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 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, lowering the chance that it was the parents’ age 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.

Still, he said, more studies are needed to confirm the 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 such births — where babies are less than two years apart — went from 11 percent of all births in 1995 to 18 percent in 2002.

The study, appearing today in the journal Pediatrics, comes just days after a new report further tarnished a 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,” he said. Instead, “science … proceeds in steps.”

Reasons behind the birth spacing-autism link aren’t clear. Parents could be 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 get help.

Or biological factors could be at play, he said. Pregnancy depletes a mother’s nutrients like folate, 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.