To properly phrase it, I believe that "You shall not commit murder" applies to both the mother and the unborn.  We cannot save either life in the long run since, in the end, everyone dies.

The position that you must choose which one to kill is the pro-death position.

The refusal to discuss this stems from the fact that I believe that "You shall not commit murder" applies to both the mother and the unborn, that I believe situation ethics to be a condemned moral calculus, and that simply noting those ends the conversation from the Catholic perspective.