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.

Genetic Factors:
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.

External Factors:
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

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