Haemolytic disease of the fetus and newborn is caused by the placental transfer of maternal IgG antibodies that are directed against fetal red (blood) cell antigens. Fetal spleen mononuclear cells phagocytose IgG coated red cells, removing them from the fetal circulation and causing anaemia and, if untreated, hydrops fetalis and kernicterus, both of which may be severe and occasionally fatal. It has been suggested that some antibodies, anti-K in particular, cause fetal anaemia predominantly through suppression of erythropoiesis rather than through red cell destruction. Antenatal screening for haemolytic disease is regularly undertaken by clinical laboratories. Maternal red cell antibodies are identified and their potential for causing disease may be determined by antibody quantification.