As regards the point at which we should accord rights to that which would eventually be a child (an embryo, a fetus, etc.), does someone who argues that a given stage is not sufficiently mature have also to answer the question of which WOULD be the critical stage? Or is it enough to say, "Well, I don't know when this thing becomes a person, but it's not a person at day 1."
I think we can leave aside all the heavy-weather issues about abortion, fetal rights and so on and go for a more general point. It's hard to see why we'd have to have a sharp answer to the question of when something acquires rights or becomes a person, or becomes depressed or becomes fluent in a language or for that matter becomes a tree, or becomes bald... for it to be okay to say: "It's not there yet." In fact, there may not even be a sharp answer to the question "What is the critical stage?" Of course, if someone had a reasonable argument for saying that an embryo is a person from day one, we'd still need to evaluate what they had to say. But they would have to do better than point out that we don't have any way to draw a bright line between person-to-be and full-fledged person.