Exactly the difficult with the "life is sacred" slogan. If a choice has to be made between lives, how does one carry it out unless there is some way of balancing lives against each other? On the other hand, one can see the logic of leaving it to God, if one believes in him, since how can we play one life against another? As the Talmud puts it when considering whether one person should be sacrificed for someone else, how do we know whose blood is redder? I think that means it is not possible to say that one person's life is more significant than someone else's.

For a consequentialist, though, this just looks like a decision not to enter into a calculation at all when there are instances where this should be done. Triage is based on the idea of the efficient use of limited resources, and it seems right to treat a patient who looks as though she might recover as compared perhaps with someone who looks as though they will not. It might still be argued that life is sacred, but interpreted in such a way that we would allow intervention based on distinctions about whose life in a particular situation should be saved.

The answer to your question depends in part on whether and how the moral status of a fetus differs from the moral status of an infant. One might reasonably think, for example, that there is no significant difference in moral status between a fetus the day before its born and an infant the day after its born. The only difference seems to be one of residence: one resides inside a uterus, and the other resides in a hospital's maternity ward. And we don't normally think that where one lives is especially relevant to one's moral status. Thus, if it's wrong to kill one, it must be wrong to kill the other. That said, one might also reasonably think that there is a significant difference in moral status between an embryo that is only two weeks old--a cluster of cells that looks something like this--and a newborn baby. And if there is a significant difference in moral status, then it is perfectly consistent on one hand to defend the right to have early-term abortions but on the other to oppose infanticide.

Of course, questions about the moral status of an embryo or fetus are in many ways the locus of ethical questions about abortion in general. (They do not play as central a role in the legal debates about whether laws banning abortion are constitutional.) Other questions might also be relevant to the ethical debate, though. Some philosophers--most notably Judith Jarvis Thomson--have argued that abortion is not always wrong even if the fetus has the same moral status as an infant. These philosophers often appeal to a woman's reproductive rights--her right to control her own body and to control whether or not she will become a mother--in order to defend the right to abortion. Thomson's rights-based defense of abortion is available online here.

The view that Matthew articulates--that the moment of birth is not morally significant in a way affects deeply the moral status of the newborn infant--is a popular one, but it has been challenged by some. For example, the feminist philosopher Mary Anne Warren argues that birth is morally significant in virtue of the newborn's expanded social relationships.

To say that newborns have a different moral status than nearly-borns does not mean that late-term fetuses ought not to be protected from harm -- but Warren's line of thought might provide a philosophical basis for concluding that newborn infants and late-term fetuses are not entitled to exactly the same degree of legal protection.

To answer your question more directly: If there is a substantive question about whether newborns and late-term fetuses have significantly different moral status, there is even a stronger case to be made that there are big differences in the moral status between newborns and fetuses that can be legally aborted. If this is correct, killing a newborn and using abortion to kill, say, a first-trimester fetus may well be morally different.

Finally, for more on Warren's view see her essay "The Moral Signficance of Birth."

There need be nothing inconsistent about this position. The first view, that abortion is morally impermissible, is a moral or ethical view. The second view, that each woman should be permitted to choose for herself whether to have an abortion, is a political view, one about what laws a state ought to have. The combination is therefore consistent so long as one denies that, if it is morally impermissible to do A, then it ought to be illegal to do A.

Why might one deny that claim? One might well ask why one should endorse it, but there is a better answer. Suppose one believed the following:

1. A law must be justifiable on the basis of principles that cannot reasonably be rejected by any citizen, where a rejection is "unreasonable" if it is flatly irrational or, more interestingly, based upon too many particulars of one's actual situation.
2. In particular, religious doctrine cannot figure in the justification of any law, since one's finding the appeal to any religious doctrine convincing depends too strongly upon one's actual situation, namely, one's particular religious affiliation.
3. In short: The brute fact of religious pluralism invalidates appeals to religion in the "sphere of public reason".
   

That's roughly the sort of view that is defended in John Rawls's A Theory of Justice, though I've adopted some terminology from Tim Scanlon in formulating the view (and so brought it closer, probably, to Rawls's later view, in Political Liberalism). The underlying idea, as applied to religion, is traceable to John Locke's Letter Concerning Toleration and Two Treatises on Civil Government. The latter is essentially the basis for the United States Constitution. (Indeed, large parts of the Declaration of Independence are based, fairly directly, upon the Second Treatise.) Indeed, one might regard (3) as a way of understanding both the basis and the content of the Establishment Clause.

If one held this sort of political view and also based one's moral objection to abortion on some religious ground—say, one's argument depends essentially upon the claim that life begins at conception, and one takes the ground for this claim to be "revealed truth"—then the combination of views mentioned is not only consistent but required.

There are many excellent philosophical discussions of abortion, and many of these do tie the question to general moral issues. One classic article is Judith Jarvis Thomson's "A Defense of Abortion", Philosophy and Public Affairs 1 (1971), pp. 47--66. Thomson's argument begins, contrary to what public political discussion might lead one to expect, by granting that the fetus has all the rights of a person. She argues that abortion is nonetheless justified because it involves a conflict of rights.

Her central example goes like this. Suppose you were kidnapped by music lovers and connected via tubes and wires to a famous violinist whose life now depends upon your remaining connected to him. If you remove the tubes, he dies; if you remain connected for nine months, you both live. Do you have a moral duty not to remove the tubes? Thomson grants that it would be very nice of you not to do so, but, intuitively, you have no such moral duty. The example is meant to be analogous to cases of rape. Here's a variation that's no longer science fiction. Suppose a woman were kidnapped and, while she was asleep, embryos were implanted in her uterus. Does she have a moral duty to carry them to term? If you don't have such a duty in the case of the violinist, how does the woman have such a duty in this case?

To extend the argument beyond cases of rape, Thomson offers other examples. There are large questions about responsiblity that arise here. Many people think that, in cases that do not involve rape, the woman bears a certain responsibility for the life of the fetus that is morally relevant. It's not an unreasonable view that there is some moral difference between the cases. Whether it is enough of a difference to make abortion morally impermissible except in rape cases is another question.

Thomson's paper is collected in several different places, including in Joel Feinberg's collection The Problem of Abortion, where you will find some critical discussion.

A much cited, more recent paper is Don Marquis's "Why Abortion is Immoral", Journal of Philosophy 86 (1989), pp. 183--202. There are two replies in the May 1990 issue of the Journal of Philosophy. Walter Sinnot-Armstrong's paper "You Can't Lose What You Ain't Never Had", Philosophical Studies 96 (1999), pp. 59--72, seems to me a balanced discussion that ends up being critical.

Why hasn't the philosophical discussion had much effect on the public debate? I expect the reason is very simple: Abortion tends to generate strong emotions, and where there are strong emotions, reason stands little chance of a hearing.