The situation in which a mother's blood type is different from that of her unborn child. In the case of some combinations of types, the mother's blood system can produce antibodies to antigens on the surface of the red cells of her fetus resulting in their agglutination. This is the case when the mother is Rh- and her fetus is Rh+. As a result of this incompatibility, the fetus can develop erythroblastosis fetalis.