During a normal pregnancy the fetus can be regarded as a transplant inside the uterus. The reason why this transplant is not rejected is still unclear. In this work various models for this immunological paradox and recent results within the field of pregnancy immunology are discussed. From an immunological point of view the development of a normal pregnancy is almost impossible. During all stages the embryo can be regarded as a transplant (4), but instead of the expected rejection an intimate parasitic graft - host relationship is established. The sperm has already been shown to be the carrier of transplantation antigens and thus theoretically capable of initiating an immune response. The fertilized egg also carries transplantation antigens on its surface after the first division. Foreign transplantation antigens release in the receiver a cell mediated immune reaction, which normally causes a rejection within some weeks.