More anecdotes - out of 30 children in my daughters class (born 2007) 1 has something which I assume is autism (haven't asked about parents thoughts about causes). 1 has Downs Syndrome. I know of no other children of any of my friends, neighbours or colleagues with autism. I have a friend with a child with Cerebral Palsy.
Taxi - your number of 8 autistic children needs to be put in context. How many children total would constitute your sample? How can you expect any extrapolation without that number. And even so I would say it's likely to be a biased sample so not a good point to extrapolate from.