Well if there is a connection between Rh- factor and autism, it would make sense that there has been a spike in autism over the past 2 generations, as Rh- females in the developed world have been able to reproduce safely with little to no risk to their offspring, since the introduction of Rhogam. In other words, since 1968, the gene pool has been flooded with more autistic genes than ever before in human history (IF there is indeed a connection). Before medical technology intervened, nature had a built-in method, so to speak, to limit the reproductive capacity of Rh- females. If an Rh- woman, hundreds of years ago, unwittingly reproduced with an Rh+ male, she was doomed to lose some or all of her babies if she became sensitized.
For instance, in my case, were it not for Rhogam (and this doesn't even account for miscarriages I had, which could have sensitized me had I not received Rhogam), while my first 2 children would have been ok (they are Rh-) my third child (Rh+) could have sensitized me, giving my 4th child (Rh+) hemolytic disease. And if I had incurred a fetal death, I probably wouldn't have gone on to have a 5th (she too is Rh-)... and so on. Apply this to countless women over generations and you can see how the Rh- factor limited reproductive capabilities.