Obese women who gain little or no weight during pregnancy have more chance of giving birth to a healthy baby, a study claims.
Overweight mothers-to-be whose weight remains fairly balanced also have a reduced risk of high blood pressure and fewer caesarean births, it added.
The findings, in the journal Obstetrics & Gynecology, show that women of differing heaviness should gain or even lose different amounts of weight.
The study analysed the pregnancies of more than 120,000 obese women from Missouri in the US to see how weight gain affects pregnancy-related high blood pressure, Caesarean delivery and birth weights.
They divided the women into weight categories, and found that the "least" obese had best outcomes when they gained 10 to 25 pounds.
But the mothers who were the heaviest had the best outcomes if they lost weight.
Dr Raul Artal, the author of the study and chairman of the department of obstetrics at the Saint Louis University School of Medicine, said: "The fear has been that not gaining weight would have a deleterious effect on the fetus.
"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."