It looks as though we're having something like a Serious Adult Discussion (SAD) about abortion right here in this space, so let me offer my answers to the questions posed (I assume in good faith) by James Davis.
1. When does human life begin? If not at conception, when? And on what scientific grounds, and by whose authority, do you make that determination?
I will stipulate that human life begins at conception. The question, however, is whether such life should have the same right to exist prior to birth as after, and if so, why? This is not a question that biological science can answer.
2. At whatever threshold you choose, what justifies taking a life a minute or so after that threshold?
The taking of such "pre-born life" can be justified on various grounds, including a religious belief that abortion is permissible prior to "ensoulment," which medieval Christian theologians claimed took place at quickening (roughly the end of the first trimester), and Mormons claim to be at implantation. Rabbinic Judaism takes the view that the pregnant woman's life is of greater worth than that of her unborn baby--such that in some (but not all) situations abortion is justified. Similarly, the non-religious justification is that the life, health, wishes, or other circumstances relating to the pregnant woman can outweigh the right to life of the blastocyst/embryo/fetus. This view is undergirded by the widespread intuition that various cirumstances (notably rape, incest, and the life of the mother) justify taking the unborn life. (See here for some recent polling on Americans' views on when abortion is justified.) It's worth noting that Roe v. Wade uses viability as the criterion for determining when the fetus' right to life approaches the mother's. My point is simply this: The legitimacy or illegitimacy of abortion cannot be settled by a determination of when human life begins.
3. Rape and incest are heinous crimes and must be stopped. But does killing a defenseless human fetus balance the scales of justice?
Not if the scales of justice are calibrated to give the fetus (or blastocyst or embryo) equal rights as the mother. If they are calibrated to leave it up to the mother to decide whether to carry such a pregnancy to term, then yes. In the survey noted above, 79 percent of Americans believe abortions should be permitted in cases of rape--a strong indication that they believe the scales of justice should be calibrated in the latter way.
4. Saving the mother’s life has been a solid criterion for decades. However, it was gradually stretched to cover emotional, vocational, financial and other factors as well. If it is reaffirmed this time around, how to prevent another slippery slope?
I'm not sure what James means by "gradually stretched." Under Roe v. Wade (which will be 40 years old next year), abortion is essentially at-will during the first two trimesters, and by far the greatest number of abortions are performed then. If Roe is overturned, then states will be free to legislate as they please on abortion, and presumably some would choose a protocol for determining the circumstances that could permit a woman to obtain an abortion--such as by requiring her to go through a serious interview by skeptical questioners. That might prevent the slippery slope that worries him.
Let me conclude with a word on ctd's tu quoque claim that I was being disingenuous in posing the questions I did. My charge that pro-life politicians and professionals are being disingenuous is that I don't believe they want to put their cards on the table--precisely because they know most Americans strongly disagree with their position. Maybe, as Michael suggests, they're just so zealous that they don't think about the implications of their position. In any event, my questions were posed in good faith, and I'd like to see someone (hey, James!) answer them.