Two comments worth highlighting. The first, from Warren Terra:
I have heard of couples who choose to adopt or to use donor sperm because they know they carry the same recessive, and I've heard of Huntingtin's carriers who choose not to have children (Huntingtin's is that rare thing, a dominant-lethal, and is not lost from the gene pool because it causes lethality long after the onset of fertility). Technically, these people are making 'eugenic' decisions.
On the other side of the argument, Downs and other spontaneous defects often aren't inheritable genetic conditions, and so aborting affected fetuses technically isn't 'eugenics'. Curiously, aborting a Tay-Sachs homozygous fetus also isn't 'eugenics', as this victim of inherited disease cannot reach adulthood and thus will not transmit the condition to the next generation (aborting a Tay-Sachs heterozygous fetus, which is not an event I've heard of, would be 'eugenics').
The point is that in the context of a political/ideological debate, 'eugenics' doesn't mean exerting decisions about your own reproduction on the basis of knowledge that your offspring will suffer from serious disabilities. It means a massive, usually state-led, movement to discriminate children, teenagers, and adults, and to sterilize and murder those of whom it disapproves. Genetics - and even the technical meaning of 'eugenics' - has little or nothing to do with it.
Allegations of 'eugenics' are just another Godwin's law violation, and deserved to be treated as such.
And this one, from Hob:
the essence of Douthat's argument is that progressives are in favor of access to abortion, and abortion can be used for eugenic purposes, therefore progressives are in favor of eugenics. This is ridiculous for reasons that have nothing to do with the motives behind particular abortions. It's like saying that if you oppose banning guns, you're in favor of bank robbery, hunting bunny rabbits, and suicide.
Also in that thread, Kathy G., who used to work for the developmentally disabled, gives a more sober and informed picture of Down Syndrome than I did. Well worth reading.