The Rh blood group system is one of thirty-five current human blood group systems. It is the second most important blood group system, after ABOem consists of 50 defined blood-group antigens, among which the five antigens D, C, c, E, and e are the most important. The commonly used terms Rh factor, Rh positive and Rh negative refer to the D antigen only. Besides its role in blood transfusion, the Rh blood group system—specifically, the D antigen—is used to determine the risk of hemolytic disease of the newborn as prevention is the best approach to the management of this condition.