Large Pregnancy Gain Ups Postmenopausal Breast Cancer Risk

SAN FRANCISCO—Women who gain 38 lb or more during pregnancy have a 40% greater risk of postmenopausal breast cancer than women who gain less weight, Leena Hilakivi-Clarke, PhD, said at the 93rd Annual Meeting of the American Association for Cancer Research (abstract 1169).

Weight gain during pregnancy, however, appears to have little effect on the risk of premenopausal breast cancer, said Dr. Hilakivi-Clarke, professor of oncology, Georgetown University.

In the study, done in collaboration with Riitta Luoto, MD, of the University of Tampere, Finland, researchers analyzed data from more than 27,000 women in Finland.

The first cohort, consisting of more than 17,360 women, contained 392 women who served as controls and 98 women who developed premenopausal breast cancer at an average age of 47. Breast cancer diagnosis and pregnancy weight gain were obtained from a questionnaire the study participants filled out from 1990 to 1993.

The second cohort of 4,090 women, included controls and 166 women who developed postmenopausal breast cancer at the mean age of 68.3 years. These breast cancer cases were identified through the Finnish cancer registry; pregnancy weight gain was obtained from cards stored in maternity centers.

"We found that women who gained the most weight during pregnancy are at higher risk for postmenopausal breast cancer," Dr. Hila-kivi-Clarke said. "Those who retained the added pounds after pregnancy are also at risk. Overall, the increased risk due to pregnancy weight gain is modest and similar to the increased risk from obesity after menopause."

Her research team has not yet investigated whether women who gain too much weight during pregnancy, and then lose all the weight, have an increased risk of postmenopausal breast cancer.

In the study, each 1 kg increase in pregnancy weight increased breast cancer risk by 3.9% when adjusted for body mass index before pregnancy (P < .035).

Half the women in the study gained the amount of weight recommended—25 to 35 lb. One third of the women gained too much weight. Weight gain of 25 to 35 pounds was not associated with either premenopausal or postmenopausal breast cancer. There is also no indication that low pregnancy weight gain might be protective, Dr. Hilakivi-Clarke added.

The effect of pregnancy weight gain on breast cancer risk may be explained by elevated estrogen levels, Dr. Hilakivi-Clarke said. During times when the breast is going through changes, as in pregnancy, estrogen may be particularly harmful, Dr. Hilakivi-Clarke said.

At least one previous study has shown that women who gain the most weight during pregnancy have higher estrogen levels than those who gain less weight, she noted. High estrogen levels during pregnancy may stimulate the growth of existing tumor cells and may also cause cancer, she said.

Dr. Hilakivi-Clarke’s study supports earlier research showing that higher body mass index before menopause confers a lower risk of breast cancer. In the study, researchers found that low body mass index in nonpregnant women tended to be linked to increased premenopausal breast cancer risk.

"At menopause, the risk conferred by body weight switches around—so that higher body mass index becomes a risk factor for breast cancer," Dr. Hilakivi-Clarke said. "Why this happens is a good question that needs to be studied."