It is important to determine the Rh factor in a pregnant woman to protect the pregnant mother from Rh incompatibility. If a mother is Rh-negative and the father is Rh-positive, chances are that fetus might be Rh-positive as well. In that case, the body of the mother can develop antibodies against the Rh antigen. These antibodies can cross the placental barrier and destroy the red blood cells of the fetus resulting in hemolytic disease of the fetus.