The recommendation builds on the agency's earlier advice that overweight women gain 15 to 25 pounds, normal-weight women add 25 to 35 pounds, and underweight women pack on 28 to 40 pounds during pregnancy.
While the adjustments for the three existing weight categories were relatively subtle, the decision to add a separate category for obese women was necessary because both obesity and weight gain during pregnancy have both surged among women across the country, the authors of the report say.
Central to the guidelines are body mass index ranges. BMI, a ratio of weight to height, is a common formula used to measure obesity.
The new guidelines use BMI ranges set by the World Health Organization and the National Heart, Lung and Blood Institute; the 1990 guidelines instead used recommendations from Metropolitan Life Insurance tables. The new ranges are more conservative, with the underweight category starting at a BMI of 18.5 instead of 19.8.
"Because of the shift in BMI in the population towards more overweight and obese, those criteria have shifted now for the obese category," said Dr. Patrick Catalano, chair of obstetrics and gynecology and professor at Case Western Reserve University in Cleveland, Ohio. Catalano was part of the committee that developed the new guidelines.
"With the new criteria, we're just trying to be consistent with what everybody else is using," he said.
The guidelines are "not dramatically different from what was reported in 1990, but they are sort of progressive," said Dr. Michael Katz, senior vice president for research at the March of Dimes, which co-sponsored the study.
"They illustrate the substantial increase in obesity," Katz said.
While the guidelines are specifically intended for pregnant women, they may have implications as well for those hoping to get pregnant.
Catalano said it will be easier to achieve the new recommendations if women attempt to get within normal BMI range when they're trying to conceive. This will result in better outcomes for both mom and baby, he said, since it is "remarkably clear that pre-pregnancy BMI is an independent predictor of many adverse outcomes."
IOM Guidelines for Pregnancy Weight Gain May Encourage Some
Interventions in diet and exercise -- both before and during pregnancy -- will be essential in helping women meet the guidelines, especially those who are obese, he said.
"The idea is that it will require an effort by many people," Catalano said. "It's not just something that one health care provider during pregnancy can do. It includes a host of other people, including a nutritionist, dietician, and even an exercise physiologist."
Women's Health Experts Split on New Guidelines
Katz said the aims of the report are on target, but "the effects are very difficult to achieve."
"Here the motivation is stronger because [pregnant women] are strongly motivated to protect their child," he said. "But to have a concerted effort like this will be difficult."
While Katz said the new guidelines are more conservative than those of the past, others say they are not conservative enough.
"It's been well shown that such a patient can lose weight in pregnancy and still be fine," said Dr. Jacques Moritz of St. Luke's-Roosevelt Hospital in New York. "[The guidelines] are still concerned with small babies. That was a problem when women smoked and drank in pregnancy. Now it's just the opposite."
"Women don't need to eat for two but for 1.1," he said.
The authors of the new recommendations noted that good outcomes for both mother and child are achieved "within a range of weight gains." Also, different factors for individual women should be considered, they said.
That means the guidelines should be used "in concert with good clinical judgment as well as a discussion between the woman and her healthcare provider about diet and exercise," they said.
For example, a patient with a BMI of 30 can gain closer to the 20 pounds at the higher end of the range, said Dr. Laura Riley, medical director of labor and delivery at Massachusetts General Hospital in Boston. On the other hand, a pregnant woman with a BMI of 40 "should get closer to the 11 pounds," she said.
Riley said that the major issue behind the new report is that women need to pay closer attention to the guidelines during pregnancy.
Adherence to Pregnancy Weight Gain Guidelines Key
"Most women don't listen to the guidelines that were out in 1990," she said. "If they had listened to those guidelines, we would have better outcomes at this point."
The report also calls for further study of pregnancy in obese women, as well as the impacts of gestational weight gain on outcomes for mothers and babies.
More Guidelines for Special Cases
Other updates include:
Though there was not as much evidence as there was for singletons, the researchers made provisional guidelines for women giving birth to twins. Normal-weight women should gain 37 to 54 pounds, overweight women should gain 31 to 50 pounds, and obese women should gain 25 to 42 pounds when they're having twins. There was insufficient information for underweight women.
Shorter women don't need to gain toward the lower end of the range for their pre-pregnant BMI. The researchers said there was not sufficient evidence to continue to support this 1990 recommendation.
There is no continued support for women under 20 who become pregnant to gain on the lower end of their range, as younger adolescents often need to gain more to improve their chances of a healthy birth.