Certain hormones, including estrogen and progesterone, have been shown to play an important role in the development of breast cancer . Levels of these hormones, as well as a few others that have been linked to breast cancer, are many times higher during pregnancy than they are during any other time in a woman’s life. These so-called “pregnancy hormones” are primarily produced in the placenta, and their levels may be indirectly estimated by the weight of the placenta immediately after birth.

In an article published in the November 16, 2005 Journal of the American Medical Association , researchers examined the link between pregnancy hormones and the mother’s subsequent risk for developing breast cancer. They found that women with heavier placentas (and, presumably, higher levels of pregnancy hormones), had a modestly increased risk of developing breast cancer.

About the Study

The researchers analyzed data from the Sweden Birth Register on women who delivered single births between 1982 and 1989. The Register noted placental weight, birth weight, and gestational age. The women were followed from their first birth until they were diagnosed with breast cancer, died, or until the study’s follow-up period ended in 2001.

Of the 314,019 women included in the study, 2,216 developed breast cancer during the follow-up. Compared with women who had a placental weight of less than 500 grams (g), women who had a placental weight of 700 g or more had a 38% increased risk of breast cancer. A total of 121,285 women had two consecutive single births during the study period; of these, 881 developed breast cancer after the second birth. Compared with women whose placentas weighed less than 500 g in both pregnancies, women whose placentas weighed 700 g or more in both pregnancies were more than twice as likely to develop breast cancer.

This study had an important limitation. Because the scientists were unable to directly measure hormone levels, they had to rely on indirect markers of hormone levels such as placental weight, which may not have accurately represented actual hormone levels.

How Does This Affect You?

This study demonstrated that women with higher placental weights were at an increased risk for developing breast cancer. Because placental weight was used as a marker for levels of pregnancy hormones, this study indirectly adds weight to the idea that the excess production of certain hormones during pregnancy may influence a woman’s risk for breast cancer.

For now, these study results do not translate into practical advice, largely because a woman cannot—and probably should not—try to influence the size of her placenta, which provides oxygen and nutrients to her unborn baby. If you are already at increased risk for breast cancer, get regular mammograms and talk to your doctor about whether hormonal therapy (when you are not pregnant) to prevent breast cancer is appropriate for you.