For women who have given birth, the risk of breast cancer is directly associated with the weight of the placenta in two consecutive pregnancies, according to a report in the Journal of the American Medical Association this week. Women with higher placental weight in prior pregnancies are at increased risk for breast cancer, the study indicates.

"Our finding of a positive association between placental weight and breast cancer risk may reflect that exposures to elevated levels of hormones influence the risk of breast cancer," Dr. Sven Cnattingius, from the Karolinska Institute in Stockholm, and colleagues note in the report.

Hormonal factors play an important role in the development of breast cancer, the authors explain. "The role of estrogens in breast carcinogenesis is well established, and serum estrogen levels are at least 10 times higher during pregnancy compared with other times of life," they add.

The study included 314,019 women who delivered a single infant between 1982 and 1989 and were followed until 2001 or until breast cancer developed or death occurred. During follow up, 2,216 women (0.7 percent) developed breast cancer, of whom 2,100 (95 percent) were diagnosed before age 50 years.

The investigators found that the risk of breast cancer was significantly greater in women with placentas weighing between 500 and 699 grams in their first pregnancy and at least 700 grams in their second pregnancy compared with those who had two consecutive pregnancies with placentas weighing less than 500 grams.

The risk of breast cancer was doubled among women whose placentas weighed at least 700 grams in both pregnancies.

A high birth weight (4000 grams or greater) in two successive births was also tied to an elevated breast cancer risk, but the association was no longer statistically significant after adjusting for placental weight and other factors.

The authors note that the "underlying biological mechanisms responsible for the observed associations may not only be limited to a direct growth enhancing effect on breast cells during childbearing, but also may be due to maternal characteristics or genetic factors associated with placental growth."

SOURCE: Journal of the American Medical Association, November 16, 2005.

Reference Source 89
November 16, 2005