Geraldine Dawson, chief science officer for Autism Speaks, praised researchers for using such a large database to uncover the impact each parent's age can have on autism risk. But she also urged older mothers not to worry unnecessarily.
"We need to look at this finding in context," Dawson said. "This study is important, but we are not going to find one factor that can explain this dramatic increase in the prevalence of autism. We are going to find multiple factors, and advanced maternal age appears to be one of them."
The new research contradicts an earlier study out of Israel that found the children of men over age 40 were at higher risk of autism. In that study, fathers over 40 were six times more likely than fathers under 30 to have a child with autism.
But subsequent studies did not confirm those findings. The Israeli study included only four women over age 40 whose children had autism -- too small for an accurate analysis, Shelton said. The new California study included more than 12,000 cases of autism, and 501 women who gave birth over 40 and whose children were diagnosed with autism.
Researchers do not know why their study found that children with an older father but relatively young mother were more apt to develop autism, but they suspect it's more than a statistical blip. "We feel it's indicative of some underlying biological process," Shelton said.
Mothers under age 30 have only a minimal age-related risk of having a child with autism, Shelton noted. It's possible that the impact of the father's age is more pronounced when dad is older and mom is much younger, whereas the risk conferred by older mothers "drowns out" any age-linked risk from fathers.
Autism is a developmental disorder that causes problems with social and communications skills and repetitive or restrictive behaviors. Because the condition has a wide range of symptoms and degrees of sever
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