-Can anybody explain the gigantic difference between -a baby of age one minute and a fetus that is one -minute from birth?
Or the difference between the baby and a chimpanzee that can communicate in sign langugage or a brilliant dolphin or all sorts of beings that have "near human" characteristics. The reality is that "humanness" is not existentially a yes-no question but a continuum of complicated emotional connections to have we identify as that personhood. Jains are the only complete purists -- they won't even do agriculture because you have to kill bugs in order to till the soil. (Of course they let others defile themselves spiritually so they can eat the products of that bug-killing).
Birth is a dividing line not for the "rights" of the fetus but for that of the mother. Murder is bad but so is slavery, and enforced pregnancy is a form of slavery on women. One can grant some level of personhood to a fetus, at least on the order of animals, yet say those "rights to life" fall compared to the right of freedom for the mother. Yet if the child is months premature, we still grant a strong right to life to the child and demand medical treatment because their are no other rights in conflict.
So the issue is not talking about the personhood of the fetus at any point but how its rights stack up against other claimants, particularly the mother most effected by a pregnancy. Absolute line drawing gets you almost nowhere in such a situation.
-- nathan