[Y]ou find birth a comfortable point [for granting the legal right to life] at least in great measure because you put the point of personation at such a late point in development. Even a lot of pro-choice people would put it at least a bit before birth; and most people would be extremely uncomfortable putting it in 'later infancy'.
Nobody wants to sanction infanticide, of course, and we all agree it's bad to inflict pain on any sentient being; but I wonder whether it's really so unusual to (after reflection) conclude that a baby only becomes a person in later infancy. I will assume, as per my recent post 'Evaluating Life and Death', that personhood tracks whether one is apt to be harmed by an early death. So a newborn is a person iff it is (apt to be) harmed by an early death. Do most people really think babies are harmed by death?
Now, it's a tragedy for the parents when a newborn baby dies. So to prevent this from confounding our intuitions, consider an orphanage. Suppose one day all the newborn babies in an orphanage spontaneously (and painlessly) disintegrate. Assume nobody else cares. Has any harm occurred? Does this make the world a worse place? It doesn't seem so to me. (But maybe I've simply internalized my theory too well. So let me ask: what's your intuition?)
If the example instead involved, say, four year old children, then it would be obvious that their deaths harmed them (assuming they otherwise would have grown up to live decent lives). This is because four year olds are obviously people, who own their futures and may suffer the loss of them. But in the case of newborns, with no self-conception, and no hopes or dreams for the future, it seems equally obvious that the opposite is true. Disintegration makes no difference to them; it doesn't harm them; there's no loss. What this indicates is that they are not yet people with futures of their own.
Of course, that's not to say that infanticide is okay. We've all internalized the rule that extends a right to life at least to the point of birth, and that's for the best. It's a good rule, and so an appropriate component of our practical morality, which determines right and wrong. But that shouldn't stop us from recognizing that the true point of personation is in fact a fair bit later.