CHICAGO -- 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 that there's a single cause for all of them.

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

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.

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 fraternal.

The new study found, as expected, high rates of shared autism disorders for identical twins: 77 percent of male twin pairs and 50 percent for female pairs had autism in both twins.

Surprisingly, it also found fairly high rates of fraternal twins both having autism spectrum disorders: 31 percent rate for male fraternal twins and 36 percent for female fraternal twins.

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 research.