Low birth weight affects a child's risk of having an autism spectrum disorder (ASD), according to a study conducted by a Northwestern University researcher.

After studying 3,715 pairs of twins that include one twin with an ASD, Northwestern researcher Molly Losh learned that a weight difference of at least 15 percent or 400 grams made the smaller twin three times more likely to have an ASD.

Losh directs Northwestern's Neurodevelopmental Disabilities Laboratory and teaches at Northwestern's School of Communication.

The twins were born between 1992 and 1995 and are part of the Swedish Twin Registry's Child and Adolescent Twin Study, which is directed by Paul Lichtenstein at Sweden's Karolinska Institute. The twins in Losh's study were the same sex, but included both identical and fraternal pairs.

"This is the first study of its size that links low birth weight with the potential for autism," said Losh. "Looking at twins is a good way to study autism because the unaffected twins serve as the controls."

The causes of autism are complex, but this study gives scientists one more clue, said Losh. "We already knew the genetic link, that you are more likely to have autism if you have a family history of it or have relatives that family members described as 'anti-social' and 'aloof.' This study helps us narrow down the environmental factors. But there are still a lot of unknowns."

ASD is an umbrella term that includes developmental disabilities that can cause "significant social, communication and behavioral challenges," according to the Centers for Disease Control and Prevention. Lack of social interaction is the common thread, although severity ranges from mild to severe.

The syndrome is an equal-opportunity condition, crossing racial, ethnic and socioeconomic lines. Its various forms are four times more common among boys than girls. They affect 1 in 110 children, the CDC said.

Children diagnosed with an ASD are more common today, but the CDC said scientists are not sure if that is due to better diagnoses or an increased rate of the disorders.

Years ago, said Losh, parents were blamed for autism. "Mothers were called 'refrigerator moms' because they were accused of being cold with their babies," she said. "In fact, I see just the opposite. These parents are doing all they can to help these children."

Losh's study will be published in a spring 2012 issue of the journal Psychological Medicine.

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Low birth weight may increase autism risk, study finds