The most ardent pro-choicers have long acknowledged that abortion is, in fact, killing. Yet your common, everyday, non-activist pro-choice person will spam every abortion-related message board on the internet (and even some non-abortion-related ones) with euphemism, equivocation, and personal insult to the contrary. It almost makes one wonder what would happen if they took apologetics instruction from the doctors they are defending. C'mon, guys, get with the program; the pro-abortion-choice mantra is: "Yes, of course it is killing; it's a sad and tragic thing, but women and their pregnancies are complicated, and so it's a necessary evil sometimes."
The average (reasonable) person does not agree with that statement. In fact, the average person's conscience recoils at the idea of killing any innocent person - for any reason. They are only able to tolerate abortion because they have deceived themselves as to what it truly is. Abortionists and the radical activists who defend them, however, have employed no such basic biological deception. Their deception is of a higher order: one that somehow imagines that we must ever kill one innocent person to protect another. Most people would not accept such a deception, but fortunately for the pro-choice side, there are a variety of self-contradictory deceptions to choose from, and each person receives the one she finds easiest to accept.
What does all of this have to do with the natural law? Well, the concept of natural law states that rules for how humans ought to live can be observed in human nature itself. It is obvious to most people that killing fellow humans does not produce good outcomes. It deprives the victim of a happy future; it tears apart relationships; and it leaves regret and emotional wreckage in its wake. Few would deny these things. But in order to get to the point where we are applying natural law theory to abortion, we must first acknowledge the simple biological reality that abortion equals killing a fellow human being. It is amazing how difficult it can be for some people to make that very simple first step, but if those doing the killing and those who defend them have made it, how hard could it be for the rest of us?