Autism is a condition for which we are still unable to attribute any cause to. Research is still going on to find out more about the factors that may be responsible for the condition. 
According to a research report in American Journal of Epidemiology some possible cause of autism are related to the process of birth and related environment. Some of these factors that may increase the child being born with autism are the following: 

Breech position of the baby

Low Apgar score, Apgar score being the index used to assess the condition of a newborn five 
minutes after birth 

Birth before 35 weeks of pregnancy

Parental history of schizophrenia or similar forms of psychosis

Parental history of affective disorder, which includes some psychoses, depression, and bipolar disorder

However, researchers also said that these are mere risk factors that can increase the chance of a baby being born with autistic disorder and are not the absolute causes of the disease.