Many factors can affect a parent's choice in spacing the birth of their kids, but new research suggests another factor be taken into consideration: children born less than two years after the previous child have up to three times more chances of becoming autistic.

Researchers, with the goal of determining how pregnancy and birth intervals of siblings affect the risk of babies developing autism spectrum disorder (ASD), gathered data on more than 45,000 children born between 2000 and 2009 at the Kaiser Permanente Northern California (KPNC).

Data included length of the inter-pregnancy interval (IPI), or the time between the birth of the first and second child, and whether the children had ASD according to the International Classification of Diseases.

Factors such as the mother's weight and condition during pregnancy, whether the second baby was born to term, birth weight and history of stillbirths and other adverse events during pregnancy of the first born were also considered during analysis. The team's findings were published in the Pediatrics journal on Sept. 14.

The team found that, out of the chosen study group, 1.2 percent, or 577, of the second-borns were diagnosed with ASD and associated disorders. Second children born less than one year or more than six years apart had two to three times more chances of having ASD compared to those born with a three- to four-year interval.

"We had the ability to look at a number of factors that might explain the findings and they actually didn't explain them," said Lisa Croen, director of the Kaiser Permanente Autism Research Program and author of the study.

"With all the data we had, we could not explain away what we found. So now we have to discover what it is about the short interval or long interval between pregnancies that increases the risk of autism in the second child."

Incidentally, these findings back up the World Health Organization's guidelines on spacing births to prevent prematurity, low birth weight and other delivery problems.

"Our results support that recommendation that when families are able to plan, it's best to space kids out at least two years apart," said Croen.

While the study showed a possible link between pregnancy and IPI, more data is still needed to understand the nature of this relation. Croen said that there is much to learn about autism and that there may be several other factors involved in its causes that are yet to be discovered.

"Autism is a complex disorder with many factors and manifestations, and it would be wrong to conclude that spacing is the only thing that matters," said Croen.

Autism is an umbrella term for several neurodevelopmental disorders. According to the National Institute of Neurological Disorders and Stroke, these disorders are characterized by communication and social difficulties and restricted, repetitive movement.

It is estimated that one child out of 88 around 8 years old may develop ASD, and that boys are four times more at risk to acquire it than girls. There is currently no cure for ASD, but ongoing research is being carried out to understand more about the condition. Therapeutic options for ASDs include medication, family counseling and skills-oriented education in special schools.