"While many people recommend that weight loss in pregnancy, particularly for very obese women is ok . . . (there) may be adverse effects," said Dr. Patrick Catalano, director of the Center for Reproductive Health at MetroHealth in Cleveland, Ohio.
"We don't have much data, in particular on body composition changes in overweight (or) obese women who lose weight," said Catalano, who led the new study. "Maybe we need to be a little bit more careful before we are just glib about saying it's ok."
Research has offered evidence of many risks posed by obesity to mother and fetus during pregnancy, up to and beyond the point of delivery.
Obese mothers are at raised risk of early spontaneous abortion and fetal birth defects. Later on, there is a higher chance of gestational diabetes and preeclampsia. At delivery, obese mothers are more likely to need a cesarean-section and to have postpartum wound infections.
The newborns of obese mothers are at risk of being overly large for their gestational age and that has been tied to childhood obesity.
The U.S. Institute of Medicine (IOM) issued guidelines in 2009 recommending that obese women gain between 5 and 9 kilograms (11 to 20 pounds) during pregnancy, which is somewhat less than the gain recommended for women who start pregnancy at a normal weight.
However, some researchers have suggested that for obese women, little to no weight gain, and even weight loss, is preferable during pregnancy to minimize the risks associated with obesity.
The American Congress of Obstetricians and Gynecologists recommends that individualized care be given to overweight or obese women who wish to gain less weight than recommended by the IOM.
Yet, little is known about the fetal health risks associated with weight loss or limited weight gain by overweight or obese women during pregnancy.
To investigate, Catalano, who was involved in developing the 2009 IOM guidelines, and his co-authors examined the effects of any weight loss or a weight gain of less than IOM's minimum 5 kg (11lbs).
They looked at 1,241 full-term singleton pregnancies among overweight and obese women.
Most (85 percent) of the women gained more than the IOM's minimum 5 kg during pregnancy, averaging a gain of 14.4 kg (32 lbs).
In contrast, 15 percent of the study participants had weight gains below IOM's minimum, averaging a gain of just 1.1 kg (2.4 pounds) while pregnant.
Infants born to women who gained little or no weight during pregnancy tended to be small for their gestational age, and had less lean body mass and less fat mass than infants born to women who gained more than 5 kg, Catalano and his team report in the American Journal of Obstetrics and Gynecology.
These infants also had a lower percentage of body fat, a smaller head circumference, lower birth weight and were smaller in length than infants born to women who gained more than 5 kg during pregnancy.
The results held when the authors took into account the mothers' pre-pregnancy weight, smoking status, glucose tolerance (a measure of pre-diabetes or diabetes) and other factors that may have influenced the infants' development.
Infants born to 46 women who lost weight during pregnancy were also small for their gestational age and had decreased lean and fat mass, lower birth weight and a lower percentage of body fat.
In contrast, infants born to women who gained more than the IOM minimum were more likely to be large for their gestational age. Among infants born to women who gained more than 5 kg, 13 percent were large for their gestational age. In comparison, 7.5 percent of infants born to women who gained 5 kg or less were large.
"Everyone agrees having a very big baby is not good," Catalano said. However, "the loss of lean mass may have long-term consequences" as well, he said.
What's the bottom line for obese and overweight women "Lose weight between pregnancies," Catalano recommends. "We just don't know if it's safe to do during pregnancy."