The Bible is not explicit about when exactly human life begins. Don't misunderstand me - the Bible is clear that human life begins in the womb, prior to birth. However, the Bible does not explicitly say "life begins at the instant of fertilization." Moreover, such a view encounters some problems. One of those problems is the case of identical twins and - in rare cases - identical triplets (see this cute example).
These identical siblings come from the same fertilized egg. Which of the triplets then began life at fertilization? And did the others begin life at all, since they only had a separate existence after fertilization? Or all of the triplets one person? The first question seems inscrutable, and the other questions seem to suggest that life does not have to begin at fertilization.
Of course, God knows the future and knows whether the egg will ultimately split and form two or more babies. So, God could give a single fertilized egg several souls at once. The problem with this ad hoc response is that if God can give multiple souls, then God also could give zero souls to a fertilized egg that he knows will not implant or will otherwise die. Neither of those ad hoc explanations is from the Bible.
I recognize that for "conservative" Roman Catholics, the view today is that life begins at the moment of fertilization (this differs from earlier views that life begins at a quickening time during the pregnancy). I don't think that Roman Catholics, however, have a compelling response to the problem of identical twins/triplets.
None of the above necessarily implies a particular way of handling the abortion question. It does suggest that we might want to be cautious about being dogmatic about the very earliest stages of pregnancy, namely the time during which it is possible for a single egg to become multiple children.
Likewise, by the same token, it makes sense for us to cautiously over-protect unborn children - perhaps even attempting to legally protect fertilized human eggs prior to any division. Since we are not sure when exactly God gives a human soul, it makes sense conservatively to protect from the fertilized egg onward.
One may object that the Bible speaks of "conception" and people possessing various qualities from the time they were "conceived" in the womb. This itself does argue for a relatively early time of human life within the pregnancy. Nevertheless, remember that we should not read back our understandings of human biology anachronistically onto the text. Thus, we should not assume that "conception" in these texts refers to the moment of fertilization.
In any event, I post this not to throw cold water on the pro-life movement, nor to discourage people from protecting the unborn. Rather, I post this to encourage pro-lifers to be more cautious and circumspect in their rhetoric - focusing on what we know (that unborn children are human beings with souls) rather than on what we do not know (namely the precise moment of ensoulment).