A woman's chance of having a child with autism increase substantially as she ages, finds a new study which also found the risk from older dad was less than previously thought.
U.S. researchers analyzed more than five million births and found that a 40-year-old woman's risk of having a child later diagnosed with autism was 50 per cent greater than that of a woman between 25 and 29.
But the researchers note that even with those higher risks, the risk of a woman over 40 having an autistic child was still less than four in 1,000.
The study looked at the health records of all 5.6 million births in California between Jan. 1, 1990 and Dec. 31, 1999, focusing on diagnoses of autism before age six. In total, more than 13,000 diagnoses were found. The study then focused on the 12,159 autistic children for whom information on both parents' ages was also available.
They found that the incremental risk of having a child with autism increased by 18 per cent for every five-year increase in the mother's age.
They also found that the risk of having a child diagnosed with autism was 36 per cent higher for fathers older than 40 compared to fathers in their 20s.
Interestingly, the research suggested that the father's age appears to matter most with young mothers. Among children whose mothers were younger than 25, autism was twice as common when fathers were older than 40 than when dads were in their 20s.
The findings contradict a 2006 study of children born in Israel that suggested paternal age played a much larger role, noted the study's lead author, University of California at Davis researcher Janie Shelton.
"Although fathers' age can contribute risk, the risk is overwhelmed by maternal age," said the study's lead author, University of California at Davis researcher Janie Shelton.
Older mothers are known to face increased risks for having children with genetic disorders, and genes are thought to play a role in autism.
The study was released Monday in the February issue of the journal Autism Research.
Recent data suggest about autism rates appear to have increased in recent decades. But many experts point out that the rise may reflect better awareness of autism and a broadening of the definition of autism spectrum disorder rather than a true increase in affected children.
While births to older mothers also have risen in recent years, that likely accounts for only a small part of the increase in cases, said study co-author and UC-Davis researcher Irva Hertz-Picciotto.
Hertz-Picciotto, professor of public health sciences, said the reason that having an older parent places a child at risk for autism is not known.
"We still need to out what it is about older parents that puts their children at greater risk for autism and other adverse outcomes, so that we can begin to design interventions," she said.
