For decades, most autism experts blamed the brain disorder on faulty genes. Now, groundbreaking new research is offering further evidence that factors during pregnancy and birth may play a larger role in the disorder than previously thought.

Dr. Peter Szatmari, an autism researcher who is the head of child psychiatry and behavioural neuroscience at McMaster University in Hamilton, Ont., believes that with every new study that emerges, scientists get closer to understanding what causes autism.

"We're way closer than we were five or 10 years ago," Szatmari told CTV's Canada AM Wednesday.

One study advancing the knowledge into the complex brain disorder was published this week in the Archives of General Psychiatry.

The study looked at the prevalence of autism by studying 192 sets of twins where at least one of the twins was affected with autism.

Some of the twins were identical -- meaning they came from one fertilized egg and were genetically identical -- and some were fraternal, twins. Fraternal, or non-identical, twins share no more genetic material than siblings born years apart.

As expected, the study found high rates of shared autism disorders in the identical twins: 77 per cent of the male twin pairs and 50 per cent for female pairs had autism in both twins. (Autism is far more prevalent in boys than girls for reasons still not understood.)

But in a surprising finding, the researchers also found fairly high rates of shared autism in the fraternal twins: 31 per cent rate for male fraternal twins and 36 per cent for female fraternal twins.

That rate was much higher than previously thought. Previous studies have estimated that about 10 to 25 per cent of siblings of children with autism are likely to be diagnosed themselves with the disorder.

The finding of so many fraternal twins who both had autism puts a new spotlight on the influencing environmental factors that may be at play during pregnancy.

Researchers aren't sure what those factors could be, but have a few theories, including: stress or illnesses during pregnancy, the parents' ages, or medications taken during pregnancy.

Dr. Szatmari, welcomed this latest research.

"This twins study out of California really has emphasized for us in a new and powerful way the extent to which some environmental risk factors that are shared between twins might play a role in the etiology of the disorder," he said.

Szatmari says this new research brings scientists closer to understanding the interplay between genetics and environmental factors.

"The key was to first understand the genetics of the disorder, which we've made great progress in doing in the last little while," he says.

"And then once you understand the inherited factors involved in the disorder, it became a lot easier to then piece together the environmental risk factors that might interact with that genetics susceptibility."

One of the environmental factors might be a mother's use of antidepressants during pregnancy. Another study published this week in the same journal found an increased incidence of autism among children born to mothers who took SSRI antidepressants (selective serotonin reuptake inhibitors) during the year before birth, particularly in the first three months of pregnancy.

"We don't understand the mechanism whereby certain medications might be involved in causing autism," commented Szatmari.

"But I really want to emphasize that the magnitude of that risk is really pretty small. And the risk of not being on antidepressants during pregnancy, if you really need them, can be substantial. So if any mums are concerned about taking antidepressants, it's really important to talk to their doctors," he said.

Another factor that might be involved is the use of assisted reproduction. In vitro fertilization and fertility drugs often result in twin pregnancies, which might explain the high rate of autism among fraternal twins.

Low birth weight and prematurity are also more common among twins and those factors appear to be involved too, some research has found.

As well, parents who seek fertility treatments also tend to be older, which in itself might influence the development of autism.

Szatmari says all this research points to maternal-fetal interaction possibly playing a more important role than we had previously appreciated.