WASHINGTON -- Closely spaced pregnancies were associated with an increase in the odds of a second child being diagnosed with autism, according to a study involving California children.
The study, which the journal Pediatrics published online Monday, showed the sooner conceptions followed the prior birth of a sibling, the greater the likelihood of the second child having an autism diagnosis.
The study looked at more than 660,000 second-born children in California between 1992 to 2002. The study measured the time the second child was conceived relative to the first child, and then looked at autism diagnosis of the second sibling.
The study found that second children who were conceived less than 12 months after the first child's birth were three times more likely to be diagnosed with autism than children spaced further apart. Second children conceived less than two years after the first had almost twice the odds of receiving an autism diagnosis.
Autism diagnoses were determined by looking at records of the California Department of Developmental Services, However, researchers cautioned that additional studies need to be done to confirm the findings.
Still, one of the researchers, Peter Bearman, a sociologist at Columbia University in New York, said "we see this really profound association" that remained when other factors, such as the age of the parents were looked at.
"We don't know what causes this," he said.
One possibility is that closely spaced pregnancies deplete the mother of important nutrients. But the other possibility is that parents are more aware of the development of a second child and can spot when the child seems to be delayed compared to the first, Bearman explained. This might allow a child with autism to be diagnosed sooner.
But the actual number of autism cases seen in the study was less than 1%, with 3,137 cases out of the 662,730 second-born children that were in the study.
The Centers for Disease Control and Prevention estimates that as many as 1 in 110 U.S. children have an autism spectrum disorder, which includes autism, Asperger's syndrome and pervasive developmental disorders.
There's no known cure for autism, which is characterized impairments in social interaction, communication and unusual behavior and interests, but therapy and medication can improve symptoms.
Bearman's study didn't include first-borns with autism. Other research has suggested people who already have a child with autism have a higher risk of having another child with autism or an autism spectrum disorder.
Bearman explained that several environmental, social and genetic factors likely play a role in the development of autism.
The debate about one factor - childhood vaccines - still remains. Last week researchers further discredited a 1998 paper that linked the measles, mumps and rubella vaccines to autism, saying the paper was based on falsified data.
Pediatrics' study was funded by the National Institutes of Health and the Robert Wood Johnson Foundation.
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