An unwanted fetus is therefore a trespasser on her property (even if an unwilling one)
Therefore, the woman is justified in killing the fetus if that is the only viable way to remove it from her property, just as a landowner would be justified in using violent force to remove a trespasser if and only if all nonviolent attempts had failed.
For those of us with a libertarian bent (or an interest in libertarian views), I recently encountered a compelling argument based on the non-aggression principle which both (1) presumes that abortion is categorically the killing of a human being, and (2) endorses a pro-choice position. Block and Whitehead, “Compromising the Uncompromisable: A Private Property Rights Approach to Resolving the Abortion Controversy” (PDF).
Essentially, the argument goes:
A woman’s womb is her property
An unwanted fetus is therefore a trespasser on her property (even if an unwilling one)
Therefore, the woman is justified in killing the fetus if that is the only viable way to remove it from her property, just as a landowner would be justified in using violent force to remove a trespasser if and only if all nonviolent attempts had failed.