Mother is Rh- i.e. dd (Rhesus An has 3 components C, D, E with D as the major component). Baby is Rh+ i.e.DD or Dd.
Initial pregnancy usually induces IgM which does not cross placenta, but next pregnancy induces IgG which crosses placenta easily causing hemolysis of fetal RBC.
Prevented by giving Rhogam to Rh- mother at 28 wk GA and at birth of Rh+ baby.