I think abortion is generally unethical, but conditions like Down syndrome may be exceptions. Why bring someone into the world who will only be a burden? Especially if they're going to live their entire lives with difficulty and suffering. Down's syndrome carriers in particular seem to be happy enough though, maybe? And maybe living a life dependent on others is no worse than being self-supporting because by not participating in the workforce you're doing less damage to the earth?
But still, back to the difficulty, suffering, being picked on, etc. associated with some conditions. It seems unethical to bring a child into the world in those cases, and not to mention that it would be a terrible burden on the parents.
On the other hand, most people suffer more than they are happy, a good proportion of people would rather not be living, so if abortion is ethical for disabilities maybe it's also ethical in the general case? Maybe it's even unethical not to abort?
Also, we would never even consider killing an infant, toddler, or whatever because they have a disability, so what's the difference if they're a fetus? They're still a living human; they're just not out of the womb yet. And yes, it depends on how far along in the pregnancy they are, but there's no definitive way to say at what point along the gestation it's no longer "okay" to abort, so it's best to err on he safe side, just like you wouldn't shoot a gun at someone if you weren't sure whether it's loaded. On this grounds you could argue that it's never "okay" to abort no matter what the condition. But my moral feeling is that it's justifiable, maybe even ethically preferable, to abort someone with a seriously debilitating genetic condition or birth defect. And feelings are often better-informed than rationality. And what are right and wrong ultimately based on if not feelings anyway?
Another thing to consider is whether people/spirits choose to be born into a certain context before they're born. If a being chose to be born knowing that he'd be disabled but knowing that the challenge would benefit him in some way, then it's definitely not very nice to abort him/her. On the other hand if we choose the context we're born into then the being must have known he/she'd likely be aborted when that's the case, so maybe that's what he/she is actually banking on? It's hard to say how much we really know about the life we'll be born into before we're born on whether we even choose to be born. Also, in one of Neale Donald Walsch's books in the dialog series, God says that it's lacking in compassion for people to have children in some particular country where they're very poor and can't afford to feed their children.. for him to say that implies that it's not always the case that the being chooses to be born knowing ahead-of-time the context they'll be born into and that it's in their best interest. (I know you think all this talk about spirits and conversations with God is bullshit, but I don't want to censor half of my opinion.)