There's still no definitive answer as to what causes it...lots of theories and suspicions and even some old wives tales floating about but nothing definitive...this little paragraph sums it up
"Experts are still uncertain about all the causes of autism. In all likelihood, there are multiple causes rather than just one. It appears to be that a number of different circumstances including environmental, biological, and genetic factors set the stage for autism and make a child more likely to have the disorder"
https://www.google.co.uk/amp/www.dailymail.co.uk/sciencetech/article-4534