After having a short conversation about the issues addressed in my last post, I've come to the opinion that the question may need to be re-framed as a discussion over whether the fetus is entitled to fundamental rights, to what extent and why? Or, put more broadly: at what point in a human being's development does it acquire fundamental rights? If a fetus has these rights, it is immoral to kill it even to save the life of the mother (unless one can support the argument that the mother's rights override the rights of the fetus).
Following this line of thought I tend to believe that the only legitimate dividing line that one can propose, on the basis of science and reason, is the point at which the fetus becomes viable, i.e. is able to live independent of its mother. Once a developing human has the technical/biological ability to survive as an independent entity it deserves the fundamental rights inherent to every human. Therefore, it naturally follows that, just as infanticide is illegal, it should be illegal to abort a fetus that is sufficiently developed as to be capable of surviving outside the womb. Once we have decided that an entity has human rights we cannot circumstantially violate these rights. Therefore, considerations of harm to the mother, incest or rape, cannot come into the issue.
As to the issue of early-pregnancy abortion, I believe my formerly expressed opinion is sound: it is morally questionable, I would not engage in it or advise others to do so, but I believe it should be legal since more harm would result from its being outlawed. Or, to express it in the present context, I do not believe the pre-viable embryo possesses full human rights, but I can see that I may be wrong or it may possess some rights. Therefore, on this issue I reaffirm my former conclusion: early-pregnancy abortion should be discouraged but remain legal.