Erythroblastosis foetalis occurs:

Explanation

A protein named as rhesus antigen, is present on the surface of red blood corpuscIes. The population having this protein are called Rh positive `(RH^+)` while others who do not have this protein are known as Rh negative `(Rh^-)` (dominant). 
Erythroblastosis Foetalis occurs due to Rh incompatibility between the foetus and mother during pregnancy. Rh antigen 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 and during her subsequent pregnancies the Rh antibodies from the mother (Rh-ve) leak into the blood of the foetus (Rh + ve) resulting in the destruction of foeta| RBCs. This could be fatal to the foetus or could cause severe anaemia and jaundice , even may lead to the death of the foetus.