Low birth weight is a contributory environmental factor to the risk of autism spectrum disorder (ASD), a new study suggests. Studying discordant twins -twins where only one has ASD- it was found that birth weight was a "very strong predictor" of autism spectrum disorder, lead author of the study Molly Losh explained. Previously it was found that when one identical twin has ASD the other is more likely to have it as well. "Because identical twins share virtually 100 percent of their genes, this is strong evidence for the role of genetics in autism," she said. "Yet it is not 100 percent the case that ASD affects both identical twins in a twin pair." However, this study shows that when only one twin is affected, environmental factors play a role either independently or in conjunction with autism. A previous study believed that pregnancy and birth experience are also environmental factors effecting the development of autism. Find out about Barchester's support for adults and children with a wide range of Autistic spectrum conditions.