Shorter Gap Between Pregnancies Linked to Increased Autism Risk

Study supports role for environmental risk factors during pregnancy.

ByABC News
January 7, 2011, 12:14 PM

Jan. 10, 2011— -- 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."