The rising prevalence of autism in the United States suggests that environmental risk factors growing in prominence are at play. New research adds to a growing body of evidence that the risk is conferred well before affected children show symptoms, such as impairments in communication and social interaction -- during pregnancy.
According to the study published in Pediatrics, children conceived within one year of a sibling were three times more likely to be diagnosed with autism than those conceived after three years or more. The association held true even when the study authors controlled for variables such as parental age, preterm birth and low birth weight – all factors known to increase autism risk.
"We've identified a really robust association," said Peter Bearman, director of the Lazarsfeld Center for the Social Sciences at Columbia University and senior author of the study. "When you see something so robust and so stable, it provides an important clue as to what we should be looking at next."
The risk of autism among children conceived 1-2 years after an older sibling was almost double, the researchers reported.
The study focused on over 660,000 second-born sibling children born in California between 1992 and 2002. During that period, the proportion of births occurring within 24 months of a previous birth increased from 11 percent to 18 percent, according to the researchers.
"Closely spaced births occur in some part because of unintended pregnancies but also by choice, particularly among women who delay childbearing," they wrote.
The mechanism by which closely spaced pregnancies may boost autism risk remains unclear, but the authors offered two possible explanations: Autistic behaviors might be more noticeable when there's an older sibling close in age for comparison; or a biological factor, such as maternal depletion of nutrients like folate, -- important for brain development -- could put the developing fetus at risk.
"I think it's likely both," said Bearman. "The next step will be to decompose this and figure out which piece of puzzle is explained by what."
Over the last 40 years, the prevalence of autism spectrum disorders – a group of conditions that includes autism, Asperger syndrome and pervasive developmental disorder – has risen 10-fold to roughly one in 110 children, according to CDC data from 2006.
Work done by Bearman and colleagues has shown that awareness increases the odds of a child receiving a diagnosis of an autism spectrum disorder. But awareness unlikely to account for the dramatic, ongoing rise in prevalence, according to Dr. Thomas Insel, Director of the National Institute of Mental Health.
"You might have called these kids something different before, but you would have called them something," said Insel. "I think awareness contributes on the modest end."
As for Bearman and colleagues' theories about the association between closely spaced pregnancies and autism risk, Insel is skeptical.
"Most parents don't forget when their children started talking," he said, explaining that an extra year or two between children is unlikely to make missed milestones go unnoticed. "I don't think that's it."
But the folate depletion hypothesis doesn't stack up either, Insel said.