defected fetus
A person born with a severe disability still will, at some point of his life and quite early if I may say (not more than 20 years needed), obtain an ability to simply kill themselves. If living as a cripple was oh so terrible as you make it sound to be, then we wouldn't even be seeing cripples doing what they can to endure and live however they can, now would we? Instead, from war veterans to people who had a freak work-related accident that had made them cripples, will often times choose to live with it, coping however they can, and even go out of their way to live their lives to the fullest (from their perspective).
And what is so terrible in allowing such child to be born? Yes, a world of pain and suffering awaits them, but over time - decades if need be - they may yet discover something to give them sufficient comfort and enjoyment in life. Alternatively, if they fail to do so, well they can simply kill themselves (and damn themselves in the process, but that's another topic). An average human lifespan is around 80 years, and like I said 20 years is enough for such people to make a choice: end it all, or endure. You want to deprive them of said choice simply because it is convenient for you to do so, or you subscribe to what is essentially a "projected morality", where your beliefs & experiences you deem as absolute and needing to be applied to everyone else.