Conditions for autism may develop even before birth, in the second and third trimester of pregnancy, a new study has found.
The study points to a disruption in the development of cortex, the outer layers of the brain which happens during the second and third trimester.
For the study that involved post-mortem brain tissue, researchers examined donated samples from 11 children, aged between 2 and 16, with autism and equal number of children without the condition.
They used special techniques to detect and visualise specific types of neurons in the brain's outer layer.
The scientists discovered dense patches in the cortex containing irregular shaped neurons residing in the wrong cortical layers, according to media reports.
These patches were 5 to 7 millimetres (about 0.2 inches) long and were found in the frontal and temporal cortexes of 10 of the 11 children with autism but only one of the 11 unaffected children, the study claimed.
"These patches are not like a lesion, or loss of cells. The cells are there, but they haven't become what they were supposed to, in the layer they were supposed to be in," said Eric Courchesne, a professor of neuroscience in University of California, San Diego.
The patches were found in frontal and temporal cortexes, areas that are important for social interaction and language, he added.
The study appeared in the New England Journal of Medicine.