Tween girls, aged 9 to 12, face an especially high risk of getting fat, says a new study in the Journal of Pediatrics.
The girls are more likely to become overweight in those years than when they become teenagers, researchers found.
The study did not explain why tween girls faced the weight gain nor did it examine boys in the same age group.
However, the results do highlight the consequences of gaining weight at such a young age. Earlier research has shown an increase in blood pressure and cholesterol levels in tweens.
Overweight youth also risk developing diabetes which can pave the way for heart disease and other ailments when they become adults. They also face 10 times the risk of growing into a fat adult.
Dr. Denise Simons-Morton of the National Institutes of Health, which funded the research, says parents should pay particular attention to increasing waistlines and poor dietary habits within the preteen age group.
"It seems to be a particularly vulnerable period," Simons-Morton told The Associated Press.
In the U.S., 17 per cent of youth are obese and millions more are overweight in the general population.
The study examined more than 2,300 white and black girls and measured their height, weight, blood pressure and cholesterol every year from age 9 to 18. Researchers followed up on the participants into their 20s to check on their weight.
By age 9, 7.4 per cent of the white girls and 17.4 per cent of the black girls were overweight. Over the next three years, between 2 and 5 per cent of the remaining girls became overweight.
After the girls turned 12, new cases dropped to between 1 to 2 per cent a year.
Simons-Morton said the concentration of weight gain in the tween years may be the result of less physical activity and from youth avoiding parental dietary advice while eating like their friends do.
