if you look at it from a coldhearted non emotional place a child in the womb is more valuable than the mother in that the mother has already lived some of her life whereas the child still has its entire life ahead of it. Its like the difference between a new car and a used car. A 2010 model will be worth more than a 1990 model.
From a Catholic point of view I think Muck described the reasoning and the circumstances behind the decision the best. Not really much to add to that