Babies born weighing more than 9lb 14oz or under 5lb 5oz have a higher of developing autism Study into foetal growth and autism has found birth weight plays role Babies that are abnormally heavy or underweight are at 62% greater risk Experts think that the placenta may disrupt development of autistic baby News comes after another study linked abnormal placenta cells to autism By Rachel Reilly The new study has shown that babies born weighing more than 9lb 14ozs are at a significantly higher risk of autism Babies born either very small or very large have a higher risk of developing autism, according to the largest ever study into the issue. Researchers found that bigger babies - those born weighing over 9lb 14oz (4.5kg) - showed a higher incidence of autism, as did smaller infants who were born weighing less than 2.5kg or 5lb 5oz. 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 (ASD). The research, led by the University of Manchester, also confirms 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 other people, making it difficult to communicate or express emotions.There is no known cure. One child in 100 has the condition in the UK and researchers believe the condition has its origins in both genetic and environmental causes. 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 nearly 600,000 children from birth to age 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 - those born weighing over 9lb 14oz (4.5kg) - showed a higher incidence of autism, as did smaller infants who were born weighing less than 2.5kg or 5.5lb. Autism is a condition that makes interacting with the outside world difficult and often overwhelming The data showed that a baby who had poor fetal growth would have a 63 per cent greater risk of developing autism compared to babies of a normal birth weight. A baby who was large at birth would have a 60 per cent greater risk. This effect was independent of whether or not the baby was born pre or post-term. Professor Kathryn Abel led the research published in The American Journal of Psychiatry this month. She said: '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 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. We now need more research into fetal growth, how it is controlled by the placenta and how this affects how the brain develops. One of the key areas to research is maternal condition and healthy growth. The news comes after a recent study carried out by Yale University and the University of California found that placentas with abnormal cells called trophoblast inclusions raised the risk of developing autism by up to 97 per cent. She said: 'The processes that lead 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.