What are the environmental factors that contribute to autism?

Prenatal factors . Include: a low folate (folic acid) level at the time of conception, drugs or alcohol from 3 weeks' conceptional age-delivery, maternal "flu"/viral illness with fever for a week, maternal obesity ;/or diabetes, advanced maternal age ; advanced paternal age if mother is young. These epigenetic factors alter function but not structure or sequence of fetal genes ; increase the risk of ASD in offspring.
We don't know. There are a lot ot theroies out there about this problem, but nothing definitive at this time.