In the largest study of its kind, researchers have shown that the risk of autism increases for firstborn children and children of older parents. The risk of a firstborn with an autism spectrum disorder triples after a mother turns 35 and a father reaches 40.
Although the study was not designed to uncover the cause of the disorder, the findings suggest avenues of research to explore, including the role of environmental toxins.
"Is this pure genetics? Or a toxic phenomenon?" said Darold Treffert, former president of the Wisconsin Medical Society, a psychiatrist at St Agnes Hospital in Fond du Lac and an expert in savant syndrome. Treffert was not involved in the study.
"I think we're bombarded with all sorts of stuff. And we know from experiences such as thalidomide that there are specific times during development of specific risks with specific chemicals. The problem is there is just so much out there."
Thalidomide was a drug taken by pregnant women in the 1950s and 1960s that caused severe birth defects in their children.
The autism study, led by University of Wisconsin-Madison epidemiologist Maureen Durkin, looked at more than 1,200 cases of autism, or 50% more than any previous study. The research team looked at more than 300,000 US births.
The team found a 20% increase in the risk of autism with each 10-year increase in the parents' ages. Also, they found a couple's fourth child has half the risk of the first, regardless of the parents' ages.
Although debate exists about the prevalence of autism in the US population and whether it is on the rise, the Centres for Disease Control and Prevention says the disorder appears in one in 150 births and is increasing.
Regardless of its prevalence, Durkin and Treffert say, the link between age and the risk of autism is not surprising. They pointed to several developmental disorders, including Down syndrome, for which risk rises as the parents' age increases.
"This a trend that I'm concerned about: The increase in developmental disorders in general and the rise in premature births", which are related to autism, Treffert said.
Durkin said the study shows there probably is an ideal window in which to have children - when parents are not too young or too old. But regardless of age, she said, the chances are still very low.
Durkin said her research will hopefully lead to other studies designed to uncover the causes of the disorder. She said the observations that parental age and birth order are involved bring to mind several likely causes.
For instance, she said, factors that might influence the disorder in the children of older parents include age-related genetic and chromosomal damage, environmental toxins and the effects of infertility treatment.
James Crow, a retired professor of genetics at the University of Wisconsin, thinks the genetic hypotheses can be ruled out because the kinds of genetic problems that occur with age are not the same for men and women. So, if autism were caused by an age-related genetic mutation, then the study would show that it is only the age of the father that causes the disorder.
That's because men produce sperm throughout their lives, while a woman's eggs are developed before she is born. And if the disease were caused by chromosomal damage, as occurs in older women's eggs, then the risk would be dependent on only the mother's age.
Crow said the other two possibilities are more likely. As for what is occurring in firstborn children, he said the most likely explanation is just a statistical artefact caused by "stoppage". Parents whose first child is autistic generally do not continue to have other children. The correlation seen with firstborn children is simply a result of parents not having more children, Crow said.
Although Durkin didn't test for this, she believes it's unlikely because most parents do not know their child is autistic until the child is 2 or 3 years old. She said many couples have had, or already are expecting, their second child when they realize their first is autistic.
Other theories include the firstborn's exposure to toxins. The chemicals a woman has acquired over her lifetime are either released directly into the foetus or passed through her breast milk as she nurses. The firstborn soaks up more of those stored chemicals.
