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.
''The data are provocative because they suggest there may times when the breast is particularly susceptible to increased estrogen levels,'' said Dr. Joyce O'Shaugh-nessy, 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.
Many women fail to take off all the extra pounds after they give birth. 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,'' she said.
Hilakivi-Clark 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.
Her study was based on a follow-up of 4,020 postmenopausal women in Finland, 185 of whom developed breast cancer at an average age of 58. The researchers checked medical records to see how much the women gained during pregnancy.
Adequate weight gain during pregnancy is essential to the baby's health. Guidelines from the Institute of Medicine say how much women need to put on depends on their body mass index, a widely used measure of fatness.
A woman who is underweight should gain 28 to 40 pounds. One who is normal weight should gain 25 to 35 pounds. Someone who is overweight should add 15 to 25 pounds. And obese women should put on no more than 15 pounds.
Statistics from the Centers for Disease Control and Prevention suggest most pregnant women probably put on more than this. In 2000, the median weight gain was almost 31 pounds. Twelve percent put on 46 pounds or more.
Pregnancy itself can protect against breast cancer, though only if women get pregnant by age 20. Those who get pregnant after age 30 actually have a higher risk than women who never have children. Breast-feeding also is modestly protective.
EDITOR'S NOTE: Medical Editor Daniel Q. Haney is a special correspondent for The Associated Press.
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