What causes autism? Scientists still don't have an answer, but two new studies suggest that conditions in a mom's womb may trigger the developmental disorder. Heredity is considered a major factor that triggers autism spectrum disorders, but scientists have long wondered what roles - if any - environmental factors play. Scientists used California health records to identify 192 pairs of twins - fraternal or identical - where at least one was affected by autism. Using diagnostic techniques that included directly observing the children, the scientists found 77 percent of male identical twins and 50 percent of female identical pairs both had autism. Those findings weren't too surprising, considering identical twins share the same genes. But what surprised researchers were the high rates of autism spectrum disorders they found in pairs of fraternal twins: 31 percent rate for males and 36 percent for females. Fraternal twins, from two fertilized eggs, share no more genetic material than any other siblings. But since they share the same womb, that could play a role, said Dr. John Constantino, professor of psychiatry at the Washington University School of Medicine in St. Louis, who wasn't involved in the new research. Constantino calls the research a key finding that puts a spotlight on pregnancy as a time when environmental factors might exert their effects."