A special case of Rh incompatibility (mismatching) has been observed between the Rh−ve blood of a pregnant mother with Rh+ve blood of the foetus. The Rh−ve mother is not usually exposed to the Rh antigen of the foetus during the first pregnancy as the foetal and maternal blood are normally kept well separated by the placental barriers. However, at the time of the birth of the first child, there is a possibility of exposure of the maternal blood to a small amount of Rh+ve blood from the foetus. Hence, the body of the mother starts preparing antibodies against Rh antigens in her blood.
These antibodies (a type of IgG antibody) could cross the placental barriers in subsequent pregnancies and cause haemolysis of the Rh+ve red blood cells of the foetus. This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby (HDN- a haemolytic disease of newly born). This condition is called erythroblastosis fetalis.