Why do you buy the argument it's a clump of cells until, say, five weeks? How do you rigorously define the dividing line for you?

From my viewpoint, it's a fetus until it can survive outside the womb, which seems to be around 22-24 weeks with current tech. Prior to that point the mother's rights must be paramount. After that, it's a more difficult problem that really needs to be answered by society (and it isn't now.)