SAN FRANCISCO -- Putting on excessive weight during pregnancy appears to increase women's chances of having breast cancer after they go through menopause, according to a new study.
While adequate weight gain is essential for the baby's health, the new work suggests that putting on too many extra pounds may be risky.
Doctors have long known that obesity increases a woman's chance of breast cancer. In fact, staying slim is one of the few things a woman can do that clearly lowers her risk.
Until now, experts assumed that all weight gain is bad. But the latest study, presented Tuesday, suggests that piling on the pounds during pregnancy may be especially hazardous.
The study found that women who put on more than 38 pounds during pregnancy had a 40 percent increased risk of developing breast cancer after menopause, although the risk before menopause was no higher than usual.
Fat cells produce estrogen, and many believe the extra hormone is what puts overweight women at higher risk of breast cancer. Dr. Leena Hilakivi-Clarke, who directed the latest study, said that getting a burst of estrogen during pregnancy may be especially bad.
"Timing of estrogen exposure is important in terms of its effect on the breast," she said. "During times when the breast is rapidly developing, estrogen might be particularly harmful. Pregnancy is one such period."
Hilakivi-Clarke, a researcher at Georgetown University, presented the findings at a meeting in San Francisco of the American Association for Cancer Research.
"The data are provocative because they suggest there may be times when the breast is particularly susceptible to increased estrogen levels," said Dr. Joyce O'Shaughnessy, a breast cancer specialist at Baylor-Sammons Cancer Center in Dallas.
However, Dr. Eugenia Calle, director of analytic epidemiology at the American Cancer Society, questioned whether weight gain during pregnancy is any worse than weight gain in general, especially if women fail to take off all the extra pounds after they give birth.
"Pregnancy is a time when many women accumulate excess weight that they don't then lose," she said.
Calle said women who put on an extra 30 to 50 pounds during adulthood face approximately double the usual risk of breast cancer after they reach menopause. "The message to women is to maintain their young adult weight through life."
Hilakivi-Clarke said her team has not yet examined whether women who gain extra weight during pregnancy and then take it all off have an increased risk of later breast cancer.
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