Marriage between Rh+ man and Rh− woman, sometimes may cause into Rh factor incompatability. If the blood of the foetus is Riband that of the mother is Rh. Rh factor, antibody is formed in mother's blood. Hence, when mother's blood circulates in the foetus, it causes clumping of the RBCs of foetus and this is erythroblastosis foetalis. In this first pregnancy is safe, because of the concentration of Rh factor antibody in mother's blood is negligible but the second child dies either in the foetus or just after birth.