Children born less than two years after their siblings were considerably more likely to have an autism diagnosis compared to 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.
The study, appearing Monday in the journal Pediatrics, comes just days after a new report further tarnished a British researcher's 1998 paper linking vaccines to autism, this time calling the paper a fraud based on altered facts.
Bearman contrasted the new research to what he called the "junk science" behind the notion that vaccines cause autism.
"One of the things that leads people to think that junk science is science is the idea that science solves all problems with a single bullet," Bearman said. Instead, "science is very slow and proceeds in steps."
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.
Dr. Diane Ashton, March of Dimes' deputy medical director, called the study results an interesting finding that she hasn't seen in prior research. The results will have to be replicated, she said, but her organization already suggests at least a year between pregnancies.
"That is to allow the mother to rebuild depleted nutritional stores and decrease the risk for low birth weight and prematurity. Surely this evidence would provide additional reasons for those recommendations to be made," she said.
The March of Dimes also recommends that all women of childbearing age take a daily multivitamin containing folic acid, an artificial version of folate. Since half of pregnancies aren't planned, the recommendation includes women who aren't trying to get pregnant.
The new study was funded by the Robert Wood Johnson Foundation and the National Institutes of Health.
Bearman hesitated to give advice to parents planning families because the results are so new and unconfirmed. Older parents may not want to wait two or three years for a second child because of other health concerns, he said.
"The advice for parents is to pay attention to the science," Bearman said.
American Academy of Pediatrics: http://www.aap.org