Father's age linked to autism risk, but overall risk is small
Men in their 40s are more likely than those in their 20s to father a child with autism or schizophrenia, but the overall risk is still low – about two per cent – according to a study published online in Nature last month.
“The majority of babies born to 40-year-olds are healthy, but the study helps explain some of the increased risk in a huge population – which helps explain some of the rise in autism rates in the last couple of decades,” says Dr. Evdokia Anagnostou, a child neurologist who leads a clinical program in autism research at Holland Bloorview.
Researchers in Iceland examined genetic material from 78 parent-child trios, where parents with no signs of mental illness gave birth to a child who later developed autism or schizophrenia, and a control group.
They found that as men aged, they transmitted more random mutations to their child at or near conception. The average child born to a 20-year-old father had 25 mutations linked to the father. This increased by two mutations a year to reach 65 for children of 40-year-old men. The mother’s age had no impact on risk.
“Lots of random genetic changes happen with age and the majority mean zip,” Evdokia said. “It’s the rare ones that lead to syndromes like autism.”
However, the more mutations a child has, the more likely he or she is, by chance, to have one of the rarer, disabling ones.
The study is important because it helps explain the biology of autism, which may aid in treatment. “If we understand the pathways from genes we discover we know where to focus our efforts.”
Evdokia emphasized that autism is “a multi-factorial disease that can’t be prevented at this point. Paternal age can now be seen as one of many contributors to risk – but it doesn’t explain the majority of risk.
“There are many, many reasons why you may end up with a child with autism and if the father is older the child gets this extra little hit. But most of the time a child needs to have many hits to get autism.”
Other factors that increase risk include a father's genetic makeup, such as having a sibling with autism; the fetal environment, including infections during pregnancy and exposure to fertility drugs; and possibly some environmental toxins.
“Some kids will have enough of a genetic hit to cause autism and some will have a genetic risk that interacts with one or more environmental factors.”
Evdokia said that the study is helpful in explaining the increase in autism rates in a huge population – but doesn't identify father's age as a major risk factor in individual cases.
“If someone asks me ‘Do you think I should not have a baby because I’m 40?’ I would say you absolutely should have a baby. The majority of babies born to 40-year-olds are perfectly healthy.”