"Our results suggest that certain perinatal factors are associated with maternal breast cancer risk," Dr. Kim E. Innes of the University of Colorado Health Sciences Center in Denver and Dr. Tim E. Byers of the University of Virginia Health System in Charlottesville conclude.

"The pattern of these associations," they add, "offers indirect support for a role of gestational hormones, and particularly gestational estrogens, in the etiology of breast cancer among young women."

SOURCE: International Journal of Cancer, November 1, 2004.

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