A much larger or much smaller birth weight than average may be associated with an increased risk of autism, according to a large new study.
Researchers examined data from more than 40,000 children in Sweden, and found that those who weighed more than 9.9 pounds or less than 5.5 pounds at birth were more likely to have autism than those with a normal birth weight.
Specifically, smaller babies had a 63 percent greater risk, and larger babies had a 60 percent greater risk. The link between birth weight and autism risk was independent of whether or not a baby was born premature or past the normal delivery date.
Autism is a neurodevelopmental disorder that affect a person's ability to communicate and interact socially.
The study, published recently in the American Journal of Psychiatry, is believed to be the first to show a link between larger babies and increased autism risk and confirms earlier research showing that low weight babies are more likely to develop autism.
"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," study leader Kathryn Abel said in a university news release.
Abel is a professor at the Center for Women's Mental Health and Institute of Brain, Behavior and Mental Health at the University of Manchester, in England.
"Anything which encourages abnormalities of development and growth is likely to also affect development of the baby's brain," she said. "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."
While the study found an association between having a high or low birth weight and having autism, it did not establish a cause-and-effect relationship.
"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," Abel concluded.