The Moral Personhood of Twins
The other thing is that, though I'm not sure when during pregnancy ensoulment would happen, there is a point where I can't imagine it would have happened yet - post-fertilization and pre-implantation. At that point, not only don't you have the least trace of a nervous system, you also still have the possibility of twinning. That you could have a unique person, already possessed of an individual nature and human rights, who could turn into two people tomorrow, seems to me improbable.
I've heard similar concerns about twinning before. As I said in my earlier post, I would put the "personhood" line much farther along. But I don't see twinning as a useful argument for rebutting this earlier line -- largely, I think, because of what I conceive "personhood" to consist in.
In his book Reasons and Persons, philosopher Derek Parfit discusses several thought experiments in which a matter transporter a la "The Fly" is invented, and then rigged to beam a person to two places at once. He then asks what happens to the original person's identity -- is one of the beam-ees the "real" person and the other a copy? Are they both copies and the original has been destroyed? His conclusion (such as it is -- Parfit is much more inclined to raise interesting issues than to offer clear solutions), which I agree with, is that each of the beam-ees shares the pre-beaming identity, while being two separate persons now. Our language just has trouble wrapping around the situation because we haven't ever had to deal with this kind of identity-branching in the case of humans.
Presumably nobody would say that the beaming should lead us to conclude that the pre-beamed person was not, morally speaking, a "person" after all. They were a person, with all the rights etc. that entails, that branched into two people, each with all rights etc. But if we can say that, then the possibility of twinning in the womb would be equivalent, and therefore couldn't disprove the pre-twinning personhood of the fetus.
I think the idea that twinning presents a problem for moral personhood is rooted in the idea of the soul as a separate, unique object fastened onto a human body -- a sort of metaphysical RFID chip. This is a common assumption. It's what allows us to talk of souls switching bodies or leaving the body altogether. Some people go so far as to imagine that there's a fixed quantity of such souls in existence, warehoused in heaven somewhere, and we have a duty to crank out more babies so that God gets to use them all*. This sort of conception does pose a problem in cases of twinning or Parfit's matter transporter. But if instead we see the soul (or just moral personhood, if you think retaining the traditional religious terminology will make it harder to change the conception) as an emergent property of the body and mind's functioning, then person-branching (and even perhaps person-merging) does not present an especially difficult metaphysical problem.
* I was once planning to write a SF story about an epidemic of soulless babies (biologically alive, but inert and vegetable-like) that occurred because overpopulation had led the universe to run out of souls. If nobody has written something like this, feel free to steal the idea.