Autism may be linked to unusually rapid brain growth during the first year of life, say researchers.
Autistic children's brains grow faster
They found that children with a small head at birth, followed by a sudden and excessive increase in head circumference during their first year, seemed to be more at risk of subsequently developing symptoms of autism.
The more excessive the growth, the more severe the later symptoms of autism were likely to be.
The finding may help experts to predict the onset of autism well before symptoms become apparent.
This could be important as treatment, such as speech and social skills therapy, is more effective if it is started at an early stage.
Currently, the disorder is not typically detected until the ages of two to four, when a child develops behavioural signs and symptoms, such as delayed speech, unusual social and emotional reactions, and poor attention to and exploration of the environment.
The researchers, from the University of California, San Diego, believe that excessive brain growth does not allow enough time for a child to properly process the experiences and emotions that guide and shape normal behaviour.
It is thought under normal circumstances the brain makes and breaks internal connections in response to these external stimuli.
Eventually, this leads to the development of the highly sophisticated thought processes which we take for granted, but which may be lacking in people with autism.
Lead researcher Professor Eric Courchesne said: "During this period of important learning and plasticity, when the brain is experiencing the world and deciding how to construct itself, it's growing too fast in the infant with autism.
"Without the guidance of experience and learning, the brain may be creating abnormal connections that make it very hard for autistic children to make sense of the world they live in."
The research focused on 48 children diagnosed with symptoms of autism.
Comparisons with children free of signs of autism showed that on average, the children's head size was in the bottom 25% at birth.
But by the time they had reached the age of 12-14 months, their average head size was in the top 15%. From then on, brain growth slowed.
At present, it is not known what causes this sudden spurt of growth.
But Professor Courchesne said: "Once we know what causes this specific growth defect, it may be possible to use biological treatments to successfully intervene before the overgrowth begins, thereby allowing the brain and behaviour to develop down more normal paths."
Stuart Notholt, of the UK National Autistic Society, told BBC News Online several previous studies had noted a significant proportion of people with autism had a larger head circumference or brain volume.
The new research differs from previous work in that it charted the brain growth of individual infants over a period of time.
"The points they make about identifying infants as early as possible for possible diagnosis and early intervention by monitoring the rate of brain growth seem very illuminating," he said.
The research is published in the Journal of the American Medical Association.