The rate of autism in second siblings whose mothers became pregnant with them less than a year after giving birth to the older sibling, was much higher than those with a 3 year (or more) gap.
The researchers say that it could also be that parents can more easily recognize warning signs of autism when they have more recently watched another kid pass through developmental stages.
"There's one possible explanation, (which) is that there is some biological factor" such as a mother's nutrient levels or stress that makes a second child more at risk for autism when siblings are closely spaced, Dr. Keely Cheslack-Postava, the study's lead author.
Another explanation "is just better diagnosis and better picking up on symptoms," Cheslack-Postava said, "in which case it would be an advantage to be more closely spaced."
The trend could, however, be explained by social factors, Cheslack-Postava said. Parents may be more likely to take a younger sibling to the doctor to be checked out for autism if they remember that only a year or two ago, when their older sibling was the same age, he or she was much farther along in development in certain areas.
Future studies will need to look more closely at specific factors that could explain the link between close pregnancy spacing and autism risk, Cheslack-Postava said. For now, it's too early to make recommendations to parents about how to space their children if they are trying to minimize the chance that a child will have autism, she said.
"There are a number of reasons for why people would choose to have children closer together or farther apart," Cheslack-Postava said. "It's a very individual decision including many factors." And the findings, she said, shouldn't have any impact on that decision.
What are your thoughts on the age gaps between siblings?