Autism risk grows steadily with fathers' increasing age, but accelerates with mothers' age after 30. These charts show generalized additive model estimates of the probability of ASD in the Stockholm Youth Cohort. Dashed lines show 95 percent confidence intervals.

It has been generally accepted that older parents are more likely to have a child who develops an autism spectrum disorder (ASD) than are younger parents. Now a study provides insight into the role played by both paternal and maternal age.

The study, published in the February 2014 issue of the International Journal of Epidemiology, found that fathers’ and mothers' advancing ages have different impacts on a child’s risk. The rise in ASD risk with parental age was greater for older mothers as compared to older fathers.

“The open question at hand really is, what biological mechanisms underlie these age effects?” said Brian K. Lee, PhD, an assistant professor in the Drexel University School of Public Health and senior author of the study.

Complicated relationship

The risk of having a child with ASD had a more complicated relationship to age in women than in men – whose risk of fathering a child with ASD increased linearly with age across their lifespan.

Among women giving birth before the age of 30, the risk of ASD in the child showed no association with age -- it was simply very low. But for babies born to mothers aged 30 and older, the chance of developing ASD rose rapidly with the mother's age.

Lee noted that the non-linear maternal age effect that is relatively stronger than the paternal age effect on ASD risk has been observed in previous studies, but has not received much attention.

Large sample

In this study, Lee and colleagues analyzed a large population registry sample of 417,303 children born in Sweden between 1984 and 2003, adjusted for numerous possible factors that could vary with parental age and also influence risk, such as family income and each parent’s psychiatric history. The study also used a case-finding approach, to identify more ASD cases than other studies might, based on all pathways to care in a socialized health system.

A goal was to study these parental age effects in more detail by looking at possible differing risks of ASD with and without intellectual disability – one of the most serious comorbid diagnoses with ASD, with a significant impact on functional status in life. This was the first population-based study with an ASD sample large enough to study ASD risk in populations of children with and without intellectual disability.

“When considering risk factors, we can’t necessarily lump all ASD cases together, even though they fall under a broad umbrella of autism,” Lee said. “We need to keep an open mind in case intellectual disability might be a marker of a different underlying mechanism.”

The finding that ASD with intellectual disability had a stronger association with older parents, compared to ASD without intellectual disability, supports continued investigation of possible different mechanisms.

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