a revulsion for the practice of eugenics
Really? Wow. You and I are just miles apart, then. IMHO people get an emotional response to eugenics that defies rational explanation. So long as it's not compulsory, and so long as there is sufficient genetic diversity, I hope and believe that eugenics will have a place in our future society.
As for why I believe it's not yet a baby, that's hard to explain. Why would you believe it is? At what point do you believe life begins? For me, it utterly defies both logic and common sense to believe that it starts at conception. At some point later, the baby emerges from the womb, and it's equally unrealistic to claim that it's not alive at that point. Somewhere during those 9 or so months, there must have been a point[1] where it changed from not alive to alive. Not a baby to baby. Where is that? I can't give you a definitive answer, and it will vary on a case by case basis.
[1] Actually, I don't believe there's a single point. I see it more as a gradual transition over a period of time. Which is in part why the question is so hard to answer.