At the scene of a homicide, investigators find two distinctly different DNA samples but only one body, so they conclude that either (a) there was more than one victim, or (b) somehow the crime scene was compromised, contaminated. Can you think of any other possibilities? Actually, I can think some, but they all involve the known fact that each individual --- including, we now know, each member of a pair of identical twins --- has his or her own, unique DNA. From the moment of conception through to death, a person's DNA is unique to that person. Also from the moment of conception, the DNA of an embryo is different from that of its mother. So, cells from an embryo are not merely a cluster of a mother's cells in her uterus; it has different DNA --- it is a distinctly different individual --- from the mother. But is it a human being? I, personally, would no more call a zygote or an embryo a human being than I would call an acorn an oak tree. But I understand the reasoning of those who would prefer to err on the side of caution and would answer that question in the affirmative. I, personally, am comfortable with the idea of a fetus' becoming a human being at the point of viability, i.e., the "interim point at which the fetus becomes ... potentially able to live outside the mother's womb, albeit with artificial aid," according to the famous Roe v Wade decision of the U.S. Supreme Court.
Oversimplifying things, then, we say: The fertilized cluster of cells in the host-mother's body is a zygote, which develops into an embryo; in its earliest stages of development, we refer to the embryo as a blastocyst, and later (after nine weeks or so) as a fetus. Once the fetus has developed to the point that it could function outside the mother's uterus, I refer to it as a "pre-born baby."
Now, I don't know at what point the fetus becomes a baby, but I do know that at some point before it is born, that little being develops fingers and toes, eyes, lungs, a brain, and, probably, the ability to experience pleasure and pain. That is definitely the case once it develops to the point that it could possibly survive outside its mother's uterus. Again, no one knows exactly when the developing person achieves viability, and it's for that very reason that many people choose to err on the side of caution and say that from the moment of conception this being is a human, with all human rights. What we do know is that from the point of conception the developing being has its distinct DNA, so it is not simply a cluster of the host-mother's cells. I understand the reasoning behind the recognition of this cluster of cells as a human being, and I do not condemn it. I just don't agree wholeheartedly.
I have never met anyone who was really pro-abortion. But, I am not uncomfortable with aborting zygotes. I am a little less comfortable with aborting pre-viable embryos; I am not at all comfortable with the idea of aborting viable embryos, even those conceived by incest or rape, or who show signs of significant mental, physical, or emotional impairment. That last point is one of those many positions I now hold which are subject to adjustment as I learn more; I am always open to changing even a dearly-held belief if I have sufficient reason to do so.
But don't women have rights, you ask? Of course, they do. But, let's remember: In my opinion, once the individual in the uterus has become viable, it becomes a person whose rights have to be recognized, acknowledged, respected, and protected. Again we say: This person has DNA that is distinct from that of its mother; it is not simply a cluster of the mother's cells.
I also want to point out one thing on which we should be very clear: At no point have we brought religious considerations into this discussion; nowhere have I said a word about a "soul" or any of that. I happen to be an active Christian, but on this issue (as on several others), I am not on the same page as my beloved and rather progressive Episcopal Church. That's completely irrelevant, though; for scientific and ethical reasons, completely aside from any religious issues, I am both pro-choice (dealing with the being in utero before it achieves viability, when only the host-mother's personal choice might be important), and pro-life (dealing with the being after it has achieved viability and is indeed a pre-born baby, with certain human rights of its own). I respect the views and sensibilities of those who prefer to afford full human rights even to the zygote, but I do not respect the views of those who feel that the rights of the woman to make decisions about her body trump those of the pre-born baby; that baby has rights, too.