LONDON — Women who gain too much weight during pregnancy have big babies, putting their children at risk of becoming heavy later on, a new study says.

American researchers followed all births in Michigan and New Jersey between 1989 and 2003. They then focused on women who had more than one child, to exclude the possibility that women who were genetically predisposed to be obese were simply passing those genes onto their babies.

Among the more than 513,000 women and their 1.1 million infants studied, scientists found that women who gained more than 53 pounds during their pregnancy made babies who were about 150 grams heavier at birth than infants of women who gained only 22 pounds.

The study was published online today in the medical journal Lancet and was paid for by the U.S. National Institutes of Health.

"It's never too early to start preventing obesity," said Stephan Rossner, a professor in the obesity unit at Karolinska Hospital in Sweden who was not connected to the study. "It may be uncomfortable for mothers to eat less and change their lifestyle, but after nine months they will get a great payoff for their children."

In the U.S., more than a third of women of normal weight and more than half of overweight and obese women gain more weight than their doctors recommend.

The Institute of Medicine, an independent, nonprofit organization that advises the U.S. government, says normal-weight women should gain 25 to 35 pounds during pregnancy, while overweight and obese women should gain 11 to 25 pounds.