Children born to obese women are more than twice as likely to be overweight by age 4, according to a new study that indicates prevention efforts should begin at � or even before � birth.

While obesity is known to run in families, the study appears to be the first to follow children from birth until preschool to see how early the problem develops, said the study’s author, Dr. Robert C. Whitaker, a pediatrician at Princeton, N.J.-based Mathematica Policy Research.

The study of nearly 8,500 women found that by 4 years of age, 24 percent of children were obese if their mothers had been obese during the first trimester of pregnancy, compared with 9 percent of children whose mothers had been of normal weight.

After the researchers took into account such factors as birth weight and the mothers’ race, education level, and smoking during pregnancy, children with obese mothers were found to be twice as likely to be obese at age 2 and 2.3 times as likely at age 4.

The research did not seek to determine why the risk of obesity increased when the mother was overweight. Whitaker said likely factors include genetics, influences in the mother’s uterus during the nine months of pregnancy, and eating habits and physical activity levels at home.

The study was funded by the U.S. Department of Agriculture and appears in the July issue of Pediatrics. It collected data on poor women and children enrolled in an Ohio welfare program.

Previous research has indicated overweight women run a higher risk of developing gestational diabetes and of having babies with heart abnormalities and other defects. That research, plus the latest study, indicates women who are planning to become pregnant should try to reach an ideal weight before conception, Whitaker said.

“It’s an issue for both the mother’s health and the child’s health. Those are not easy to separate,” he said.

Dr. Rebecca Unger, a pediatrician in the Nutrition Evaluation Clinic at Children’s Memorial Hospital in Chicago, questioned whether the study � which involved only poor children � could be applied to the general population.

Still, she said the research underscores the importance of trying to prevent obesity by identifying risk factors early in life.

Other studies have shown that overweight children are likely to grow up to be fat adults.

Unger said it is easier to prevent or treat obesity early in life � when parents can reduce the amount of juice a child drinks, or take away a bottle from a toddler � than to keep a teenager from eating fast food daily with his friends.