Tammy Movsas of Michigan State University in East Lansing, Mich., said autistic children born either pre-term or post-term are more likely to injure themselves compared with autistic children born on time.
The study did not uncover why there is an increase in autistic symptoms, the reasons may be tied to some of the underlying causes of why a child is born pre-term -- prior to 37 weeks -- or post-term -- after 42 weeks -- in the first place.
The research published online in the Journal of Autism and Development Disorders suggested the length of a mother's pregnancy is one factor in affecting the severity of the disorder.
"We think about autism being caused by a combination of genetic and environmental factors -- with pre-term and post-term babies, there is something underlying that is altering the genetic expression of autism," Movsas said in a statement. "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. This change in environment may be part of the reason why there is a difference in autistic severity in this set of infants."