I believe that "purist" anti-abortion activists would be perfectly happy with a definition that says any fertilized egg gains legal protections. If that effective bans several forms of birth control like the pill or IUDs, they'd have no problem with that. In terms of broad-based public opinions, a purist stance would never gain traction, even among otherwise anti-abortion activists. Thus, that side of the aisle is all about compromise.

On the flip side of the aisle, the pro-choice "purists" would be perfectly happy with a definition that says that legal protections don't start until the baby is born, and anything before that is between a woman and her doctor. This argument would similarly fail to gain a majority of public opinion, and thus we see concessions with regard to third trimester abortions.

Ultimately, like everything, the abortion issue boils down to politics. "Right" and "wrong" are all relative to what sort of a compromise can achieve sufficient political traction.