There is no single cause of autism because there is no one type of autism. Scientists have identified a number of rare gene changes, or mutations, associated with autism. However, most cases involve a complex and variable combination of genetic risk and external factors that influence early brain development.
Research has identified more than a hundred autism risk genes. In around 15 percent of cases, a specific genetic cause of a person’s autism can be identified.
If there is a genetic predisposition to autism, a number of external factors further increase a child’s risk. The clearest evidence of these environmental risk factors involves events before and during birth.
Advanced parental age (mother and father) at time of conception
Maternal illness during pregnancy,
Extreme prematurity and
very low birth weight and
certain difficulties during birth like oxygen deprivation to the baby’s brain.
Mothers exposed to high levels of pesticides and air pollution may also be at higher risk of having a child with ASD.
Please note that these external factors, by themselves, do not cause autism. but, in combination with genetic risk factors above, they appear to modestly increase risk.
Source: AutismSpeaks, Wikipedia