Eugene Volokh has an excellent post up about how science can help us answer questions, but only morality can tell us which questions to ask.
What rule we should use for deciding when someone should have the legal right not to be killed is not a scientific question. Applying the rule may be a scientific question; if we decide that only entities that have consciousness have the right not to be killed, then science can tell us whether John Smith has consciousness. But deciding on the rule is simply not a scientific issue: It's a matter of moral judgment, which science isn't equipped to provide. Science can't tell us whether the legal right not to be killed vests at conception, at viability, at consciousness, or at birth; nor can it tell us when the right dissipates.
Quite right. Additionally, as science has advanced and given us more insight into the workings of the womb, more people are beginning to realize exactly what abortion entails -- and they're rejecting it on moral grounds.