I was asked recently if abortion is ever justified. That depends on what is meant by "abortion" (because I've noticed people use the term in two ways with regard to this matter). Let me explain.
I think it's always wrong to intentionally kill an unborn human being. But there may be (extremely rare) cases in which the unborn child must be surgically removed or expelled from the mother in order for the mother to survive, and if this happens before viability the child will die (e.g., in the case of an ectopic pregnancy or cancerous uterus).
I think this is morally justified (in fact, obligatory) given two requirements:
(1) If no action is taken, both mother and child will die.
(2) The intent of the act is not the death of the child, but to save the mother by correcting a life-threatening pathology, with the foreseeable (but unintended) consequence that the child will die. This precludes most abortion techniques, which involve dismembering or suctioning apart the unborn human being (where killing is intended, as an end or as a means to an end); the child must instead be removed or expelled with care, consistent with his or her status as a valuable member of the human family, and allowing for the possibility of survival if the child is old enough (the doctor should always do his or her best to save both patients).
When we treat both mother and child as valuable persons (the pro-life view), it becomes clear that this is the correct course of action. If two people are drowning in a lake, and I am only able to rescue one, it is better to rescue that one than to let both drown. It is better to save one life than to let two die.
Does removing the child to save the mother qualify as an abortion? Only insofar as "abortion" refers to the premature ending of a pregnancy (the pregnancy is aborted), rather than the intentional killing of the developing child (the baby is aborted). On the latter definition, abortion is never justified; on the former definition, "abortion" (sometimes called indirect abortion) to save the mother's life, even with the unintended consequence of the child's death, is morally justified.
This, I think, is the proper application of pro-life principles to a terrible situation.
