I would think that during the first trimester it should be ok. Because during that time, a fetus is just that, a fetus. It hasn't developed any farther than a bunch of cells. Its not a baby and doesn't have intelligence yet.
When you are cooking eggs for breakfast do you consider it killing a chick, or nothing like that at all? The egg is about at the same point as the fetus during those first months. nothing dveloped, just a bunch of cells.
correct me if im wrong, this is what I heard from a friend and it makes perfect sense to me :)