Several people in comments of previous posts assert that they are comfortable with both defining a fetus as human and also supporting the right to abortion on demand. As I've said, I vigorously defend abortion rights. I could not however endorse abortion if I believed a fetus to be human.
Humans have human rights, and the most basic right-- the foundational right-- is the right to live. I don't think that the state or any individual has the right to take the life of another person. (I am sympathetic to merciful euthanasia, but of course in that situation there is the informed and free given consent of the person who wishes to die.) Yes, it is a terrible burden to ask women to undergo pregnancy against their wishes. People should indeed have the right to control their own bodies. But I cannot imagine that this right trumps the right to live. We mostly like to pretend that our rights are absolutes, but sadly real human life dictates that at times, they run up against each other. At these times, we invent a hierarchy as best we can. And as horrific as a forced pregnancy seems to be, it doesn't seem right to me that it would be worse than ending life. Look, say we're talking about a female fetus, and suppose for reasons of argument I grant that a fetus is human. When that fetus grows into a woman, she could become pregnant and desire an abortion. Her right to acquire that abortion seems compelling. But could it be more compelling than a right to life that allowed her to grow to that age in the first place? No, I simply can't reconcile support for abortion with belief that a fetus is human. That maybe logically unsatisfying; I guess it's just a first principles thing.
Again, though, I believe that a fetus is not human. This interpretation is supposedly susceptible to the complaint that defining life as beginning at birth is arbitrary. (This is a principle line of argumentation in Ramesh Ponnuru's book.) Maybe so. The law, however, is a series of arbitrary decisions, as I've said before. Steal a certain dollar amount of merchandise, and you might receive a misdemeanor. Steal just about that amount, and you might receive a felony. If someone has sex with a girl a day after her eighteenth birthday, there are no consequences. If that same person has sex with her the day before her eighteenth birthday, technically, he could be subject to imprisonment and a lifetime on the sex offender registry.
Decisions we make about law and rights seem arbitrary because we've got to muddle through in a world that is remarkably resistant to bright-line rules and easy interpretation. As far as things go, I find defining the beginning of life at birth to be remarkably sensible. And though traditionalism is never the last word on things, I think there's a lot of evidence that we have traditionally defined the beginning of life at birth. There are also many pragmatic advantages to that, due to the difficulty of understanding when conception happens, the (still) relatively high chance of miscarriage for any particular pregnancy, incompatible philosophical and moral arguments about viability, or the soul, or sufficient resemblance to an adult human.... Muddy waters, of course, don't necessitate adhering to any particular position on the issue. Someone could come up with many reasons to support the vision of life beginning at conception. Ultimately, however, I think most people are comfortable regarding the fetus as non-human, and while appeals to democracy are never dispositive, they are sort of all we have, when we argue public policy.
As an aside, the "you don't understand because you are not a woman" argument doesn't thrill me, I'm afraid. It's no more moving than, say, "you don't have a right to argue about tariffs because you are not in the manufacturing business", etc. Democracy requires a certain degree of false universalism among the citizenry. That's imperfect, but is I'm afraid an inevitable and necessary consequence of rule by the people.