Women who gain too much weight during pregnancy may not only have bigger babies, but bigger teenagers as well, a study suggests.

Researchers at Harvard Medical School found that among nearly 12,000 children and teenagers they studied, those whose mothers gained more than the recommended amount of weight during pregnancy were 42 percent more likely to be obese.

The risk was independent of other factors the researchers examined, including mothers’ pre-pregnancy weight, family income and parents’ education.

Some past studies have linked excessive weight gain during pregnancy to a higher risk of obesity in childhood. These latest findings add to evidence that the fetal environment may have a “sustained effect” on children’s weight regulation, Dr. Emily Oken and colleagues report in the journal Obstetrics & Gynecology.

For women, they say, the study underscores the importance of going into pregnancy at a healthy weight, and then gaining only the recommended amount.

In the U.S., the Institute of Medicine (IOM) suggests that normal-weight women gain 25 to 35 pounds during pregnancy. Women who were overweight before becoming pregnant are encouraged to gain a little less - 15 to 25 pounds - while underweight women should put on 28 to 40 pounds.

The current study included 11,994 children between 9 and 14-years olds whose mothers were part of the Nurses Health Study II, a long-range health study of female nurses from across the U.S. The researchers found that 6.5 percent of the children were obese.

Oken’s team found that when mothers exceeded the IOM guidelines for pregnancy weight gain, their children’s weight also tended to climb.

Compared with their peers whose mothers followed the IOM guidelines, those whose mothers gained too much weight were 42 percent more likely to be obese by the time they were 9-to-14 years old.

Researchers suspect that excess pregnancy pounds may affect fetal development in a way that makes children more susceptible to excessive weight gain.

Animal research has found that overeating during pregnancy alters the expression of genes involved in fat regulation in offspring, and seems to affect the appetite-control centers of their brains as well.

SOURCE: Obstetrics & Gynecology, November 2008.