How does the position that, from conception on, humans have a right not to be killed mesh with Mormon theology? At first glance, you'd think that because Mormons are strongly and almost uniformly pro-life, (both culturally and as a mater of doctrine) this conception-is-the-beginning position would mesh well. However, Mormon doctrine, which allows for abortion in cases of incest and rape, is inconsistent with life beginning at conception. Also, Mormons are allowed to use the pill.
Ponnuru argues that drawing any line after conception leads to a strange, unworkable duality. While specifically addressing the argument that humans acquire the right not to die when they acquire some higher level of brain function, he writes:
First: By treating human organisms and "persons" as separate, though mostly overlapping, categories, it assumes that a distinction can be made between a person and the body that person merely "inhabits." The "person" is an aware, conscious "self" that floats above the body, as a sort of ghost in the machine. An embryonic (and fetal, and infant) body comes into existence before this person does, and the person can die before the body does. But this dualism is untenable. It contradicts everyday experience: We sense and perceive, which are clearly bodily actions, but also engage in conceptual thinking, which cannot be reduced to bodily actions; and it is clearly the same subject who does both types of things. The dualist who utters his idea refutes in the act of voicing it. We are (among other things) our bodies. p.86
I guess you could say he recognizes some separation when he says "engaging in conceptual thinking . . . cannot be reduced to bodily actions." So we are more than just our bodies, but also our bodies. Still it seems to me that Ponnuru decries the idea that we are somehow separable from our bodies, and that is exactly what Mormons believe: body and spirit are separate (duality). Our spirit enters our body at (or probably some time before) birth and exit our bodies at death.
Accepting this duality, Mormons can then draw a principled line after conception. The question for a Mormon is not: when does "biological" life begin, but instead when does the spirit enter the body? Drawing the line there means Mormons can allow for some abortion before the spirit enters the body, for instance in cases of rape and incest. It also means Mormons can support embryo destructive research if the embryo is destroyed before the spirit enters the body.
Still there are problems with this line. For instance, if it's okay to kill an embryo before the spirit enters, why is it not okay to have an abortion for other reasons, (like birth control) before the spirit enters the body? Also when does the spirit enter the body?
There may be an answer to the first question. Mormons believe that the we come to earth to get a body. Clearly what is forming inside the mother at conception is a body, so perhaps there is still some moral reason not to disrupt this process or destroy this body even though it doesn't contain a spirit. Perhaps the reason that abortion is "like murder" , but not actually murder, is because abortion destroys the forming body, which is clearly an important part of our souls (body and spirit together). if we are prohibiting abortion for this lesser reason, then we might also permit abortion in cases where there is compelling reason, like to find a cure for disease or to restore choice to a woman who was raped. But if this is the case, what is the compelling reason for allowing abortion in the case of incest? The mother was not denied her initial choices. Or what is the reason for allowing Mormons to use the pill? Convenient birth control doesn't seem like a great reason to allow for even pre-spirit abortions.
And the second question is harder. If we suppose that Mormons permit abortion in cases of rape and incest because the abortion happens before the spirit enters the body, Mormons should still prohibited abortion after the spirit enters the body. So when exactly does that happen? Mormons doctrine allows women who fit within one of the exceptions to make the decision to have an abortion based on their own revelation. Not exactly a bright line.