“…Notwithstanding low A and B expression on fetal RBCs, possible minor ABO donor–fetus incompatibility, in theory, can cause additional fetal cell hemolysis and hemolytic transfusion reactions. As is known from ABO hemolytic disease of the newborn, the IgG fraction of these antibodies will not lead to severe hemolysis during pregnancy, because complement is seldom activated . It is unlikely that the passively transferred IgM A and B antibodies are capable of activating the fetal complement system, because as estimated from CH50 lytic potential, this is impaired during fetal life and still relatively immature at birth .…”