Well, for one thing, autism is polygenic and multifactorial (lots of different genes can cause it, and lots of different combinations of genes can cause it)... so you couldn't test for it like you could test for Down syndrome. Even with Down syndrome the test isn't that accurate; with autism, you'd probably have positive results consisting of three or four NT babies for every autistic baby, and you couldn't tell which was which. And with DS, if you have the extra chromosome you'll always end up with DS; with autism, if you have the genes you'll probably only end up with autism about 70-90% of the time--and there's no telling whether you'll be profoundly autistic or be a little-professor Aspie or an artist savant or anything else on the spectrum. Look at identical twins: Yes, both twins are almost always autistic; but the type and severity of autism varies between the twins. There's your environmental influence. You can't predict by a prenatal test what kind of autism a child will have, because it simply isn't determined at that point. It's all in how you develop.
Eugenic abortion is... ugh. It gives me the creeps. I don't care if you're pro-choice or pro-life; the decision that a life isn't worth living when you have a disability, when it would be worth living if you didn't, is just eugenics plain and simple, based on prejudice and bad science.
If you are going to have a child, you should accept the child as they are--brown hair or blonde, athlete, intellectual, poet, or social butterfly, girl or boy, black or white, disabled or not. Once you start saying that some children are more acceptable than others, you are making judgments about the value of human life.
If you say, "Well, I can raise a child, but I'm not ready for a disabled child," then think twice about having that child. Any child could get knocked on the head in a car accident, or break their neck falling downstairs, or just have a disability that isn't obvious until after they're born. Disability is very common. If you aren't prepared to love and raise a disabled child ("prepared" in the sense of "willing to do what it takes" rather than "already an expert", of course)--if you're not prepared to raise a disabled child, you're not prepared to raise any child at all.