Rhesus incompatibility describes the situation where the mother’s and the baby's blood groups are different in terms of their Rhesus factor (ABO incompatibility refers to differences between their major blood groups). The Rhesus factor is part of our blood grouping characteristics, and we are either positive or negative. If the mother is rhesus negative, and the baby is rhesus positive, then the mother can produce antibodies if there is mixing between the two circulations. The antibodies can cross the placenta and cause haemolysis, or break down of the foetal red blood cells, resulting in Rhesus haemolytic disease of the newborn. There is a greater risk of this with each successive pregnancy, or if there have been episodes of bleeding during the pregnancy, since this increases the risk of mixing of the two circulations. The development of antibodies can be prevented by using anti-D immunoglobulin, routinely offered to all rhesus negative mothers during pregnancy and after delivery if the baby is rhesus positive.