Women who have heavy placentas during consecutive pregnancies face an increased risk of breast cancer, researchers have found.
A large study of more than 314,000 Swedish women suggests the breast cancer risk rises for women whose placenta weighed between 500 grams and 600 grams in their first pregnancy and 700 grams in the next pregnancy, or vice versa.
"In the present population-based cohort study, mother's risk of breast cancer increased with placental weight," wrote lead author Sven Cnattingius and colleagues in the study, published in the Journal of the American Medical Association. "Compared with women who had a low placental weight (499 g or less) in two successive pregnancies, the risk of breast cancer was doubled among mothers whose placentas weighed 700 grams or more in both pregnancies."
The investigators note the association between placental weight and breast cancer risk may be due to the increased levels of hormones in the placenta during pregnancy.
"Our finding may reflect that exposures to elevated levels of pregnancy hormones influence the risk of breast cancer," the authors write. "The role of estrogens in breast cancer is well established and serum estrogen levels are at least 10 times higher during pregnancy compared with other times of life."
The study analyzed women who gave birth to a single infant between 1982 and 1989. Participants were tracked until 2001, or until they either developed breast cancer or died. Of the entire group, 2,216 women developed breast cancer, which is 0.7%.
In addition to hormones, the researchers suggest factors such as the mother's height, body mass index, smoking and hypertensive (high blood pressure) disease during pregnancy may all influence both placental weight and the risk of breast cancer.
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