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."
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."
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.