Close birth spacing may raise autism risk

LOS ANGELES 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.

The study involved more than half a million children from California between 1992 through 2002.

It's not exactly clear why close birth spacing may increase a child's risk for autism. Researchers say more studies are needed.

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.

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 appears in the journal Pediatrics.