Dr. Raul Artal, who has conducted extensive research on obesity during pregnancy, says Institute of Medicine recommendations followed worldwide by obstetricians encourage obese women to gain at least 15 pounds during pregnancy and specify no upper limit for weight gain.

"Pregnancy has become over the years a state of indulgence and confinement," Artal wrote in an editorial in the medical journal Expert Review of Obstetrics and Gynecology.

"Pregnancy is an ideal time for behavior modification that includes physical activity and with proper medical supervision it can be safely prescribed."

The institute guidelines were not grounded in scientific evidence and focused on preventing low birth-weight deliveries, which generally occur when women who are underweight and of normal weight don't gain enough weight during pregnancy, Artal says.

"If overweight pregnant women control the number of calories they consume and maintain a physically active lifestyle, they can fight problems that commonly come from obesity, such as hypertension and diabetes," Artal said.