The number of children with developmental disabilities has increased by 17% in 12 years, driven largely by big jumps in diagnoses for autism and attention deficit hyperactivity disorder, research shows.

More than 15% of school-age kids — about 10 million children — had a developmental disability in 2006-08, according to a study released Monday in the journal Pediatrics. That's up from 12.8% in 1997-99.

The study suggests that three or four children in a typical elementary school classroom have development disabilities. Some of the increase may be the result of the rise in premature birth, which leaves some babies with lasting impairments, says study author Sheree Boulet of the Centers for Disease Control and Prevention. One in eight babies today are born prematurely, according to the March of Dimes.