TUESDAY, Nov. 15, 2005 (HealthDay News) -- The heavier the placenta during pregnancy, the greater a woman's risk of breast cancer before menopause, a new Swedish study suggests.
"Compared with mothers whose placenta at first delivery weighed less than 500 grams, women whose placenta weighed equal to or greater than 700 grams had a 38 percent increase in risk of breast cancer," said study author Dr. Sven Cnattingius, a professor at the Karolinska Institute in Stockholm.
Results of the study are published in the Nov. 16 issue of the Journal of the American Medical Association.
While scientists don't know the exact cause of breast cancer, hormones clearly play a role in risk, according to Cnattingius. And during pregnancy, he added, hormone levels surge.
"Hormones during pregnancy are primarily produced by the placenta. And the levels of, for example, estriol -- the most potent estrogen -- are reported to increase with placental weight. We therefore hypothesized that a mother's risk of breast cancer may increase with placental weight," said Cnattingius.
The placenta is the temporary organ that supplies nourishment to the fetus.
To test this hypothesis, the researchers reviewed data from more than 300,000 Swedish women included in the Sweden Birth Register from 1982 through 1989. The study included follow-up information on the women through 2001, or until they had been diagnosed with breast cancer or died.
Fewer than 1 percent of the women -- 2,216 -- developed breast cancer. Almost all of the women -- 95 percent -- were diagnosed before the age of 50.
The study confirmed the researchers' hypothesis. Women whose placentas weighed 700 grams or more had a 38 percent increased risk of breast cancer, compared to women whose placentas weighed less than 500 grams, according to the study. A woman with a placenta weighing between 500 and 699 grams in one pregnancy and 700 grams or more in another pregnancy had an 82 percent increased risk of breast cancer, compared to women who had two pregnancies with placentas weighing less than 500 grams.
When compared to women with two pregnancies with placentas weighing under 500 grams, women whose placentas weighed 700 grams or more in two pregnancies had double the risk of breast cancer, the researchers said.
"The risk of (predominantly premenopausal) breast cancer among mothers increase with placental weight," Cnattingius concluded.
The researchers also confirmed previous findings that an earlier maternal age was somewhat protective against breast cancer. Those whose first pregnancies occurred when they were 19 or younger had about a 10 percent reduced risk of breast cancer, while those who waited until they were 35 or older to have their first child faced 20 times the average breast cancer risk.
Commenting on the placenta findings, Debbie Saslow, director of breast and gynecological cancer control for the ACS, said, "This is an interesting study, but it's not cause-and-effect, it's an association. The risk reported is a really small relative risk, and it's not going to change any lifestyle recommendations."
The only women who might want to consider these findings are those who have tested positive for breast cancer genes, Saslow added.
If you know you're already at a high risk for breast cancer, and if you don't feel strongly about when in your life you have children, it might be better to have them earlier, she said. But, that doesn't mean you couldn't have children later in life either, she added.
What women can do to help prevent breast cancer is get enough exercise, eat right and don't drink alcohol excessively. Besides potentially lowering your breast cancer risk, you'll also lower your risk of heart disease and diabetes by taking these steps, she said.
More than 200,000 American women are diagnosed with breast cancer each year, according to the ACS. Over a lifetime, 13 percent of U.S. women will develop the disease, according to the society.
To learn more about breast cancer risk and how pregnancy influences that risk, visit the National Cancer Institute.