or hemolytic disease of the newborn
antibodies to an {{link=antigen">antigen in her fetus's red blood cells.
An Rh-negative woman (see Rh blood-group system ) with an Rh-positive fetus whose ABO blood group (see ABO blood-group system ) matches hers is likely to have an immune reaction after the first such pregnancy, which sensitizes her when fetal red blood cells enter her bloodstream, usually during labour. If blood typing shows incompatibility, an anti-Rh antibody injection given to the mother after the birth can destroy the fetal red cells, thus preventing trouble in a future pregnancy. If amniocentesis detects products of blood destruction, Rh-negative blood transfusions to the fetus before birth or exchange transfusion after it may save the baby's life. ABO incompatibilities are more common but usually less severe.