A new study is warning that children with autism face a heightened risk for obesity and they start packing on the pounds at much younger ages than their typically-developing peers.

Among children on the spectrum ages 2 to 17, over 33 percent are overweight and 18 percent are obese, according to findings published online Monday in the journal Pediatrics.

By comparison, the study found 31.8 percent of kids in the general population qualified as overweight and 16 percent as obese.

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While obesity typically becomes more of a concern as kids get older, researchers found much higher rates of unhealthy weight in those with autism ages 2 to 5 as compared to typically-developing children. The differential between the two groups disappeared for kids ages 6 to 11, and then surfaced again with 12 to 17-year-olds on the spectrum significantly more likely to be overweight or obese.

“These findings raise the question of whether there are different trajectories of weight gain among children with ASDs, possibly beginning in early childhood,” wrote Alison Presmanes Hill of Oregon Health & Science University and her colleagues in their findings.

For the study, researchers looked at weight and height for more than 5,000 kids on the spectrum who participated in Autism Speaks’ Autism Treatment Network at 19 sites across the United States and Canada. Statistics were then compared to data collected on over 8,800 similar-age children in the general population through the National Health and Nutrition Examination Survey.

Sleep problems common among kids with autism may play a role in driving up their risk for becoming overweight, researchers said. What’s more, children with the developmental disorder who are older, of Hispanic or Latino ethnicity and those who have parents with lower levels of education are also at heightened risk, the study found.

“The findings suggest that health care providers should talk with families early about the risk of unhealthy weight in ASDs, particularly when other comorbid conditions exist,” the study authors wrote.