Children with autism appear to have significantly more brain cells than their typically developing peers, new research shows, offering evidence that the developmental disorder originates prenatally.

The finding comes from a small, preliminary study published Wednesday in the Journal of the American Medical Association that looked at the brain composition of seven boys with autism and six typically developing males ages 2 to 16 who had died.

Those with autism had 67 percent more neurons in the prefrontal cortex of the brain, the area that handles everything from language and communication to social behavior, mood and attention. They also had heavier brains compared to the other boys their age.

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The extra brain cells found in the children with autism are particularly significant, researchers say, because these neurons are produced before birth suggesting that autism may onset prenatally.

“An excess of brain cells was found in each child with autism that we studied,” said Eric Courchesne, the lead author of the study and the director of the Autism Center of Excellence at the University of California, San Diego. “While we think that ultimately not every child with an autism disorder will show this, our study does suggest that an abnormal excess of cells may be quite common among children with autism.”

Courchesne and his colleagues acknowledge that their study sample is small, but say that it’s difficult to obtain brain samples from young people with autism who have died. Nonetheless, they say their findings could have big implications.

“This is an exciting finding because, if future research can pinpoint why an excessive number of brain cells are there in the first place, it will have a large impact on understanding autism, and perhaps developing new treatments,” Courchesne said.