The findings from US-based care consortium Kaiser Permanente support the World Health Organization's recommendation of spacing pregnancies a minimum of two years apart.
Study lead Ousseny Zerbo said: "Interpregnancy intervals – the time from the first birth to conception of the second child – may be a factor in autism risk
"While additional research is needed, our study adds to the growing body of evidence about various risk factors for autism."
The cause of autism is unknown, but research has identified a number of different genetic and environmental factors that may play a role in its development. Previous studies have linked short pregnancy intervals with increased risk of psychiatric and neurodevelopmental disorders, including autism. Longer interpregnancy intervals (IPI) have also been linked to adverse perinatal outcomes, including low birth weight, small for gestational age and preterm birth, as well as increased autism risk.
Study findings showed that autism was diagnosed in 0.81% of second-born children following IPIs of 3-4 years. In second-born children with IPIs of less than 6 months, the prevalence of autism was 2%; for intervals of 6-8 months, 1.75%; and for intervals of 6 years or more, 1.8%.
Unlike other studies, this one was able to rule out the possibility that several other autism risk factors explained the findings. These factors included ASD status, prematurity, low birth weight, Caesarean-section delivery of the first-born child, maternal weight changes between pregnancies, and maternal antidepressant use three months before the pregnancy of the second child.
To help determine how various autism risk factors may be linked to family genetics or environmental issues, a new effort is underway at Kaiser Permanente in Northern California called the Autism Family Biobank. The biobank will collect genetic information from 5,000 member families with a child on the autism spectrum.
Lisa A. Croen, PhD, research scientist with the Kaiser Permanente Division of Research, added: "We don't understand why factors such as interpregnancy interval may increase the risk of autism. We're hoping that efforts like the new biobank can point us toward the answers."