"Otherwise healthy" is doing a lot of heavy lifting, there.
Any pregnancy can result in the birth of a baby with disabilities. (Any pregnancy can also end in a mother having disabilities she didn't have before, and not enough is talked about that.) But its reasonable for every pregnant woman to assess the risks ahead of her, and take action accordingly. I mean, there's tons of threads on MN of the "should I have a third DC?", with the answers being at the "can you fit them all in the car?" level. People have children - or don't have children - for all kinds of reasons, some big, some small.
There's lots of reasons that women dont want to have a disabled child that aren't to do with the worth, or otherwise, of the child, and more to do with their own circumstances. I am an older mother - an "elderly primagravida", in the wildly flattering medical lingo - and had a discussion about what I would do if my DS's nuchal fold test came back positive. My DS was my first, so we would have kept him, no hesitation at all. But if I'd become pregnant a second time, and the nuchal fold test came back positive, I'd have aborted, because I would not have wanted to leave my first DC with the responsibility for a disabled sibling after I'd gone, and because I would not have been able to make financial provision for two kids in the same way, which matters if one is unlikely ever to be able to support themselves financially. Other women may make completely different decisions based on their own circumstances, and that's fine, too.
Every child a wanted child. (And I'd say that most of the pregnancies that end up in late term abortions are wanted, even so, which is incredibly hard on the pregnant women who are put in the position of making the best choice for themselves and their families that they can, under difficult circumstances.)