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 (24 kilograms) during their pregnancy made babies who were about 150 grams (0.3 pounds) heavier at birth than infants of women who gained only 22 pounds (10 kilograms).
The study was published online Thursday 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 (11 to 16 kilograms) during pregnancy, while overweight and obese women should gain 11 to 25 pounds (5 to 11 kilograms).
Heavier babies have a significantly higher risk of staying heavy throughout their lives, said Dr. David Ludwig, director of the Optimal Weight for Life program at Children's Hospital in Boston, one of the study authors. Big babies also have higher chances of developing problems later in life including asthma, allergies and even cancer.
Ludwig and his co-author found most women tend to put on similar amounts of weight for each pregnancy, though they are often heavier when they become pregnant with subsequent children.
Previous studies have shown pregnant women who pack on the pounds suffer from complications like diabetes and high blood pressure, but little research has shown what those extra pounds could mean for babies. In addition to bumping up their chances of becoming obese later, large babies are also more likely to get stuck in the birth canal or need a cesarean section.
Ludwig said when pregnant women overeat, some of those extra calories overstimulate the fetus' growth.
"The fetus is developing in an abnormal metabolic environment where there is excess blood sugar," he said. "That could alter the development of tissues, organs and perhaps even the wiring of the brain that regulates appetite and metabolism."
Neal Halfon, of the Center for Healthier Children, Families and Communities at the University of California, said obesity prevention in the womb wasn't about encouraging pregnant women to trim down but improving their diet and exercise.
While obesity is caused by many different factors, some experts said the link between birth weight and obesity later in life should make women more conscious of how much weight they gain while pregnant.
"This is an extremely important message," said Arne Astrup, a professor of nutrition at the University of Copenhagen. "If mothers are not careful, they could in some way program their children to be obese or diabetic before they are even born."