n who had placentas weighing less than 500 g in 2 consecutive pregnancies, the risk of breast cancer was increased among women whose placentas weighed between 500 and 699 g in their first pregnancy and at least 700 g in their second pregnancy (or vice versa), and the corresponding risk was doubled among women whose placentas weighed at least 700 g in both pregnancies. A high birth weight (4000 g or greater) in 2 successive births was associated with an increased risk of breast cancer before but not after adjusting for placental weight and other covariates. Compared with women who had a placental weight of less than 500 g, women who had a placental weight of at least 700 g had a 38 percent increase in risk of breast cancer.

"Our finding of a positive association between placental weight and breast cancer risk may reflect that exposures to elevated levels of pregnancy hormones influence the risk of breast cancer. 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," the authors write.

"In addition, placental weight appears to be a better indicator of the hormonal milieu than birth weight or other included birth parameters. 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," the researchers conclude. (JAMA.2005; 294:2474-2480. Available pre-embargo to the media at www.jamamedia.org)

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Contact: Sven Cnattingius, M.D., Ph.D.
sven.cnattingius@mep.ki.se
JAMA and Archives Journals
15-Nov-2005

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