What physiological circumstances lead to erythroblastosis foetalis?

The antigens of the foetus do not get exposed to the ${{Rh}^{-}}$ blood of the mother in the first pregnancy. However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the ${{Rh}^{+}}$ blood from the foetus. In such cases, the mother starts preparing antibodies against Rh antigen in her blood.
In case of her subsequent pregnancies, the Rh antibodies from the mother (${{Rh}^{-}}$) can leak into the blood of the foetus ${{Rh}^{+}}$). This could cause severe anaemia and jaundice to the baby. This condition is called erythroblastosis foetalis.