"Would you like your baby to be ravaged by rubella, blind, with impaired cognitive capacity, and partially crippled?" Most parents, offered this choice PRIOR to conception, would of course say "No, I'll take the healthy baby." I can't quite picture anyone, in that context, responding "Oh, but that's unfair, there should be some rubella-ravaged babies in the world TOO. They have a RIGHT to be born, so some parents have to be brave and committed enough to accept them."
Likewise, if parents could select which egg to impregnate, which sperm would win the race to give its peculiar DNA to a new zygote, would prospective mother and father CHOOSE the egg and sperm which would form a zygote exhibiting Down's Syndrome? Not at all. They would carefully select for 23 healthy pairs of chromosomes. It is possible that some of the paid staff of Down's syndrome advocacy organizations might actually object. “Why this unseemly prejudice against a natural condition? These genes have a right to expression in the post-partum population.” That sounds dangerously close to the muddled mental meanderings of Richard Dawkins: selfish genes indeed.
Somehow, when the issue is a post-conception decision whether to terminate a badly damaged pregnancy, a fair number of sincere voices opine “you can't destroy that child.” Only in the most technical biological sense is it a child. There was a time when prospective parents had no way to know, prior to delivery, how healthy the child was. During the past fifty years or more, many means of testing and examining the zygote, blastocyst, embryo, and early stages of fetal development, have been discovered and devised. There are clear, objective standards for assessing whether the tissue from which a child is growing is either badly misconstructed, or severely ravaged. It is only sensible, at this stage, to remove the damaged tissue and start over.
Parents who are firmly committed to the understanding that a new human being exists, with full right to protection, from conception onward, have every legal and moral right to carry such pregnancies to term. In truth, the entire community will bear some of the unavoidable costs of that decision. But very few people would intervene in such a private individual decision, merely because the resulting child might “become a burden to society.”
Some parents sincerely believe that it would be an act of unspeakable cruelty to bring such a child into the world, if it can be prevented early enough in pregnancy. In fact, the number of babies born with Down's Syndrome has been markedly reduced, in the United States, because a tremendous number of parents make precisely that decision. It is a sensible, compassionate, and rational decision, albeit some parents make a very different choice.
Imagine a soul, awaiting attachment to a new human baby, in whatever metaphysical muster zone such souls gather in. This soul observes that the intended mother is carrying a fetus which is badly deformed. The soul cries out in agony, “No mommy, I don't want to attach to that body. I want to live life to my full potential. Take that damaged tissue out and make me a new healthy body.” Nobody knows of course if that is how attachment of the soul works. Nor do we know that it is not. It is a possibility worth considering.
The counter-argument is often framed in reference to a severely disabled child already born. The simple truth is, if the child has been born, then abortion is not an issue for that child. Further, by the time the question is posed in this manner, the child has a name, a history, an identity, a place in a family. Our culture and our laws have always protected babies delivered by live birth. Rightly so. If a person is born with disabilities, they are here, they have to make the best of it, and everyone tries to accommodate the disability to help them make the best of it. But there is nothing good about the disability. If the person concerned could get the disability taken away, most would do so. Disabilities are a pain, they hamper the person who has one, they put an extra strain on their relations with others, however accepting and loving. It is good to cope successfully, even to triumph over disability. It is even better not to have one.