I was helping out with a local group here once and also just other friends of friends stories. So most of the background I've heard is a combination of Mum's having certain medical conditions like low thyroid, and traits running in both families
and
different stresses during pregnancy
so my neighbour's brother's son, the Mum walked a lot in summer, and the doctor's warned her not to;
acquaintance who had a son, where she worked long hours in their business;
stress but feeling that the cause was her son's vaccinations.

I don't know if there's much to these theories though. imho it's more genetic and the stress might bring the autism traits out a lot more??