They were at lower risk for developing high blood pressure and delivering by cesarean section and more likely to have a normal-weight baby.
The findings, in the October issue of Obstetrics & Gynecology, fuel growing concern that national weight-gain guidelines for pregnant women are outdated and don't take into account the nation's obesity epidemic.
Current recommendations developed by the Institute of Medicine in 1990 say women should gain at least 15 pounds during pregnancy, and the guidelines place no upper limit on pregnancy weight gain.
The study found that women of different weights should gain or even lose different amounts of weight.
"The fear has been that not gaining weight would have a deleterious effect on the fetus," said Dr. Raul Artal, study author and chairman of the department of obstetrics, gynecology and women's health at Saint Louis University School of Medicine.
"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He added, "The guidelines are outdated and we have to change them."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 said the research is the largest population-based study to look at the effect of weight gain during pregnancy among obese expectant mothers.
Relying on birth certificate data, the study analyzed the pregnancies of more than 120,000 obese women from Missouri to see how weight gain affected pregnancy-related high blood pressure, cesarean delivery and the baby's birth weight.
The study looked at birth outcomes for pregnant women in three levels of obesity - from not as heavy to heaviest - and came up with optimal ranges of weight gain.
The least-heavy obese women had best outcomes when they gained 10 to 25 pounds.
The next heavier group of obese women had the best outcomes if they gained zero to 10 pounds.
The heaviest obese women had the best outcomes if they lost weight.
The study's strengths are its size - drawing from an entire state - and that it is the first to look at different levels of obesity, "not a lumping of all obese women together," said Dr. Emily Oken, a Harvard University Medical School professor and researcher on obesity, nutrition and weight gain in pregnancy.
Oken also believes it's time to review the guidelines written almost 20 years ago that she says were based on data from the 1970s and 1980s when obesity rates were much lower.
The Missouri study found that the least-heavy obese women who restricted their weight gain were at somewhat higher risk to have a low birth-weight baby.
But obese women who gain too much are also at risk for low birth-weight babies, Oken said, adding "It is a balance."
In the coming weeks, the Institute of Medicine, a private organization that advises the federal government, is expected to begin the lengthy process of gathering scientific evidence to decide whether the guidelines should be changed, said spokeswoman Christine Stencel.
Under the institute's 1990 guidelines, those with a "normal" body mass index - a combination of height and weight - were encouraged to gain 25 to 35 pounds. Women with a higher BMI have a lower target - 15 pounds only for the most obese women. Women with a lower BMI should gain more weight during pregnancy - up to 40 pounds.
Stencel said the guidelines were written at a time when the bigger concern was undernutrition.