I draw the line at it's having a functional nervous system. Before that, it's just meat, but it's also an outgrowth of the woman's body, and therefore it's her prerogative to decide it's fate. We shouldn't even be having this discussion, safe and effective birth control pills for men need to be invented and mass produced.
Young people under the influence of powerful hormones, cannot be expected to control themselves. It's unrealistic to rely on moral fortitude. Evolution has favored too long our animal instincts to procreate. Now, it's too much, we have become a cancer on the Earth. That's not justification to cause people harm, but we have to realize that legal abortion is a pro-life point of view. Of course, with a monotheistic viewpoint, you think mankind is the primary purpose of the Earth.