Severely obese women should lose weight during their pregnancy, instead of adding on pounds to avoid problems during birth, researchers found.

The St. Louis University study found that by limiting the weight gain of pregnant women who are obese significantly decreases their chances of developing preeclampsia, high blood pressure brought on by pregnancy. They are also less likely to require a cesarean delivery and are more likely to have a baby of normal weight.

Researchers studied the effects of weight gain during pregnancy of more than 120,000 obese expectant mothers in Missouri. The study, published in the Obstetrics & Gynecology, found that these women don’t have to gain extra pounds in order to have a successful pregnancy.

Women with a Body Mass Index (BMI) of 35 are recommended to gain fewer than the currently recommended 15 pounds, guidelines that were first recommended by the Institute of Medicine in 1990. A BMI of 18.5 to 24.9 is considered normal weight.

“This study confirms what we’ve suspected all along—that obese women don’t have to gain any weight during their pregnancy,” said Raul Artal, M.D., study author and chairman of the department of obstetrics, gynecology and women’s health at Saint Louis University School of Medicine. “Guidelines for nutrition during pregnancy at that time were based solely on expert opinion and not on scientific data. Obesity was not the problem it is now. Fifty percent of Missouri’s population is either overweight or obese. The problem is also prevalent in many other states in the country.”