ABO incompatibility can result when the fetal blood type differs from the mother's blood type.
A, B and O are the major blood types. In these cases, the mother creates antibodies against the fetus' incompatible blood type. These antibodies cross the placenta into the fetus' blood stream where they begin to destroy the fetus' blood cells. ABO incompatibility is similar to Rh incompatibility but generally creates much milder problems than Rh incompatibility. Severe ABO incompatibility problems may require an exchange transfusion .