While I believe that an unborn baby probably is a person, I think that this actually weakens the position of the Pro-life argument rather than strengthens it. It plays into the hands of abortion advocates because it enters into ambiguous territory, where the waters are murky, when there is no need to do so.
The standard for what constitutes personhood is not a clear-cut issue and some conceptions of personhood wouldn't include a fertilized embryo. Because 'personhood' is a disputed concept which can be interpreted in a variety of ways--many of which include appealing to features that a 1 day old fetus doesn't posses, such as consciousness--submitting to 'personhood' as the proper standard for determining whether a being is worthy of legal protection against murder is an unnecessary concession on the part of the Anti-abortion position. It opens a door for the Pro-abortion position that should remain closed.
Alternatively, the standards for defining what constitutes a unique human life are unequivocal and a matter of scientific fact. If it is never acceptable to destroy an innocent, unique human life then abortion is murder. And a fertilized embryo, at the moment of conception, is all of those things factually: unique (it has its own DNA), innocent (clearly), human (product of the human reproductive process) and life (biologically). This standard affords the pro-abortion position no wiggle room as they literally, as a matter of fact, can't deny any of the descriptors that have been applied to the fetus. Therefore they are forced to concede that they think it is acceptable to forcibly destroy a unique, innocent human life--which any sane, moral person should loathe to admit.
Framing the debate in this way forces proponents of abortion to face just exactly what it is that they are advocating directly, without the vague, murky language of 'personhood' confusing the issue. If an honest person, when confronted with this argument, still wants to say "Yes, it's OK to destroy a unique, innocent human life as long as that life is very young" then I think we have come to a proper impasse. We can now, with a very clear conscience, write off this person's opinion as morally depraved.
This may sound a bit harsh, and I know that for some it is hard to accept that such a contentious issue could possibly be this straightforward with no real grey areas. But I've heard every argument in favor of abortion and none of them serve to refute the logic that I've just laid out.
As for the 'positive' arguments in favor of legal abortion, I can't find any that would be significant given the weight of the argument already laid out:
- The 'right to privacy' cited in Roe is pure judicial invention and is rightly worthy of scorn.
- Arguments that highlight the social benefits of abortion are moot because, even if the social benefits are great, that could also be true for infanticide, euthanasia and even genocide. In any of those cases we recognize that any social benefits derived from mass killings are irrelevant because the individual's right to live trumps all social considerations.
- Focusing on the mother's rights gets you no where because--once we've established that the fetus is a unique, innocent human life--the right to life trumps the 'right to choose'.
- Arguments relating to the mother's health, rape and incest are advanced on behalf of situational ethics and are not arguments in favor of keeping abortion illegal as such, but only arguments that these special considerations must be taken into account when they arise. Duly noted, but these can't be arguments in favor of legal abortion any more than the fact that sometimes killings happen in self-defense is an argument in favor of legal murder.
Slavery used to be a contentious issue in the U.S. Whether the Earth was flat or not used to be a contentious issue. Whether the Earth was the center of the universe used to be hotly contested. I truly believe, and believe that I can thoroughly demonstrate, that the issue of abortion is more qualitatively similar to these types of issues than truly ambiguous issues--such as what makes a 'person'.