It's wrong to kill babies. It's also wrong to deprive women of their freedom of choice over their own bodies and subject themselves to a painful pregnancy for any reason but the welfare of others. If you take the middle ground of these two, it's to acknowledge that one or both of these injustices happen. Abortion's the greatest debate in history because, either to a religious person, or a secular person, life is nonetheless sacred. You can't redefine life through existential debate because, well, that's denying your own existence and your right to life.
To allow abortion in certain cases and deny it in others is to acknowledge abortion as the slaughter of the innocent and as a woman's right. Then, in the cases where it's denied, you are forcing a woman into having her life changed by an unwanted child for no reason but your bossy own, and when abortion's granted you are killing a human that has done nothing to deserve it.
In the US today, abortion is legal, but when you accidentally terminate a wanted pregnancy through negligent driving it's considered manslaughter. This inconsistency leads to only one conclusion: a fetus is human if and only if its mother wants it. Am I only human because my mother and others continue to love me?
There is no middle ground when you are talking about life; it didn't work for Plato, and it didn't work for Descartes.
