You see, mammalian development is a tricky thing. I've studied it, although I am no expert in it. During the very early stages of Mammalian development, the embryo consists of undifferentiated stem cells. They are called stem cells because from these cells, other more differentiated cells may form (this involves DNA methylation, super coiling, uncoiling and other methods of gene switching and regulation). But there is no dispute that at the very early stages of mammalian development, the embryo consists of nothing more than a couple of hundred undifferentiated stem cells. These stem cells eventually differentiate into more complex tissue layers (germ line, mesenchymal, what have you), but during the early stages they haven't yet.
So killing or destroying an embryo is not the same thing as killing a person. Let me ask you this: if you smash an acorn seed, have you killed an acorn tree? No? Of course not. Same thing with an embryo - if you destroy an embryo, you are merely destroying a microscopically small undifferentiated ball of cells. These cells might have BECOME a human, but are not a human yet. You can't really point at a microscopic ball of undifferentiated cells and say 'That's a human!', because it's not. It's very clearly not. Anyone who has ever seen an embryo under a microscope, like I have during prac courses, can't actually truthfully call it a person, because it so clearly not a person - no brain, no nerves, no organs, no limbs, no skin.... it's about as "human" as a collection of skin flakes. Sure, it might have more potential to develop INTO a human, but at the embryo stage it is no more self-aware, cognizant or capable of feeling than a cell culture. Yet I don't hear anyone complain when I kill and grind up literally millions of HeLa Cells and extract the RNA for use in experiments. People quite clearly recognize that a cell culture is just a bunch of cells. Well, an embryo during early stage development is also just a mass of cells.
Same thing with very early stage foetuses. They don't yet have brains. Brain development does not begin until quite some time after conception. I don't have the exact figures of when brain development begins - I'm not a developmental biologist, just a lowly molecular guy, and it is different for each foetus, but it doesn't happen right away, and the foetus's brain takes a while to fully form. That's science. Anyone even REMOTELY educated in biology cannot disagree with those facts. Brain development does NOT begin at conception and foetuses sometimes do not have brains until quite a bit further on in the development process.
However.... at some stage before birth the brain has mostly formed. We know this because premature babies with mostly functioning brains have been born - my brother was one of them. I know for certain that a week before full term.... most foetuses have mostly or fully formed brains. I can't say for certain whether or not they are fully developed, but I'd wager that they probably are. Shouldn't we err on the side of caution in these cases? If the brain has formed.... aren't they people? I feel very, very, very, VERY uncomfortable with destroying a human brain, any human brain. The destruction of a brain is, in my opinion, the same thing as killing a human. What makes us human are our cognitive abilities - it's what separates us from the animals, it's what gives human life meaning. If late term foetuses have mostly or fully formed brains.... aren't they human? Aren't they?
I'll consent to late-term abortions in the case of rape, incest or if the mother's life is threatened. But for any other reason..... I don't know. A brain is a brain whether it is in the womb or outside the womb.
Again, I'm not a developmental biologist. I went into the molecular/cellular side of things because I find it just as interesting and less messy. So I can't provide a definite time limit on when abortion switches from being gravy beans to being not-gravy-beans (infanticide or manslaughter). Development is a complex, messy affair and different cases develop differently. We don't all begin to form brains at exactly the same time (although there is a general time frame at which brain development begins).
I would say... and this is just going by the seat of my pants here.... that abortion or embryo destruction at anytime before 2-3 months is absolutely fine. After that..... I think we should, if it's possible, scan the foetus to determine how much brain/neural tissue has formed, to decide if it's okay to abort or not. But if the foetus is one or two months away from delivery, if it does not threaten the mother's life and if it does not have significant genetic or developmental abnormalities that preclude a worthwhile or long term life.... then I say we should call it a human (albeit a human-in-the-womb) and prevent abortion.
I feel uncomfortable at conferring personhood on a being just because it emerges from a vagina. I confer personhood when the brain mostly (or fully) forms, no sooner, no later than that.