Not necessarily. Normally miscarriages occur becuase there is something already wrong with the fetus or wrong with the pregnancy in general. Now getting hit in the stomach may not do a thing to the fetus and then again it might. There is no telling in this kind of situation because it will be different for every girl on what could happen.
My neighbor slipped on a sidewalk and fell hard right on her stomach and she was about 6 months pregnant. Nothing happened to the baby,she continued and carried him to full term. Now I have another friend who was in a fender bender. She was belted in but for some reason she micarried one of the twins she was carrying.
It is a roll of the dice basically. But dont try it if you think you are pregnant because you could put your own life at risk. If you dont want to be pregnant then go through the proper channels to end the pregnancy safely.