My mother has runs a special needs unit of about 3 classes in a normal government primary school in Western Sydney for about 5 years and has been a teacher of "normal" kids for about 30 years before that. She is convinced that the percentage of special needs kids (autism, downs etc) as compared to "normal" primary school kids is rising due to advances in medical technology. She feels that 30, 40 or 50 years ago a lot of the kids she teaches would have died due to complications at childbirth related to their conditions whereas with better medical technology today more survive. This judgement is just based on her experience only. Whether its true or not I don't know but she has been teaching kids for over 35 years.