A potentially life-threatening condition that can develop when a mother with Rh-negative blood has a baby with Rh-positive blood. Rh-positive and Rh-negative indicate the presence or absence of an inherited blood characteristic known as the Rh factor (named after the rhesus monkey, the species in which the factor was originally identified). If Rh-positive blood mixes with Rh-negative blood, the Rh-negative blood develops antibodies to the Rh factor and attacks it. Such a situation can arise when a woman with Rh-negative blood is carrying a fetus with Rh-positive blood. Small amounts of blood from the fetus leak across the placenta during pregnancy and birth and stimulate an antibody response (Rh isoimmunization) in the mother that attacks the baby’s blood cells. Usually the disease occurs in a subsequent pregnancy, after the woman develops antibodies to the Rh factor.