Now, a study has confirmed that older mothers are more likely to give birth to a child with autism, too.
The authors of the epidemiological study, published February 8 in Autism Research, examined the parental age of more than 12.000 children with autism and nearly five million "control" children between 1990 and 1999, all living in California.
The researchers found that mother over 40 had a 51 percent higher risk of having a child with autism than mother 25 to 29, and a 77 percent higher risk than mothers under 25.
(2) Autism a developmental disorder characterized by impaired social interaction and communication appears to be on the rise.
The U.S. centers for Disease Control and Prevention now estimates that as many as one in 110 children in the U.S. has an autistic spectrum disorder a group of developmental disorders including autism, Asperger's syndrome and pervasive developmental disorder.
The prevalence of autistic spectrum disorders in California in 2007 was 12 times that from 1987, representing in average annual growth of 13 percent, according to a report from the California Department of Developmental Services.
Only a fraction of these extra cases can be explained by changes to diagnostic criteria and earlier diagnoses.
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(3) Maternal age is also increasing in the U.S.A. California-based study reported a three-fold increase in the number of births to woman ages 40 to 44 between 1982 and 2004.
But this trend toward delayed childbearing accounted for less than 5 percent of the total increase in autism diagnose in California over the decade.
According to the study a finding that surprised Janie Shelton, a doctoral student in University of California Davis's Department of Public Health Sciences and the study's lead author.
"I would have expected to see more of a contribution, because age is a risk factor and woman are having kids later," she says.