The main reason of erythroblastosis foetalis is :

Rh-anomaly takes place during pregnancy, if the mother is Rh- negative, but the foetus is Rh-positive. In such case, Rh antibodies are produced in the mother’s blood against the foetal antigen, when it gets mixed with the foetal blood during pregnancy. The first pregnancy, may be normal. But, this condition can complicate the second pregnancy and cause haemolysis in the second child. This condition is called erythroblastosis foetalis.