On a separate note, it's worth noting that many jurisdictions make a distinction between early- and late-term abortions. I can't see any reason for this except the idea that human life increases in value steadily as it grows rather than at a specific time. (Well, that combined with the fact that it's usually possible to get an abortion before late-term, and also easier to perform, so there's relatively little reason to have late-term abortions... which raises the interesting question of why people ban them; I wonder what sorts of situations result in people wanting abortions late-term. Finding out about health issues with the mother or fetus?)

Anyway, my main point with saying the above was to note that the use of birth as a set point isn't nearly as common as these sorts of discussions make it out to be.