Study finds link between close birth spacing, autism

MONDAY, JAN. 10, 2011

CHICAGO – Close birth spacing may put a second-born child at higher risk for autism, suggests a preliminary study based on more than a half-million California children.

Children born less than two years after their siblings were considerably more likely to have an autism diagnosis compared with those born after at least three years.

The sooner the second child was conceived, the greater the likelihood of that child later being diagnosed with autism. The effect was found for parents of all ages, decreasing the chance that it was older parents and not the birth spacing behind the higher risk.

“That was pretty shocking to us, to be honest,” said senior author Peter Bearman of Columbia University in New York. The researchers took into account other risk factors for autism and still saw the effect of birth spacing.

“No matter what we did, whether we were looking at autism severity, looking at age, or looking at all the various dimensions we could think of, we couldn’t get rid of this finding,” Bearman said. Still, he said more studies are needed to confirm the birth spacing link.

Closely spaced births are increasing in the United States because women are delaying childbirth and because of unplanned pregnancies. Government data show the number of closely spaced births – where babies are less than two years apart – is rising, from 11 percent of all births in 1995 to 18 percent in 2002.

Reasons behind the birth spacing-autism link aren’t clear. It could be that parents are more likely to notice developmental problems when siblings are very close in age, Bearman said. When 2-year-old Billy isn’t developing like 3-year-old Bobby, parents might be more likely to seek help.

Or biological factors could be at play, he said. Pregnancy depletes a mother’s nutrients like folate, a B vitamin found in leafy green vegetables, citrus fruit and dried beans. Prior research has tied close birth spacing to low birth weights and prematurity, possibly because of lack of folate.

“And it could be a combination of effects – not a single explanation but a combination of dynamics,” Bearman said.

The researchers looked at births from 1992 through 2002 in California. They analyzed data on second-born children born to the same parents whose older siblings didn’t have autism. The information on autism diagnoses came from the state’s Department of Developmental Services.

The overall prevalence of autism was less than 1 percent in the study. Of all the 662,730 second-born children in the analysis, 3,137 had an autism diagnosis. Of the 156,034 children conceived less than a year after the birth of their older siblings, 1,188 had an autism diagnosis – a higher rate, but still less than 1 percent.

Children with Asperger’s syndrome and pervasive developmental disorders, milder forms of autism, weren’t included. Government studies indicate about 1 in 100 children have autism disorders, including the milder forms.

The new study was funded by the Robert Wood Johnson Foundation and the National Institutes of Health.


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