SAN FRANCISCO -- In a new twist on the dangers of extra weight, a study found that women who gain too much during pregnancy face an increased risk of breast cancer later in life.

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.

But the latest study suggests that piling on the pounds during pregnancy may be especially hazardous.

The study, presented Tuesday, found that women who put on more than 38 pounds during pregnancy had a 40 percent increased risk of developing breast cancer after menopause. 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 study, said that getting a burst of estrogen during pregnancy may be especially bad.

"During times when the breast is rapidly developing, estrogen might be particularly harmful. Pregnancy is one such period," she said.

Hilakivi-Clarke, a researcher at Georgetown University, presented the findings at a meeting in San Francisco of the American Association for Cancer Research.