I've never really understood the claim that we cannot tell when a fetus is human.
It seems pretty straightforward to me.
First, lets talk about what we mean when we say "human". Why is it murder to kill a human, and not to kill something not-human, say, a fish?
What is it that we should value when we talk about humanity such that it is a crime to harm it?
I think it is our consciousness and our ability to experience pain because of that advanced level of cognition that is (as far as we know) actually unique to being a human being is the thing that we value. Now, there are some interesting argument around this of course, but in this context, I think it is pretty clear what it is that we consider to be relevant - human levels of conscious thinking and potential for suffering.
And we can tell when a fetus starts exhibiting human like brain activity. It can be pretty simply measured. It is around 28 weeks. I am pretty happy with tossing in a margin of error there, and calling it the current SC standard actually rather reasonable. '
I actually reject what I see as Meri's argument (which was Roe's argument I think, that the SC rejected as well) that a womans right to an abortion was absolute. I do think that there is in fact a balance to be struck between the individual rights of the mother, and societies interest in the prenatal health of this human being. And I think the argument that there is no possible way to have the view that abortion should be regulated in good faith, ie, based on concern for the health of this fetus rather than a desire to control women, is simply crazy. I am very content and comfortable saying that I think abortion can be progressively more restricted as the pregnancy progresses, AND that we can and should do a much, much better job providing better heath care and health services to children and potential parents.
Great article on the development of the human brain in vitro:
https://www.nytimes.com/2005/06/19/books/chapters/the-ethical-brain.html