Women who tend to keep a tight rein on their eating gain more weight during pregnancy than their peers who are more relaxed about eating, new research confirms.

Women are getting this message - "oh, now you're pregnant, you're free to eat whatever you want," said Dr Anna Maria Siega-Riz, of the University of North Carolina at Chapel Hill.

But the Institute of Medicine now says women don't need to boost their calorie intake during the first trimester of pregnancy, while they need 340 more calories daily during their second trimester and 450 extra calories in the third.

More and more women are gaining more than the recommended amount of weight during pregnancy, Siega-Riz and her team note in the Journal of the American Dietetic Association.

There's evidence that women who restrain their eating and those who diet frequently may actually gain more weight, they add.

To further investigate how a woman's pre-pregnancy eating habits influence pregnancy weight gain, Siega-Riz and her colleagues followed 1223 pregnant women participating in a study of preterm birth and fetal growth.

All reported their weight before pregnancy, and also completed questionnaires evaluating restrained eating behaviour, weight cycling and concern with dieting.

On average, the women gained 52 per cent more weight than they needed to, the researchers found, while 63 per cent of study participants overall gained too much weight.

Thirty-two per cent of the underweight women gained too much weight, while 63 per cent of normal weight women did, 85 per cent of overweight women put on too many pounds, and 74 per cent of obese women gained excessive amounts of weight.

Women who were considered "cyclers", meaning that before pregnancy they had gained and lost five pounds or more during the course of a week at least once, gained 2 kilograms more than non-cyclers.

Among every weight status group except for underweight women, those with a high degree of eating restraint gained more weight than those who were less restrained eaters. The same pattern was seen for women who were habitual dieters.

It's possible that women who habitually restrain their eating are physiologically more vulnerable to gaining weight during pregnancy, Siega-Riz noted, but it's more likely that they see pregnancy as an opportunity to let go of these restraints.

The message from health care providers, she said, "should not be that it's okay to eat for two."

Reuters