"For a condition as complex as autism, it’s almost certain that both genes and environment play an important role. But teasing apart how much DNA contributes to the developmental condition and how much is due to environmental exposures remains a subject of much debate." (http://time.com/4956316/how-much-of-autism-is-genetic/)

Recently JAMA reported that genetics likely accounts for around 83% of the autism spectrum disorder and environmental factors probably contribute around 17% to the risk of developing autism. (https://jamanetwork.com/journals/jama/fullarticle/2654804)

"There is no doubt that environmental factors interact with genetics to cause various types of autism. But recent studies make it clear that the environmental factors are, in general, both subtle and complex. According to the National Institutes of Environmental Health Sciences, certain environmental exposures may increase the risk of autism, but they are not known to actually cause autism. "(https://www.verywellhealth.com/genetics-and-autism-4153365)

The environmental exposures include: (https://www.verywellhealth.com/genetics-and-autism-4153365)

• Advanced parental age at time of conception
• Prenatal exposure to air pollution
• Maternal obesity or diabetes
• Extreme prematurity and very low birth weight
• Any birth difficulty leading to periods of prenatal oxygen deprivation to the baby’s brain
• Prenatal exposure to certain pesticides
• Prenatal exposure to Valproate or Thalidomide
• Lack of prenatal nutrition
• 
  

BTW, I am not a pediatrician or geneticist, but just want to provide some information here.