Right, so if you don't know, the only responsible course is to err on the side of preserving life. Here's an example, should it be illegal to fire a bullet up into the air in a crowded area? Of course, because it's a reckless endangerment of life. Sure, you might not hit anything, but the fact is you don't know what you'll hit, so in the absence of such certainty, we conclude that it's irresponsible to risk innocent life in such a manner. The same argument goes with driving under the influence. Yes many people drive home drunk everyday without hurting anyone. But we recognize that it puts innocent people at risk and thus outlaw it.
If you can tell me exactly when human life begins and when it deserves protection. Then I'll agree, abortion is fine before that. But short of absolute certainty, you have to admit that abortion is potentially the murder of a child. I don't think it's worth taking the risk when a child's life is at stake.
Do you want absolute certainty again drunk driving? Then outlaw blood alcohol levels above 0.0% Want 100% certainty that nobody will ever get hit by a stray bullet? Then outlaw all firearms discharges. Obviously the law is a compromise designed to maximize the probability of a moral outcome; it recognizes 100% certainty is both physically and politically impossible.
Laws that protect certain stages of life are going to be compromises over what that stage represents to most people (or their representatives), and I don't believe that religious distinctions are going to be politically acceptable. Science also doesn't say what life stage should be protected, it is amoral. I believe the inevitable compromise will be to protect humanity.
Here's a few crude tests: can you picture holding it in your hand? That's possible with most of the pictures we've seen, but hard to imagine with a cell. Can you feel a sense of loss? I honestly don't know about that for the loss of a cell, but at the very least I think most women would not be aware of it. It would be an abstract loss for them. What would its death be like? A cell can't feel anything, won't react, and won't care. Do these single celled humans die normally? Yes, it happens all the time.