For years, overweight women who become pregnant have been advised to limit their weight gain to 25 pounds over nine months.
But on Thursday, health officials sharply reduced that number for many of them. In the first revision of weight-gain guidelines since 1990, the Institute of Medicine and the National Research Council said that women who start their pregnancies very overweight should limit their gain to 11 to 20 pounds.
About 27 percent of women of childbearing age fall into this category, defined as obese because they have a body mass index of 30 or more (for example, 5 feet 5 inches and at least 180 pounds).
The new guidelines stem from growing evidence linking weight gain before and after pregnancy to health problems for both mother and baby.
Overweight and obese women, or those who gain excessive weight during pregnancy, are at higher risk for complications that include gestational diabetes and a life-threatening blood-pressure problem known as pre-eclampsia. And children whose mothers gained too much weight during pregnancy are more likely to be overweight or obese themselves.
The guidelines for women with a body mass index under 30 do not change. Underweight women, with a body mass index of less than 18.5, should gain 28 to 40 pounds during pregnancy; normal-weight women, with a B.M.I. of 24.9 or lower, should gain 25 to 35 pounds; and overweight women, with a B.M.I. of 25 to 29.9, should gain 15 to 25 pounds.
But the guidelines do urge doctors to better counsel all women to be vigilant about their weight. As many as 70 percent of pregnant women fail to comply.
“Women must bring weight gain during pregnancy into these ranges,” said Kathleen M. Rasmussen, a professor of nutrition at Cornell University who led the committee that wrote the report. “For most women, this means gaining less than they currently gain.”
Gaining too little weight is also a concern: low-birth-weight babies are at risk for several health problems. Women carrying twins or multiples are advised to gain more weight than those carrying a single baby.
To ensure a healthier pregnancy, the report also urges doctors to counsel women to lose weight before conceiving.
Health officials say the new guidelines were necessary because of changing weight patterns among American women. Today, about 55 percent of women of childbearing age are either overweight or obese. But the 1990 pregnancy guidelines did not offer specific advice for obese women, who instead were often told to follow the recommendations for overweight women.
For instance, in the past, a 5-foot-5 woman who started pregnancy at 180 pounds would have been urged to finish her pregnancy at 195 to 205 pounds. Under the new guidelines, her recommended weight at delivery would be 191 to 200 pounds.