I have no idea. I just disagree with the pro-life people who say it happens at the moment of conception and the pro-choice people who say it happens at the moment of birth. Both estimates place importance on beings with little resemblance to persons. Innocent though they may be, newborns are not all that distinct from small animals, nor are they even very distinct from one another. They don't even have a genetic signature that makes them hard to replace, since their DNA is randomly recombined from their immediate ancestors. I realize this sounds harsh, because most people are wired to treat babies as precious resources, rather than renewable ones, and infanticide as a particularly heinous crime. Beyond appealing our parental instincts as evolved mammals, I don't see why that should be the case, compared to killing a fully developed adult. From a societal standpoint, adults are more valuable and harder to replace than babies. Potential value is simple to create; actual value is the result of hard work on the part of both the individual and the community, and intuitively should be more morally protected due to its relative scarcity, but the reverse is usually the case.
It's comes down to a fairly simple problem, since most people in this kind of discussion all concede that life is sacred and should be preserved. This leaves them to argue over a working definition of "life", instead of the value of life.
It's a silly semantic argument. The breathing and thinking human begins at conception. The breathing and thinking comes later, but if those first steps are interrupted, there is no thinking or breathing. To soothe feelings, we can pretend the early steps don't qualify as "life," and thus preserve our good feelings about ourselves. No one really wants to condone murder, or be a murderer, so we created special definitions for some actions which end the life of a human being, which spare the killer from social sanctions. We even have a legal verdict of "justifiable homocide," which is legal talk for, "Don't worry about it, he needed to be killed."
If we can justify the killing of a living breathing human, given the proper circumstances, it's plain we can do the same for the breathing human. We simply pull the line a little to the left, and it's okay. If not justifiable, at least excusable.
The rest is just the same emotional response we feel for kitten and puppy videos.