AUTISTIC children have abnormally fast brain growth that begins a few months after birth, according to research that offers the first hope for early detection of the disorder.

The causes of autism have so far been unknown, but US specialist Eric Courchesne will tell the World Autism Congress in Melbourne today he has discovered that by the time an autistic child is about four, the size of their brain is equal to that of a normal 12-year-old.

While the trigger for this growth remains unknown, the discovery means that the size of a baby's skull could be an early warning sign of autism, Professor Courchesne said.

Using magnetic resonance imaging, Professor Courchesne and his team found excessive brain growth in autistic children.

After examining their medical records they found that children who later become autistic have normal brain size at birth.