Women are advised to gain weight during pregnancy for their baby’s wellbeing but a new study suggests that pregnancy weight gain should be different for women with different weights.

Researchers at Saint Louis University School of Medicine published new findings in the October issue of Obstetrics & Gynecology.

They emphasize the need for a modernized version of the Institute of Medicine’s recommendations for weight gain in mothers-to-be. Issued in 1990, these do not take into consideration the current obesity problem that has become a national epidemic, the AP reports.

The study found that women of different weights should gain different amounts of weight during their pregnancy, while some of the more obese women would be better off losing some of the extra pounds.

The Institute of Medicine’s current guidelines say women should gain at least 15 pounds during pregnancy, without placing an upper limit on pregnancy weight gain.

“The fear has been that not gaining weight would have a deleterious effect on the fetus,” the Associated Press quotes Dr. Raul Artal, study author and chairman of the department of obstetrics, gynecology and women's health at Saint Louis University School of Medicine, as saying.

“Not only were there no deleterious effects, but there are benefits. Women, by not gaining weight in pregnancy, reduce their risk of hypertensive disorder, have less C-sections and have babies of normal weight.”

He added, “The guidelines are outdated and we have to change them.”

Artal and his team analyzed the pregnancies of more than 120,000 obese women from Missouri to see what effects weight gain had on their pregnancies, in terms of blood pressure, cesarean delivery and the baby's birth weight.

The participating women were divided into three groups, according to weight, from not as heavy to heaviest.

The researchers found that the women who gained little weight or none at all, and those who even shed some pounds (in the heaviest group), had the best outcomes, with a lower risk of developing high blood pressure and delivering by cesarean section.

A spokesperson for the Institute of Medicine, Christine Stencel, said the organization expects to commence research to decide whether a revision of its guidelines is necessary or not, the AP reports. Stencil added that in 1990, when the recommendations were written, undernutrition was a bigger concern than obesity.

The 1990 guidelines advise women with a normal body mass index to gain 25 to 35 pounds; women with a higher BMI are advised to gain no more than 15 pounds; while mothers-to-be with a lower BMI should gain up to 40 pounds.