I'm not sure what you're referring to with the transgender reference—any medical interventions trans people have are initiated by the trans person, making it completely unlike eugenics.
Edit: I should probably add that eugenics was widely popular less than a century ago in the U.S. and hasn't gone away completely. I think these examples are poor, however.
I guess I don't understand your comment; what does natural selection have to do with it?
I'm making no statements here except that the abortion of Down-positive children is in fact eugenics. Parent's comment was asking specifically how abortion is "mass eugenics." Maybe saying abortion in toto is "mass eugenics" is overly broad, but certainly it's used for eugenic purposes--Down Syndrome being one such case.