Well, the soul is the animating force behind a creature. All animals, and even plants, have a soul. The human soul happens to be a spirit as well, whereas animals and plants etc. do not have a spirit. So, being that a fetus, at conception, is animated, it would be only logical to say that the fetus has a soul. Being that said fetus is a human fetus, it would have a human soul. Being that human souls are spirits, the fetus would be a true human in essence.