Teen mothers, older parents, and so-called May-December couples appear to have a higher risk of having a child with autism, according to one of the largest studies ever conducted on parental age and the condition.

About 1 in 68 children has autism, a set of developmental brain disorders that are thought to be caused by both genes and other unknown factors.

Previous research has already noted a higher risk of autism among children of older parents. This study, which looked at more than 5 million children from five countries, found that risk as well: autism rates were 28 per cent higher when dads were in their 40s, versus dads in their 20s, and rates were 66 per cent higher among kids born to dads over 50 than among those born to dads in their 20s.

“We’ve seen this finding in the research before,” said Michael Rosanoff, the director of public health research at Autism Speaks, the organization that funded the study.

“But this study really allowed us to see this in a higher resolution and show us that this is not just mothers or fathers; in fact, it’s both,” he told CTV News Channel Tuesday.

The study looked at autism rates among 5.7 million children in Denmark, Israel, Norway, Sweden and Western Australia, including more than 30,000 with autism.The children were born between 1985 and 2004, and the researchers followed up on their development until 2009, checking national health records for autism diagnoses.

By using information from five countries, the researchers were able to create the world's largest dataset examining parental age and autism risk.

“Because we had over 30,000 children with autism in this study we were able to look at some new factors,” – including the parental age gap, says Rosanoff.

“And we were able to find that mothers and fathers who had a large gap in their ages – specifically those who were 10 years or more older than their counterpart -- those couples had a higher risk of having a child with autism,” he said.

Autism rates were highest when dads were between 35 and 44 years old and their partners were 10 or more years younger. Rates were also high when moms were in their 30s and their partners were 10 or more years younger.

As well, autism rates were 18 per cent higher among children born to teen moms than among those born to moms in their 20s.

“We don’t know why (this is), but it’s an interesting finding and the first finding of its kind,” Rosanoff said, adding that the link between teen mothers and autism was noted at every one of the five research sites.

The study authors say their findings suggest there may be multiple mechanisms at play that contribute to the link between parental age and the risk for autism, and these mechanisms need further research.

“Unfortunately, there aren’t any recommendations that can be made at this point based on these findings,” said Rosanoff.

“… But this study really allows us now to understand a specific risk factor for autism, and that’s parental age.”

The full study results appear in the journal Molecular Psychiatry.