MONDAY, Jan. 10 (HealthDay News) -- Having babies close together may triple the risk of autism in the second child, new research suggests.

The highest risk was seen in babies conceived within a year of their older sibling. Babies conceived within 12 to 23 months and 24 to 35 months of their older sibling were also at a heightened risk of autism, although the risk was not as pronounced.

Researchers looked at birth records for about 660,000 second-born children born in California between 1992 and 2002. Autism diagnoses were confirmed using records from the California Department of Developmental Services.

Second-born children conceived within a year of the birth of the first-born children were more than three times as likely to have autism than children conceived more than three years apart.

In addition, babies conceived within 12 to 23 months had nearly twice the risk of having autism, while babies conceived between 24 and 35 months were 26 percent more likely to have autism, the investigators found.

"We think this is important research because the finding provides an important clue about what may possibly be modifiable risk factors for autism," said study author Keely Cheslack-Postava, a postdoctoral research scholar at Columbia University in New York City. "But because we don't know what it is about the inter-pregnancy interval that actually matters we can't at this point say that increasing space between pregnancies is necessarily advice that should be given."

Researchers saw a similar trend among subsequent children; closely spaced third-born children were also at heightened risk of autism.

The study is published in the February issue of Pediatrics.

The reason why closely spaced pregnancies may up the risk of autism isn't known, but it could have something to do with the mother's body not having enough time to recover completely from the prior pregnancy, said Andy Shih, vice president of scientific affairs for Autism Speaks.

"When you have a child so quickly after the first, the womb environment may not have sufficiently recovered to properly support a second pregnancy," Shih said.

The researchers said the first pregnancy may deplete critical nutrients such as folate and iron, and the mother may also be more stressed during the second pregnancy.

The findings are especially important because of the trend in women having babies closer together. Between 1995 and 2002, the proportion of births occurring within 24 months of a previous birth increased from 11 percent to 18 percent.

This could be due to more women delaying first pregnancies and feeling hurried to have their second child because of worries about declining fertility, the researchers noted.

At the same time, the incidence of autism has been rising. Closely spaced pregnancies could be one, but definitely not the only, factor driving that, Shih said.

In the study, researchers took into account maternal and paternal ages, parental education and other factors that could affect autism risk or how likely a child is to receive a diagnosis of autism.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more on autism.