A new study has revealed that babies whose growth is at either extreme in the womb, either very small or very large, are at greater risk of developing autism.
It is the first time that a clear link has been made between babies who grow to above average size at birth and risk of Autism Spectrum Disorder and follows from a study of more than 40,000 child health records in Sweden.
The research, led by The University of Manchester, also confirmed earlier research which reported that premature and poorly grown, low weight babies appear more susceptible to the condition.
Autism affects how individuals interact with the world and with other people and there is no known cure. Researchers believe it has origins in both genetic and environmental causes.
Professor Kathryn Abel, from the University's Centre for Women's Mental Health and Institute of Brain, Behaviour and Mental Health, who led the research said: "The processes that leads to ASD probably begin during fetal life; signs of the disorder can occur as early as three years of age. Fetal growth is influenced by genetic and non-genetic factors. A detailed understanding of how fetal growth is controlled and the ways in which it is associated with ASD are therefore important if we are to advance the search for cures.
Researchers looked at data from the Stockholm Youth Cohort in Sweden, where early ultrasound dating provides detailed weights of the baby's progression in pregnancy. Infants and children then also take part in structured clinical assessments of their social, motor, language and cognitive abilities.
The cohort contained records of 589,114 children aged 0-17 in Sweden between 2001 and 2007. Certain child data was removed, including children too young to have a diagnosis for ASD, adopted children and non Swedish or Stockholm County residents, children not born in Sweden and twins.
From the remaining available data, researchers found 4,283 young people with autism and 36,588 who did not have the condition and who acted as the control.
The study found that bigger babies who were born weighing over 4.5kg (or 9lb 14) showed a higher incidence of autism, as did smaller infants who were born weighing less than 2.5kg (5.5lb).
A baby who had poor fetal growth would therefore have a 63 percent greater risk of developing autism compared to normally grown babies. A baby who was large at birth would have a 60 percent greater risk. This effect was independent of whether or not the baby was born pre or post term.
Professor Abel added: "We think that this increase in risk associated with extreme abnormal growth of the fetus 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. Risk appeared particularly high in those babies where they were growing poorly and continued in utero until after 40 weeks. This may be because these infants were exposed the longest to unhealthy conditions within the mother's womb," she stated.
The research was published in The American Journal of Psychiatry this month.