It depends... Are we speaking about keeping only tall blonde kids? Or aborting a fetus with 95% Down syndrome? Angelman's? Some other even worse? Stopping a possibility fatal pregnancy? Where do you put the line?
Good point well made. In the UK Down's syndrome is now commonly screened for with an option for the parents to abort and there's very few people that see a problem with that, is that eugenics?
Most people would agree "I only want a child with blonde hair" is too far, but as for preventing suffering, as you say, it's a tough line to draw.