I don't believe in a soul, nor do I believe in some inherent "right to life" that we possess. Ultimately, it's neither "good" nor "bad" whether individual humans, or even the entire human race, survive for any length of time. However, we humans place a subjective value on our lives, because although they are not objectively "good", they are nevertheless important to us.
Now, I respect the lives of others for the same reason I respect my own life. Since this life is all I have, I would like others to allow me to go ahead and live it as I see fit. In fairness and in empathy, I also choose to allow others the same considerations.
Now, I am hardly going to extend that right to a sperm cell. That would be ridiculous. It is similarly ridiculous to extend that right to a single cell that happens to have a complete set of human DNA. So at what point do I decide that I owe another human being his or her life? It's not a question that has any objective answer, so I'll answer it from the only viewpoint that I can -- a subjective one.
I disagree with anyone's wish to take away my life because I am a self-aware, rational, thinking being. If I were not, then it wouldn't matter if someone killed me -- I wouldn't know or care. For example, I would feel like an irrational idiot if I insisted that I be kept on life-support indefinitely if I were brain-dead. If I'm brain-dead, I'm not going to know and it's not going to either hurt or help me. Until I experience life and have a chance of continuing to experience it, I do not have any "right to life". It's a moot point.
We could simply avoid this issue if it were not for the fact that you cannot give full rights to two people sharing the same body. Either the mother or the baby must take precendence. Now, if the baby is not self-aware, it seems obvious to me whose rights should take precedence, since as I said, a non-self-aware being has no rights in my opinion. If the baby is self-aware, then the problem becomes very difficult -- and I think the correct decision would be the one that results in the least total harm for both the mother and the child. But remember, psychological harm to both the mother and the child, and harm to the child because of a hard life are both considerations that must be taken into account. Survival, though it is the primary evolutionary consideration, is not and should not be our only social consideration.
I fully admit that my views on this matter are subjective -- but a caveat: I don't believe that there is any such thing as a non-subjective view on this issue. Even those who claim their opinion is objective are only saying so because they think that their subjective beliefs are objectively true. In any case, I cannot say that this is a hard-and-fast moral rule that everyone must agree with, but it is a rule that I live by. However, I don't think abortions should be performed lightly. It should be a decision that the mother feels is absolutely necessary. Unfortunately, it is not possible to enforce any such rule, and so it's not a viable way to determine the morality of a given case.