The children of overweight mothers are 15 times more likely to be obese by age 6 than children of lean mothers, U.S. researchers reported on Tuesday.

The children start piling on the pounds at age 3, the team at The Children’s Hospital of Philadelphia and the University of Pennsylvania found.

The study, published in the American Journal of Clinical Nutrition, suggests that doctors need to start watching out for the children of overweight parents very early - by age 4 at the latest, the researchers said.

“We found dramatic increases in body fat between ages three and six,” said Dr. Robert Berkowitz, who led the study.

“We should be doing prevention and treatment programs at a much earlier age,” Berkowitz added in a telephone interview. “Everyone knows how difficult it is to take weight off once you are overweight.”

Berkowitz said his team is now examining the eating and exercise habits of the families they studied but said other studies clearly show genes play an important role. The dramatic spike in weight between ages 3 and 6 support this, he said.

“This suggests that some genes controlling body weight may become active during this period,” Berkowitz said.

Writing in the American Journal of Clinical Nutrition, Berkowitz and colleagues said they studied 70 children at the hospital over a six-year period.

About half had overweight mothers and half had lean mothers.

While babies, the two groups of children looked much the same in terms of weight and body fat.

But one-third of the children of overweight women gained weight quickly after turning 3.

Of the 37 children of healthy weight women, only one became overweight, the researchers said. And, as seen in adults, children from poorer families were more likely to be overweight.

Berkowitz said certain children must carry a combination of genes that make them much more vulnerable to what he calls the “toxic” Western environment, where it is difficult to exercise and food abounds.

He does not believe poor diet alone is to blame.

“It doesn’t appear to be the factor entirely because we know there are plenty of lean people who eat hamburgers,” he said. “The lean group stays pretty lean but this high risk group really takes off.”

The U.S. Centers for Disease Control and Prevention says 15 percent of U.S. children are overweight. More than 60 percent of U.S. adults are overweight or obese.