Most severely affected child in Rh-isoimmunization patients if:

Rh negative mother delivering Rh positive child for the first time gets sensitized to the D antigen of the fetus and stas developing D antibody which is fully loaded in maternal circulation. When Rh negative mother delivers her Rh positive 2nd child the antibody in the fetal circulation causes hemolysis in fetal circulation and the child is most severely affected.