Waiting too little or too much time between pregnancies may significantly increase the odds that children will have autism or other developmental disabilities, new research suggests.

In a review of existing studies on more than a million kids, researchers found that children born to moms who waited less than a year between pregnancies were nearly twice as likely to have autism compared to those who waited over three years.

The findings published online Thursday in the journal Pediatrics also suggest that such children may face an increased risk of cerebral palsy and developmental delay.

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Meanwhile, in cases where mothers waited a long time – more than five years between pregnancies – the study also found an increased risk of autism.

For the review, researchers from the World Health Organization and the U.S. Agency for International Development looked at 12 existing studies. Seven of the studies focused on the relationship between birth spacing and autism, two considered the impact on cerebral palsy odds and three looked at developmental delay.

Despite identifying an association between birth spacing and the likelihood for autism and other conditions, the findings do not indicate that birth spacing alone causes the developmental disabilities. Instead, it’s likely that other factors surrounding pregnancy timing – including folic acid levels and infertility – may play a role, researchers said.

“Because advanced maternal age is also a well-recognized risk factor for ASD, educating women and their families about healthy timing and spacing of pregnancy (20–35 years of age and 24–59 months, respectively) could contribute significantly, in both developed and developing countries, to a reduction in this neurodevelopmental disorder,” wrote lead author Agustín Conde-Agudelo of the World Health Organization’s Collaborating Centre in Human Reproduction and his colleagues in their findings.