People who are rhesus positive have a substance known as D antigen on the surface of their red blood cells. Rhesus negative people do not. A rhesus negative woman can carry a baby who is rhesus positive if the baby's father is rhesus positive. If a small amount of the baby's blood enters the mother's bloodstream during pregnancy or birth, the mother can produce antibodies against the rhesus positive cells (known as anti-D antibodies).
This usually doesn't affect the current pregnancy, but if the woman has another pregnancy with a rhesus positive baby, her immune response will be greater and she may produce a lot more antibodies. These antibodies can cross the placenta and destroy the baby's blood cells, leading to a condition called rhesus disease, or haemolytic disease of the newborn. This can lead to anaemia and jaundice in the baby.
Anti-D injections can prevent rhesus negative women from producing antibodies against the baby. Rhesus negative mothers who haven't developed antibodies are therefore offered anti-D injections at 28 weeks of pregnancy, as well as after the birth of their baby. This is safe for both the mother and the baby.