Researchers have determined in one of the largest ever multinational studies of parental age and the risk of autism that children of teen mothers, that older parents and parents with an age gap of more than 10 years have a higher risk of autism spectrum disorder than other children.

The new study, published this week in the journal Molecular Psychiatry, involved the study of more than 5.7 million children across Australia, Denmark, Israel, Sweden and Norway. Previous research had already determined an association between older paternal age and autism, but there has been a lack of definitive information on whether parental and maternal ages are independent risk factors.

Co-author Michael Rosanoff, director of public health research for Autism Speaks who funded the study, suggests that the study is unique among research conducted on ASD and parental age.

"By linking national health registries across five countries, we created the world's largest dataset for research into autism's risk factors," Rosanoff says. "The size allowed us to look at the relationship between parents' age and autism at a much higher resolution - under a microscope, if you will."

Of the millions of children examined by the study more than 30,000 had autism. Each child born between 1985-2004 had their development tracked by researchers until 2009. To separate the influence of maternal and paternal ages, the researchers adjusted their findings for the possible influence of the other parent's age.

The researchers found that the ASD prevalence was 66 percent higher in children born to fathers that were older than 50, and 28 percent higher in children born to fathers in their 40s, compared to those born to fathers in their 20s.

Similarly, ASD prevalence among children born to mothers in their 40s was 15 percent higher, however, rates were also 18 percent higher among children born to teen mothers.

"After finding that paternal age, maternal age and parental age gaps all influence autism risk independently, we calculated which aspect was most important," coauthor of the study, Dr. Sven Sandin says. "It turned out to be parental age, though age gaps also contribute significantly."

"In this study, we show for the first time that autism risk is associated with disparately aged parents," says Abraham Reichenberg, coauthor of the study and neuropsychologist and epidemiologist with the Icahn School of Medicine at Mount Sinai in New York City, New York. "Future research should look into this to understand the mechanisms."

The authors admit a number of limitations of their study. For example, there is a lack of information about some variables that could impact their findings, such as psychiatric history.

"Although parental age is a risk factor for autism," Sandin says, "it is important to remember that, overall, the majority of children born to older or younger parents will develop normally."