There are a few shades to the conversation:
1) The right of a mother to terminate a pregnancy based on early detection of Down's
2) Whether it's good that abortions lead to a decline in the incidence of Down's
3) Whether it's good to believe (generally) that people with Down's should (always) be aborted/shouldn't be born
You can have a gamut of opinions across all these different topics. I don't think there's an obviously right answer (I say "yes" to 1, but it's not about Down's but instead about not controlling women's bodies/family planning decisions), just answers that we can decide are best depending on what our priorities are. Different individuals - people with Down's, mothers of people with Down's, the rest of their families and friends and acquaintances loved ones - have different experiences that make it almost impossible for there to be a universally correct reason one way or the other.
But if one doesn't understand the embryo/fetus to constitute a human life in the first place, then what basis is there for rejecting 1, 2, or 3?
For example, with point 2), the only countervailing consideration I can think of against a reduction in the incidence of Down's Syndrome is that it would result in the loss of human life. That is, if I told you that we could reduce the incidence of Down Syndrome by killing every mother who a pre-natal screen showed was about to have a baby with Down Syndrome, then such a solution would obviously be rejected because the benefit of reducing the incidence of abortion is clearly outweighed by the loss of human life. But what if I told you that, no strings attached, we could reduce the incidence of Down Syndrome long-term without the loss of human life? I suppose there's an argument to made that every life is valuable and that those with Down Syndrome contribute to the diversity of human character, genetics, or whatever word you'd prefer, but that point usually comes up only when talking about people who
already have Down Syndrome. If I told you we could flip a switch and those with Down Syndrome would no longer be born, I believe the vast majority would accept such a proposal if there was no loss of human life. Well, for the pro-choicer, there is no loss of human life.
Similar logic applies to the other two points. If the question of life or death is not even part of an equation, then what rationale is there for continuing to propagate a chromosomal disorder? I'm not being entirely rhetorical here; there may well be something I've overlooked that strongly incentivizes the continued birth of babies with Down Syndrome. But until I'm made aware of such an oversight, I maintain that for those who are already pro-choice because they don't believe embryos/fetuses to be human, the mere fact that embryos and fetuses with Down Syndrome are now being aborted due to pre-natal screens presents no moral dilemma at all.