The risk of a child developing autism increases by as much as threefold when pregnancies occur close together, researchers said Monday.

In a study looking at birth records of more than 660,000 second-born children in California, researchers found that the likelihood of autism was nearly three times as high for children conceived within 12 months of their older sibling’s birth as opposed to those conceived after three years.

What’s more, the risk declined the farther apart the births were spaced, according to the study published online ahead of the February issue of the journal Pediatrics.

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“This finding is particularly important given trends in birth spacing in the United States,” the researchers wrote. “Between 1995 and 2002, the proportion of births occurring within 24 months of a previous birth increased from 11 percent to 18 percent.”

The study is believed to be the first to link autism to the timing of pregnancies.

The researchers did not specifically examine why an increased risk for autism occurs when a second child is conceived soon after the birth of a first. But they say it could be related to stress or due to depleted levels of folate, iron or other nutrients during the second pregnancy.