Patchy changes in the developing brain long before birth may cause symptoms of autism spectrum disorder (ASD), research suggests.
The study, in the New England Journal of Medicine, raises hopes that better understanding of the brain may improve the lives of children with autism.
It reinforces the need for early identification and treatment, says a University of California team.
US scientists analysed post-mortem brain tissue of 22 children with and without autism, all between two and 15 years of age.
They used genetic markers to look at how the outermost part of the brain, the cortex, wired up and formed layers.
Abnormalities were found in 90% of the children with autism compared with only about 10% of children without.
The changes were dotted about in brain regions involved in social and emotional communication, and language, long before birth, they say.
The researchers, from the University of California, San Diego and the Allen Institute for Brain Science in Seattle, say their patchy nature may explain why some toddlers with autism show signs of improvement if treated early enough.
They think the plastic infant brain may have a chance of rewiring itself to compensate.