@ buck: moral fall pits in either answer, i can't seem to come out with a definitive answer.
Here's my take.
When you "know" your child is going to have serious disabilities, it'd be mighty selfish to go on & bring her/him into this world.
And it's not about mommy or daddy not being allowed to raise or have kids (adoption, anyone?), it's about thinking ahead what kind of life that child will have. And the question did state "serious" disabilities.
Somewhat related: a week ago or so I was reading some interesting stuff about how many children are born (in Belgium - 100s a year) with disabilities, disfigurements, serious growth issues, etc because of the mother having recurring drug problems / (a)busing drugs during the pregnancy.
Not only are they a (financial) burden on society, but those kids will also never, ever have a serious or normal life. A good part will never grow old, those that do will most likely end up miserable. Fun for them!
So now they're starting to raise the question whether or not we should prohibit those people of having children. Yes, obviously.
Now, I know abusing drugs and having a genetic, involuntary problem is not the same thing, but here's the kicker: Looking at it from the kid's perspective, the reason why you "came out messed up" is completely irrelevant, isn't it?