Hemolytic disease of the newborn (HDN) is a blood disorder in a fetus or newborn infant. In some infants, it can be life-threatening. A, B, AB, and O are the four major blood types. The types are based on small substances (molecules) on the surface of the blood cells. When people who have one blood type receive blood from someone with a different blood type, it may cause their immune system to react. This is called ABO incompatibility. Rh incompatibility is a condition that develops when a pregnant woman has Rh-negative blood and the baby in her womb has Rh-positive blood.

HDN may develop when a mother and her unborn baby have different blood types (called “incompatibility”). The mother produces substances called antibodies that attack the developing baby’s red blood cells. The most common form of HDN is ABO incompatibility, which is usually not very severe. Other, less common types may cause more severe problems. The least common form is Rh incompatibility, which can almost always be prevented. When this form does occur, it can cause very severe anemia in the baby.