So long as we agree that innocent humans cannot be killed out of hand, we need not bring religion into the debate.* Unadorned science tells us that the zygote is a new human individual. If someone like Sindawe wants to propose religious or philosophical conjectures about when someone becomes a full person, that is his business. Such people must not be allowed to use this as justification to murder what is known to be a human being. There simply is no reason** why a pregnant women or anyone else should be allowed to decide whether another person is protected by law or not, and then kill that person on fiat. We don't allow this in other areas of the law. We should not allow it here.
* Ultimately, I think respect for human life can come only from a transcendent religious authority. But so long as we all agree that murder should be illegal, there is no need to go into that debate. Hence, there is no need for the anti-abortionist to bring religion into the argument.
**Other than rape victims, those getting abortions are simply reversing a choice they have already made - to engage in behaviors known to result in pregnancy. Carrying the child of a rapist would be a terrible thing to undergo, but it cannot justify killing that child. Even in the case of pregnancies that endanger the life (not the health) of the mother, it simply won't do to let women have total latitude to decide whether their pregnancy is too dangerous. When a woman must make such a decision, she still must be accountable to some sort of standard, just as I am held to a standard when I decide I must shoot in self-defense. I must show that I had good reason to believe my life to be in danger, and that I could not retreat. If my decision is second-guessed, so should her's be.