Obese women can safely gain just a small amount of weight when pregnant, U.S. policy advisers said on Thursday.

Women who are obese should gain only about 11 to 20 pounds during their pregnancy, the Institute of Medicine and the National Research Council panel said in new guidelines.

“It had become clear that heavier women could gain less weight and still deliver an infant of good size,” the report said.

With two-thirds of the population overweight or obese, the panel said, it is clear that new pregnancy guidelines must be geared toward heavier women.

Women of healthy weight or who are slightly overweight can gain the standard recommended amounts, said Kathleen Rasmussen, professor of nutrition at Cornell University in New York, who chaired the committee that wrote the report.

Healthy women of normal weight should gain 25 to 35 pounds during pregnancy, the same as recommended when the guidelines were last updated in 1990.

Women who gain too much weight while pregnant not only risk keeping that weight after they have the baby, but also have higher rates of some pregnancy complications, including high blood pressure and gestational diabetes.

Overweight and obese women may also endanger the baby by trying to diet while pregnant, so doctors are urged to work harder to get women to lose weight before becoming pregnant.