Gaining more than 38 pounds during pregnancy can increase the risk of post-menopausal breast cancer by 40% — about the same risk as being obese after menopause, cancer researchers reported April 9 at the 93rd Annual Meeting of the American Association for Cancer Research.

But excess weight gain in pregnancy did not increase the risk of developing breast cancer before menopause. Normal or recommended pregnancy weight gain is 25 to 35 pounds.

Leena Hilakivi-Clarke, PhD, of the Lombardi Cancer Center at Georgetown University, said that in western countries about "30% of women gain more than 38 pounds during pregnancy."

Half of pregnant women gain the recommended amount of weight, while 20% of pregnant women gain too little weight, which doesn't increase cancer risk but poses a health risk to the fetus.

Moreover, women who retain the added weight after pregnancy and carry it with them to menopause are at greater risk than women who manage to lose the excess weight before menopause.

"My advice to women who gained 38 pounds or more during pregnancy is this: lose that weight before menopause," Clarke said.

Clarke based her findings on analysis of data from two large studies of Finnish women.

The first study, pulled from a group of over 23,000 women, focused on 98 who developed breast cancer before menopause and 392 women of similar ages who did not. The second study was from a group of over 4,000 women. Clarke studied 166 women who developed breast cancer at an average age of 58 and an equal number of women in the same age range who did not.

Joyce O'Shaughnessy, MD, a breast cancer specialist at Baylor-Sammons Cancer Center and U.S. Oncology in Dallas, Texas, said the report underlines the link between obesity and breast cancer.

"From a public health standpoint, this just gives further credence to the belief that we need to focus our prevention efforts on reducing obesity," said O'Shaughnessy.

Clarke said one possible explanation for the link between pregnancy weight gain and late life breast cancer risk is that "the weight gain comes at time when the breast is already vulnerable. It is generally accepted that estrogen is linked to breast cancer risks and estrogen levels are at their highest during pregnancy."

In a related study, Margot Cleary, PhD, of the Hormel Institute, Austin, Minn., reported that when leptin, a protein linked to weight gain, is added to either normal breast cells or malignant breast cancer cells in lab studies, the leptin "promotes cell growth."

According to Cleary, "Adding leptin to breast cancer cells increased cell numbers by 160%." Leptin levels increase as weight increases, she added, "So I think there is a clear correlation between the weight gain findings and the laboratory results with leptin."

Cleary said, "A first pregnancy after age 30 increases breast cancer risk by about 40%, which is roughly equal to the risk associated with pregnancy weight gain."

Women who have never been pregnant have about a 35% to 40% increased risk for breast cancer compared to women who had at least one pregnancy.