I think you'll find that pro-choice folks are split (perhaps many different ways) on this question. Personally the way I view the problem is through the lens of minimizing harm to all parties involved -- and as such, I wouldn't say that it's "not wrong" to kill a fetus, just that in most cases it is the lesser harm than forcing a woman to carry an unwanted child and to bear the life long responsibilities that it entails.

In that lens, imagine a scenario where a woman is happy to be pregnant and excited about becoming a parent. Then she learns the child will be male, and she decides to abort it. There's a wide gulf between between the harm of "didn't want to be a parent" and the disappointment of "didn't want a son". It may help to imagine this with even more trivial features: "oh the baby is going to have blue eyes? never mind then...".

For something like trisomy-21, I find myself of mixed mind. On one hand, there are many things that can happen to a child through their life that could end up posing a similar impediment to the child and burden on their parent. On the other hand, since we're able to detect it so early, perhaps it is the best way to minimize harm.