f18
As Sisyphus says, a fertilised ovum is not alive in its own right, whereas a newborn baby manifestly is. Where to draw the line is tricky. I would advocate the point at which the central nervous system has formed (as far as basic structure is concerned; the brain carries on developing long after birth, and in a sense throughout the individual's life).
I don't believe that the circumstances of the pregnancy change the ethical status of an abortion. A rape victim can justifiably obtain abortion in any situation where any other pregnant woman can, and only in such situations.
The perfect solution would perhaps be a way for doctors to remove the fœtus without killing it, and growing it in some kind of tank. Such technology is a way off yet, methinks.
Hope this post isn't redundant.