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Which mother and fetus ABO blood types most often lead to ABO incompatibility?

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Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background. — blood group The major component of human blood is the red blood cell.

ABO incompatibility between a mother and her fetus occurs most frequently when the mother has type O blood and the fetus has either type A or type B blood.1

In these instances, the mother's immune system may produce antibodies that cross the placenta and react with the fetal red blood cells. While this condition can lead to erythroblastosis fetalis—a form of hemolytic disease of the newborn—ABO incompatibility typically results in only mild disease compared to other blood group incompatibilities, such as those involving the Rh factor.12