Having babies close together may triple the risk for the second-born child to be diagnosed with autism, according to a study published in the February 2011 issue of the journal Pediatrics. Conducted by a team of scientists from Columbia University, researchers examined the odds of autism among more than 660,000 second-born children in California. Compared to children who were conceived more than three years after the birth of an older sibling, children conceived less than 12 months after their sibling's birth saw their risk for an autism diagnosis increase more than three-fold. Children conceived between 12 and 23 months later were almost twice as likely to be diagnosed with autism, and children 24 to 35 months were one and a quarter times more likely to have been diagnosed with autism.
Further research is needed to better understand the connection between pregnancy spacing and autism, but researchers are confident that have "identified a really robust association," says Peter Bearman, director of the Lazarsfeld Center for the Social Sciences at Columbia University and senior author of the study, in an interview with ABC News. "When you see something so robust and so stable, it provides an important clue as to what we should be looking at next."
According to researchers, close pregnancy spacing may up autism risk because women are more likely to have depleted levels of nutrients such as folate and iron, as well as higher stress levels, after a recent pregnancy (affecting fetal brain development). Researchers also admit that having children so close in age could simply make it easier for parents to pick up on differences in behavior. As researchers point out, autistic behaviors might be more noticeable when there's an older sibling close in age for comparison.