In the past two decades, the prevalence of autism appears to have increased astronomically, from one in 10,000 in the 1980s to one in 88 currently, according to the Centers for Disease Control.

Why that is has led to much debate. In fact, some dispute claims of an increase in autism, attributing growing numbers to better diagnosis at younger ages as well as a much broader definition of autism spectrum disorders (ASD).

Researchers such as Drexel University’s Brian K. Lee, however, want to explore the possibility of environmental factors.

“We don’t know whether or not the increase is real, but it’s definitely worth investigating,” says Lee, an assistant professor of epidemiology and biostatistics as well as a fellow at the A.J. Drexel Autism Institute. “If there is a real increase, it’s really important to identify risk factors that may explain the increase.”

Along with Swedish collaborators out of the Karolinska Institutet in Stockholm, he is analyzing large population samples of Swedish children to identify environmental risks for autism. (Genetics alone cannot account for the rapid increase in autism seen in recent years, he says.)

Lee’s work explores many areas, including the role of infection and inflammation in early life as well as the mother’s diet during pregnancy. Another area of interest is parental age at conception.

“Now that we see an association, we want to try and explain it … instead of just guessing why we see these things.”

– Brian Lee, fellow,
A.J. Drexel Autism Institute

Most recently, he looked at parents’ age as a risk factor for autism, with a special — and novel — focus on whether age is a factor for the additional problem of intellectual disabilities in some autistic children. His findings, which were published earlier this year in the International Journal of Epidemiology, were based on data on 417,303 Swedish children born between 1984 and 2003. His work was funded through grants from Autism Speaks and the Swedish Research Council.

“Both the mother’s and father’s age matters in terms of a child’s autism risk,” Lee says. “The older the parent is, the higher the risk the child has.”

But the data also highlighted the differing impact of a mother’s age when compared to a father’s age.

Mothers show a sharp increase in bearing autistic children only after age 30. For fathers, however, the chance of having a child with autism increases linearly for each year of age, Lee says. “It’s a slow, steady climb,” he says.

The research also found that the father’s age has a bigger impact when the mother is 35 years old or younger. In contrast, a mother’s advancing years raised the risk regardless of the father’s age.

“When mothers are younger the father’s risk is more apparent,” Lee says. “But as mothers get older, their risk tends to overshadow any risk from the fathers.”

Much of the recent research on autism has focused on the father’s age and the quality of the sperm, which accumulates mutations over time. Lee, however, notes that his study concludes that “maternal age has greater implications for ASD risk than a similar increase in paternal age.”

Older parental age also correlates with autistic children with intellectual disabilities — and this study is believed to be the first of its kind to look at that aspect, according to Lee.

One reason for this association could be the greater chance of passing along genomic alterations with parental age. But another possibility is environmental factors, such as whether the child was delivered by C-section or vaginally.

“Now that we see an association, we want to try and explain it … instead of just guessing why we see these things,” he says.