The key word in the argument is "viability." If the life has grown to a point where it is viable, and can survive without the life of its mother, than it is a life in and of itself. If it is not yet viable, and requires its mother's life for survival and further growth, then it is not yet a life in and of itself, it's a part of its mother's life—and she has the right to make choices about her own life. If I want to cut off my arm, I have the right to do so because it is my arm. If my arm were capable of its own survival, either attached or detached, then I shall not have the right to destroy it, attached or detached. Hence post-viability restrictions already the law of the land, and defined further by States.