Last Updated Jun 9, 2015 5:39 PM EDT
In the largest-ever international study of parental age and autism risk, researchers have found increased autism rates among children born to teen moms and among kids whose parents have large gaps between their ages.
The study, funded by the advocacy organization Autism Speaks, also confirmed that children of either older moms or older dads have a higher risk of autism.
Researchers looked at the national health records of more than 5.7 million children in five different countries - Denmark, Israel, Norway, Sweden and Western Australia - including more than 30,000 with autism.
"There's no other data set like this out there," study co-author Michael Rosanoff, Autism Speaks' director of public health research, told CBS News. "We've seen in previous literature that older parents are at a higher risk of having a child with autism, but what we're unsure of is if it's the father's age that increases the risk, if it's the mother's age, or if it's both. This study was able to look at the effects of maternal age and paternal age both independently and jointly."
The results, published today in the journal Molecular Psychiatry, found that autism rates were 66 percent higher among children born to dads over 50 years old, as compared to dads in their 20s. Autism rates were 15 percent higher when moms had children in their 40s and 18 percent higher for children of teen moms, when compared to those born to women in their 20s.
CBS News medical contributor Dr. Holly Phillips explained that "paternal age - the age of fathers when they have children - showed the strongest link. Children who were born to fathers over 50 had a 66 percent higher rate of having autism. If the fathers were in their 40s, there was a 28 percent higher risk of autism."
Phillips added, "One of the more shocking aspects of the study is, we're seeing these links on both sides of the spectrum," with increased risks among teen mothers as well.
Autism rates also rose when both parents were older and when there were wider gaps between the two parents' ages. These rates were highest when fathers were between 35 and 44 and their partners were at least 10 years younger, and when mothers were in their 30s and their partners were 10 years younger or more.
While the higher risk of autism among older parents has been linked to genetic mutations in the sperm or egg cells, there is currently no explanation for the increased risk among children of teen moms and those born to parents with a wide age gap.
Rosanoff hypothesized that "it may be a function of younger moms also experiencing suboptimal pregnancies for various reasons." Teen moms, he said, could have less access to prenatal care due to younger age or possible lower socioeconomic status. "This study couldn't look at these things but it points us in the direction of where the research should go."
Finally, the researchers highlighted that parental age is one of potentially many risk factors contributing to the development of autism, and not a cause of autism.
"By being able to understand some of the risk factors for autism, we can better understand what may have been some of the causal factors in an individual's autism diagnosis," Rosanoff said. "But this study tells us about risk and not necessarily cause."
Study co-author Dr. Sven Sandin, a medical epidemiologist affiliated with the Icahn School of Medicine at Mount Sinai in New York, and Sweden's Karolinska Institutet added in a statement that "although parental age is a risk factor for autism, it is important to remember that, overall, the majority of children born to older or younger parents will develop normally."