Bigger babies are at greater risk of having autism, say researchers.
There is a 60 per cent increased threat of the condition in infants who were 10lb or more at birth.
A similar increase in risk for babies with low birth-weight because their growth in the womb was poor was also recorded.
The study provides the strongest evidence so far that the danger of autism is greater in babies whose growth in the womb was either poor or excessive.
Researchers believe a poorly functioning placenta – the life support system for the foetus – may lead to abnormalities of growth in the baby’s body and brain.
Around one in 100 children develops autism but symptoms do not usually become apparent until the second year of life.
Autism – or autistic spectrum disorder – includes Asperger’s syndrome and is an umbrella term for a range of developmental disorders that have a lifelong effect on an individual’s ability to interact socially and communicate.
An estimated 600,000 Britons are affected by ASD and genetic factors play a role in its development.