It is all about minds. Life is interesting to be sure. But life is everywhere and it does not necessarily rise to the uniqueness that is humanity. Humanity is life to be sure. But it is life combined with identity. A mind. And it is the mind that raises humanity about a cluster of cells. Humanity is worth defending. Or at least that is what our human view of life concludes.
So the question becomes when does a fetus become part of humanity? When does the mind arise from the cluster of cells that is quickly becoming part of humanity? When should it be defended? And when should it be considered (sorry for the seeming unemotional connection of this phrase) a parasitic attachment to the woman that she is free to discard?
Neurologically speaking we know that a mind cannot arise from a brain until the Cerebrum is formed and functional. This does not occur until some time in the third trimester. But the existence of a Cerebrum does not mean a mind is present and active. It takes time and accumulation of experience before the processing of the brain creates enough interconnections to begin to fathom self and become aware. The Cerebrum is just the minimum biological requirement. But the brain is not just parts. It is parts in action collecting information.
So we are a bit fuzzy as to when exactly the mind arises from the brain. A erring on the side of caution suggests we pin it to when the Cerebrum becomes capable of performing the actions that can give rise to a mind. However in my research I have come across mention of the fact that the fetus' connection to the womb via the umbilical cord introduces various chems that effectively neutralize that higher functions of the brain (read Cerebrum) until such time as the cord is severed. My research on this factor is sketchy at the moment. But if it is true then there is not a mind present in a fetus until at least the moment the umbilical cord is cut. Ironically the Jews and early Christians may have had it right when they suggested that the soul (a possible equivalent of the mind) entered the body with the first breath.
My current position is that on or about the third trimester optional abortions should be strongly discouraged. If a medical condition requires an abortion then it should still proceed. And if it turns out that there is no mind present until the cord is cut then it would imply that no person is being harmed even until the moment of birth if it is terminated.
All of this is of course extremely emotionally difficult to deal with. The mind does not deal with truth as much as it deals with emotions, images, and perception. Anything that looks like a baby will be perceived and emotionally related to as a baby. I myself would find it heart wrenching to perform an abortion on a late term pregnancy. It looks so much like a person that my mind would reel at the idea of it. But our brains can when necessary turn to reason and logic and try to understand what is really going on rather than simply relying on how we feel about a thing. Our nature is to react emotionally. But we can learn to try to discern the truth in even the most overwhelming of emotional experiences.