Washington ? As if being a tween is not hard enough, scientists now call the years between 9 and 12 a time when girls are especially at risk of getting fat.

Girls are more likely to become overweight in those preteen years than when they are teenagers, researchers report today in The Journal of Pediatrics.

The study did not say why that was and did not examine boys to know whether they face a similar risk.

But it did highlight consequences of that adolescent weight gain. Chubby tweens already were seeing their blood pressure and cholesterol levels inch up, backing up earlier research that fat’s toll on the arteries begins early. Also, being overweight in childhood brought more than a tenfold risk of a youngster’s growing into a fat adult.

Parents should pay attention to creeping waistlines and poor dietary habits, particularly in this age group, said Dr. Denise Simons-Morton of the National Institutes of Health, which funded the research.

“It seems to be a particularly vulnerable period,” said Simons-Morton, who heads obesity-prevention efforts at the NIH’s National Heart, Lung and Blood Institute.

Some 17 percent of U.S. youngsters are obese and millions more are overweight, a problem affecting all ages. Overweight children are at risk of developing diabetes and they grow into overweight adults who, in turn, develop heart disease and other ailments.

The study tracked more than 2,300 white and black girls starting at age 9. Researchers measured height, weight, blood pressure and cholesterol every year through 18. Participants were called in their early 20s to check their weight.

Some 7.4 percent of the white girls and 17.4 percent of the blacks already were overweight by 9. Each year through age 12, between 2 percent and 5 percent of the remaining girls became overweight, reported Douglas Thompson of the Maryland Medical Research Institute, the paper’s lead author.

After the girls reached 12, new cases leveled off to between 1 percent and 2 percent a year.

Other research has shown that the preteen years are when youngsters switch from heeding parents’ dietary advice to eating like their friends do, Simons-Morton said. Less physical activity plays a role, too. She recalls from her own daughters’ tween years long sedentary hours on the phone and worries about getting sweaty.

“It should be cool to be physically active, and attractive,” she said.