Genetics   
While genetics doesn’t account for all cases of autism, it is certainly a factor.  Studies have shown that parents who have ASD or have family members with ASD are far more likely to have children with ASD, and if two parents have one child with ASD the chances of the same couple having another child with ASD increases dramatically.  Also, various studies have identified non-inherited or ‘spontaneous’ genetic mutations present in certain children with ASD.

Prenatal Environment  
Since genetics cannot account for all cases of ASD, it is commonly assumed that environmental causes will ultimately account for the remaining cases. Central among these is prenatal environment, which may affect a pregnancy during the critical first eight weeks of conception.  These include:

Teratogens
Environmental agents known to cause birth defects. Examples are thalidomide, valproic acid or misoprostol; or rubella infection in the mother.

Pesticides
There are instances of autism in children conceived near farm fields that use organochlorine pesticides such as dicofol and endosulfan. A relatively small number of cases where children with ASD can be proven to have been conceived under these conditions and a small control population to compare against make further studies connecting pesticides with autism challenging.

Folic Acid
An untested hypothesis proposes that Folic Acid might play a role in autism due to its modulation of gene expression through epigenetic mechanism.

Fetal Testosterone
One project has published several reports suggesting that high levels of fetal testosterone could produce behaviors relevant to those seen in autism. This remains a controversial theory.

Ultrasound
Study showed that sustained exposure of mouse embryos to ultrasound waves caused a small but statistically significant number of neurons to fail to acquire their proper position during neuronal migration.