Experts have blamed most of the risk for autism on inherited genes. Now one of the largest studies of twins and autism shifts the focus to the womb. It suggests that the mother's age and health may play a larger role than was thought.
The study found a higher-than-expected rate of autism in fraternal twins. Those are twins that aren't genetically identical and are more like regular siblings. But they do share their mother's womb for nine months.
The new study didn't try to determine what factors increase risk during pregnancy. But experts say they could include stress, diet, infections, a mother's age and medications.
The study was led by Stanford University researcher Dr. Joachim Hallmayer. It was published Monday in Archives of General Psychiatry.