There's a huge range of chromosomal anomalies. You don't see the vast majority of them because the pregnancy self terminates. It's something the human body is already doing.

People with down syndrome are great people who live rich lives. But along with developmental disabilities they suffer from a great many health problems and have severely shortened life spans. Perhaps the future is such therapies will be able to initially focus on these secondary effects.

I don't think methods of preventing chromosomal anomalies are eugenics, since such anomalies are already not inheritable.