These children also may be more likely to inflict self-injury, researchers found.
Although the reasons their symptoms are more severe remain unknown, the study suggested it may have something to do with what caused the children to be born before 36 weeks' gestation or after 42 weeks.
"With preterm and post-term babies, there is something underlying that is altering the genetic expression of autism," Tammy Movsas, a postdoctoral epidemiology fellow at Michigan State University's College of Human Medicine, said in a university news release.
"The outside environment in which a preterm baby continues to mature is very different than the environment that the baby would have experienced in utero," she said. "This change in environment may be part of the reason why there is a difference in autistic severity in this set of infants."
The researchers examined an online database of 4,200 mothers with children with autism, ranging in age from 4 to 21 years. They grouped the children into one of four categories based on the timing of their birth: very preterm (fewer than 34 weeks' gestation), preterm (34 to 37 weeks), standard (37 to 42 weeks) and post-term (more than 42 weeks).
The mothers completed questionnaires about their child's symptoms. The study revealed very preterm, preterm and post-term children with autism had significantly more severe cases than children with autism who were born between 37 and 42 weeks gestation.
Although the study found a link between length of pregnancy and autism severity, it did not show a cause-and-effect relationship.
"Normal gestation age of birth seems to mitigate the severity of autism spectrum disorder symptoms, and the types of autistic traits tend to be different depending on age at birth," Movsas said.
For babies born late, the study's authors said prolonged exposure to hormones, higher rates of problems with the placenta and Cesarean section could play a role in the severity of a child's autism.
On the other hand, the study also revealed that very preterm babies show an increase in the mannerisms associated with autism.
Nigel Paneth, an epidemiologist at Michigan State, said that although autism has a strong genetic component, the findings indicate that something about pregnancy or the perinatal period may affect how autism manifests itself.
"This adds to our earlier finding that prematurity is a major risk factor for autism spectrum disorder and may help us understand if anything can be done during early life to prevent or alleviate autism spectrum disorder," he said in the news release.
The study was published online April 3 in the Journal of Autism and Development Disorders.
No one knows what causes autism. Other research published this week found that spontaneous genetic mutations and older fathers may influence a small number of autism cases.
According to recent figures from the U.S. Centers for Disease Control and Prevention, one in 88 U.S. children has some form of the neurological disorder.
More information
The U.S. National Institute of Neurological Disorders and Stroke provides more information on autism.
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