I discussed this with a MCHN once. I honestly think that some babies are just born with it through no fault of their own or due to anything their mother did during the pg or labour etc - they are just 'wired' differently. some aquire it and some is from it being heriditary. I agree with Kirsty, that I don't think it has become an epidemic, but rather what typically defines the nature of ASD's has become more structured and tested - meaning it isn't just a stab in the dark anymore with diagnosis.

I wonder how many people do have some form of autism or other disability that was never diagnosed.
This is mainly what the MCHN and I were talking about, that it isn't that autism has become the new trendy or latest fad in medical conditions, but there is a better chance of a diagnosis and that in cases where it runs in families, once a child is diagnosed with an ASD, then the actions of another much older family member are put into perspective.

ETA - I know what I've just said doesn't directly related to birth and autism, but it is interesting that the links have been made.