I think Peter's concept of "making the primary decision" about being pro-life or pro-choice, and then ironing out the details, is a very interesting one.
Many years ago my wife and I lost a child. She was 20+ weeks pregnant, and the pregnancy became infected (long story). She went into labor, her body wanting to get rid of the infection I guess. The doctor said the only real choice was to induce labor which would lead to the death of the baby (maybe not today, but at that time). If not, her life was in jeopardy, and the baby still would not survive. There was little time for parsing the moral issues involved. I took great comfort in the fact that this was taking place in a Roman Catholic hospital; their clear theological/ethical "pro-life" position convinced me that this really was the only choice.