Of all second siblings whose mothers became pregnant with them less than a year after giving birth to the older sibling, about 7.5 in every 1,000 were diagnosed with autism. When mothers became pregnant three years or more after giving birth, about 2.5 out of every 1,000 younger siblings were diagnosed with autism.

Rates were somewhere in the middle for mothers who became pregnant between one and three years after giving birth to their first kid.

But the authors of the research, published today in Pediatrics, say they don't know if younger siblings of closely spaced pairs are actually more likely to have autism. 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, the researchers say.

"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, from Columbia University.

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.