“Our study of discordant twins – twin pairs in which only one twin was affected by ASD – found birth weight to be a very strong predictor of autism spectrum disorder,” said Northwestern University researcher Molly Losh. Losh directs Northwestern’s Neurodevelopmental Disabilities Laboratory and teaches at Northwestern’s School of Communication.
“That only one twin is affected by ASD in some identical twin pairs suggests that environmental factors may play a role either independently or in interaction with autism risk genes,” she added. “And because autism is a developmental disorder impacting brain development early on, it suggests that prenatal and perinatal environmental factors may be of particular importance.”
After studying 3,715 pairs of twins the researchers found that lower birth weight more than tripled the risk for autism spectrum disorder in identical twin pairs in which one twin had ASD and the other did not.
“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 a direction to work towards, 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.”
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 and her colleagues’ findings add to a growing body of knowledge about the complex causes of autism and suggest that birth weight could be one of the environmental features that interacts with underlying genetic predisposition to autism.