WASHINGTON (Reuters) — Obese women can safely gain just a small amount of weight when pregnant, but doctors need to do more to help women stay slim before they get pregnant, US policy advisers claim.

Women who are obese should gain about 11 to 20 pounds (5 to 9 kg) while pregnant, 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.

"In our population today, more women of reproductive age are severely obese (eight percent) than are underweight (three percent) and their short- and long-term health has become a concern, in addition to the size of the infant at birth," the report reads.

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 body mass index or BMI — a measure of height to weight — should gain 25 to 35 pounds (11 to 16 kg) during pregnancy, the same as recommended when the guidelines were last updated in 1990.

Overweight women should gain 15 to 25 pounds (6.8 to 11 kg). BMI is accepted globally as a good measure of whether someone is overweight. A five-foot, six inch (167 cm) woman weighing between 115 and 154 pounds (52 and 70 kg) has a normal BMI, according to an online calculator at www.nhlbisupport.com/bmi/.

A BMI of over 25 is considered overweight, while a BMI of 30 or more — reflecting 33 percent or more body fat — makes a person obese.

The report said doctors should record a woman's weight, height, and BMI routinely before conception, throughout pregnancy and after. 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.

"Preeclampsia is about twice as prevalent among overweight, and about three times as prevalent among obese women, as it is among normal weight women," the report said. This dangerous condition can kill a pregnant woman. The World Health Organization estimates it kills 500,000 babies a year globally.