Researchers led by Andrey Rzhetsky of the University of Chicago scoured the insurance claims of roughly one third of the U.S. population looking for incidences of ASD. They compared those rates, county by county across the country, with the rate of birth defects in the reproductive systems of baby boys. The idea was that the birth malformations would indicate whether the parents had been exposed to environmental toxins like pesticides.
Sure enough, regions with higher rates of congenital malformations also had higher rates of ASD. For every 1% increase in birth defects there was a 283% higher incidence of ASD. This suggests that whatever is causing the increase in birth defects is also causing ASD.
To be clear, this is no more than an association. We don’t know for sure that environmental toxins are responsible for causing ASD, let alone which toxins. It is interesting that the male genetic structures form in the fetus at about the same time as neurons appear within the developing brain. Thus, exposure at that time could affect both or either developing organs.
If there is a an environmental factor causing ASD, it is felt prenatally, and perhaps even before conception (if the parents' germ cells were affected). By the time the baby is born, the damage has already been done.