I think it begins at conception. At that point it has the possibility of becoming a person, and should be considered a living thing. And as several others have stated, since we don't know I would rather err on the extreme side of caution in this case and say it begins at conception.
Having carried three babies, I think its kind of silly to say the spirit doesn't enter the body until birth. You can feel the baby moving, it responds to your voice and to other loved one's voices, responds to music and sounds, and certainly with my three the exhibited parts of their personality even in-utero.
All of that said, I still support the woman's right to choose until the pregnancy is viable and can survive on its own. I personally could not have an abortion because of my beliefs, but I do not think my religious beliefs should dictate other people's actions who believe differently. They will have to face the consequences of their choices.