Waiting too few years or too many between pregnancies can greatly affect a child’s risk for autism, new research suggests.

Children conceived less than two years or more than six after the birth of an older sibling appear to be at significantly greater odds for the developmental disorder, according to a study published online this week in the journal Pediatrics.

Researchers looked at medical records for over 45,000 second-born children delivered between 2000 and 2009 at Kaiser Permanente’s Northern California hospitals. They found that risk for autism varied substantially depending on how long parents waited to have a second child.

Among kids conceived three to four years after their sibling, less than 1 percent were diagnosed with autism. By contrast, however, more than 2 percent of children conceived within six months had the developmental disorder and the rate dropped to just 1.74 percent when parents waited six to eight months to conceive.

At the same time, researchers also found heightened odds for autism among children born well after their siblings. Among those conceived more than six years after their brother or sister, 1.84 percent were diagnosed with autism.

Using medical records, researchers were able to control for considerations like prematurity, low birth weight and maternal weight-changes between pregnancies, they said.

“We don’t understand why factors such as interpregnancy interval may increase the risk of autism,” said Lisa Croen, a research scientist with the Kaiser Permanente Division of Research and a senior author of the study.

While more research is needed, those behind the study said their findings support the World Health Organization’s recommendation to space pregnancies at least two years apart.