Maximum chances of ABO isoimmunization is seen in?

TEXT BOOK OF PATHOLOGY HARSHMOHAN 6THEDITION PG NO 341 HDN due to ABO incompatibility. About 20% pregnancies with ABO incompatibility between the mother and the foetus develop the HDN. Naturally-occurring anti-A and anti-B antibodies' which are usually of IgM class do not cross the placenta, while immune anti-A and anti-B antibodies which are usually of IgG class may cross the placenta into foetal circulation and damage the foetal red cells. ABO HDN occurs most frequently in infants born to group O mothers who possess anti-A and/or anti-B IgG antibodies. ABO-HDN differs from Rh(D)-HDN, in that it occurs in first pregnancy, Coombs' (antiglobulin) test is generally negative, and is less severe than the latter. <\p>