NEW YORK (Reuters Health) - A new study suggests that kids who are born only a year or two after an older sibling might be more likely to be diagnosed with autism than those with a bigger age gap.
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, told Reuters Health.
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."
According to estimates by the Centers for Disease Control and Prevention, slightly less than one percent of kids in America have autism. Researchers have not identified a clear cause, but some think it might be linked to conditions in the womb - which could be different depending on how closely siblings are spaced.
Cheslack-Postava and her colleagues used California birth records to find all pairs of first and second full siblings born in the state between 1992 and 2002. Combining that with information on all autism diagnoses from the California Department of Developmental Services, they were able to match up in which sibling pairs one, both, or neither child was diagnosed with autism.
Because they were interested in the impact of the length between pregnancies on autism, the researchers focused on tracking second siblings that were born with or without autism following a first sibling that didn't have the condition. Out of more than 600,000 pairs of siblings in which the older sibling didn't have autism, about 3,000 younger siblings were diagnosed with autism.
When the authors controlled for certain characteristics that may affect the risk of autism - including the mother's age and race, and the child's gender and birth year - the pattern of spacing and autism rates remained the same.
The study does not prove that spacing children closer together causes autism in the second sibling, and Cheslack-Postava said there are many possible ways to explain the link - and that she "can't speculate as to what's the most likely."
One possible explanation is that mothers who got pregnant soon after giving birth the first time had low levels of certain nutrients like iron and folate, or that their bodies were still under stress from the first pregnancy, and that affected the second child's development.
While it could also be that something about parents who choose to have children close together - such as their genes or hormones - makes them more likely to have kids with autism, the authors say this is probably not the case. If it were, firstborn children in closely spaced sibling pairs would also be more likely to have autism, but that pattern didn't turn up in the results.
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.
If that were true, some younger siblings in pairs who were spaced far apart might not have been diagnosed yet by the time the researchers collected the study data.
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.