Some studies state that quite a high percentage of cases had a history of natal conditions that might have caused damage. But, in general, pregnancy may well have been unremarkable. However, the incidence of obstetric abnormalities is high. No one factor can be identified, but labour crises and neonatal problems are recorded with a significant number of children with AS. There is also a greater incidence of babies who are small for gestational age, and mothers in the older age range. It is recognised that there are three principal causes of Asperger's Syndrome - genetic factors, unfavourable genetic events, and infections during pregnancy or early infancy that affect the brain.