Erythroblastosis fetalis occurs when an Rh-negative mother who has antibodies for the Rh protein carries an Rh positive baby. This happens because the woman had previously carried an Rh positive baby, which caused her immune system to react to the baby's blood type by creating anti-Rh antibodies. Any subsequent pregnancy with another Rh positive child will cause the mother's body to mount an immune response against the unborn child. The mother's antibodies cause the child's red blood cells to be damaged. The child can become anemic and can suffer damage due to the inability of the damaged blood cells to supply enough oxygen.
This link will give you lots of helpful information about this condition.
in pregnancy... for.xmple when mother has A -ve blood group , and baby has A +ve blood.gp.....