Erythroblastosis foetalis can occur if

Erythroblastosis foetalis (Haemolytic Disease of Newborn)occurs due to Rh incompatibility if the mother is Rh-ve and the foetus is Rh+ve. Rh antigens of the foetus do not get exposed to the Rh-ve mother in the first pregnancy as the two bloods are well separated by placenta. However during the delivery of the first child, there is a possibility of exposure the maternal blood to small amounts of the Rh+ve blood from the foetus. Such a sensitized mother prepares antibodies against Rh antigens. In case of her subsequent pregnancies, the Rh antibodies from the mother of Rh-ve can leak into the blood of the foetus and destroy the foetal RBCs. This disorder is termed erythroblastosis foetalis.