Most of the risk of autism has been blamed by experts on inherited genes. Now one of the largest studies of twins and autism shifts the focus to the womb, suggesting that the mother's age and health may play a larger role than thought.

The new research doesn't solve the mystery of what causes autism. Most scientists think faulty genes and outside factors are both at work. And since autism spectrum disorders include a wide range of conditions, from mild to severe, it's unlikely there's a single cause for all of them. Finding autism spectrum disorder in so many fraternal twins shifts a research spotlight onto pregnancy, researchers say. iStock

Conditions during pregnancy may trigger autism where there's a genetic vulnerability, said Dr. Gary Goldstein of the Kennedy Krieger Institute in Baltimore, who was not involved in the new research.

"We've identified lots of vulnerability genes, but not everybody who has them gets autism," Goldstein said.

The new twins study, published Monday by Archives of General Psychiatry, used rigorous methods to diagnose autism spectrum disorders, including direct observation of the children.

Using California health records, it's the largest study to do that and the first to consider a large sample of twins drawn from a general population, said lead author Dr. Joachim Hallmayer of Stanford University in Palo Alto, Calif.

Children with autism can have trouble communicating and interacting socially. They may have poor eye contact and engage in repetitive behavior such as rocking or hand-flapping. One in 100 children have autism disorders, according to U.S. government estimates.

The new study included 192 sets of twins where at least one of the twins was affected with autism.

Some of the twins were identical and some were non-identical, or fraternal, twins.

The researchers used DNA testing to determine which twins were identical and which were fraternal. That was important because identical twins come from one fertilized egg and have identical genetic makeups. Fraternal twins, from two fertilized eggs, share no more genetic material than any other siblings.

The new study found, as expected, high rates of shared autism disorders for identical twins: 77 per cent of male twin pairs and 50 per cent for female pairs had autism in both twins. Surprisingly, it also found fairly high rates of fraternal twins both having autism spectrum disorders: 31 per cent rate for male fraternal twins and 36 per cent for female fraternal twins.

Other studies have found 10 to 20 per cent of younger siblings of children with autism are likely to be diagnosed themselves with the disorder.

Pregnancy factors and autism

Fraternal twins share the same womb, even though they don't share identical genes. That could be important, said Dr. John Constantino of Washington University School of Medicine in St. Louis, who wasn't involved in the new research.

"Finding so many fraternal twin pairs in whom both twins have autism spectrum disorders is a key finding that puts a spotlight on pregnancy as a time when environmental factors might exert their effects," Constantino said.

Those factors could include stress, diet, infections, a mother's age and medications, experts said. The new study didn't try to determine what factors increase risk.

The study was funded by grants from the U.S. National Institute of Mental Health and Autism Speaks, an advocacy group.

In another study published Monday in the same journal, researchers found a higher risk of autism among children born to mothers who took antidepressants during the year before birth, particularly in the first three months of pregnancy.

It's too early to advise pregnant women against antidepressants, however. Untreated depression also can be harmful to mother and baby, said lead author Lisa Croen of the Autism Research Program at Kaiser Permanente, a large health maintenance organization in California.