The study from the University of North Carolina in Greensboro found that women who, by exercising and dieting, lost half-a-kilo a week between the fourth and 14th weeks after birth had babies who were just as plump and tall as those of women who did not exercise or diet.

But the study was done only with women who had a body mass index of at least 25 - a category into which about half the US population falls.

"The results might be different for women with body-mass-index values of less than 25," said the researchers, led by Cheryl Lovelady.

The study was an attempt to test a 1991 recommendation by the Institute of Medicine, part of the National Academy of Sciences, which said women could safely lose up to 2 kilograms a month when breast feeding.

But in an editorial in the Journal, Nancy Butte of the Baylor College of Medicine in Houston said the babies in the study showed a wide range of weight gain.

As a result, she cautioned, "the growth of some infants might have been inadequate."

Diet and exercise programmes "may be advisably postponed until four to six months" after the baby is born "when a mother's milk is no longer the sole source of nutrition for her infant," Butte said.

"The women's reactions to the programme, such as hunger, fatigue, irritability, and psychosocial stress, should have been evaluated," she said.

As a result of their workouts - four sessions of up to 45 minutes each week - the 21 women in the exercise group saw their fitness rating increase from "average" to "excellent," a rating based on standards set by the American College of Sports Medicine. The 19 women in the control group remained at the "average" level.

(Agencies via Xinhua)