Babies who are either very small or very big when they are born may have an increased risk of developing autism, a new study indicates.

According to UK scientists, this is the largest study of foetal growth and autism to date. While previous research had suggested a link between low birth weight babies and autism, this marks the first time a clear link between large babies and autism has been made.

Autism, also known as autism spectrum disorder (ASD), is a biological disorder of the brain that impairs communication and social skills. Signs can include an absence of eye contact, apparent aloofness, avoidance of physical contact even with family, difficulty dealing with interruptions to routine and a lack of interest in other children and what they are doing.

The team from the University of Manchester looked at data from the Stockholm Youth Cohort in Sweden, which contained records of almost 600,000 children aged between zero and 17 between 2001 and 2007. These records included ultrasounds which provided details on growth while the children were still in the womb.

From these records, the scientists selected over 4,200 children with autism and 36,000 without the condition to study.

They found a higher incidence of autism among bigger babies - those over 4.5kg (9lbs, 9ozs) at birth. They also found a higher incidence among babies who weighed less than 2.5kg (5lbs, 5ozs) at birth.

From this, they calculated that large babies had a 60% increased risk of developing autism, while smaller babies had a 63% increased risk. These results were irrespective of whether the babies were born prematurely or after their due date.

"We think that this increase in risk associated with extreme abnormal growth of the foetus shows that something is going wrong during development, possibly with the function of the placenta. Anything which encourages abnormalities of development and growth is likely to also affect development of the baby's brain," the scientists said.

They noted that the risk of autism appeared particularly high in babies who were growing poorly but were born overdue, i.e. after 40 weeks.

"This may be because these infants were exposed the longest to unhealthy conditions within the mother's womb," they suggested.

The scientists said that their findings ‘have shown for the first time categorically that abnormal foetal growth in both directions increases risk of autism spectrum disorder'. They called for further research in this area.

Details of these findings are published in The American Journal of Psychiatry.