Older one I may have linked to before:
Between them, current science seems to say that until 29 weeks the fetus lacks the physical brain connections that correlate with consciousness, and they don't really start functioning until 33 weeks. Even then, the fetus is in a state of constant sedation, from various chemicals plus low blood oxygen, and "wakes up" at the first time with birth. The idea that in a world of gray areas, birth is a rare natural sharp dividing line seems to have real merit, not just convenience, allowing principled distinction between even late abortion and infanticide even if you want to discount the mother's autonomy and focus on the fetus/baby.
In a way, the 'soul' really does enter with the first breath.
(From my limited observations, that's still not much of a soul. On observing a friend's 2 month old, slightly pre-term baby, I quipped that he was barely an animal, let alone human, yet. Nurse shit cry sleep repeat.)
See the DW comments at http://mindstalk.dreamwidth.org/333425.html#comments