Which mother-infant blood type combination is at highest risk for ABO incompatibility?

The combination of mother type O and infant type A poses the highest risk for ABO incompatibility. This is due to the fact that a mother with type O blood has anti-A and anti-B antibodies present in her serum. Therefore, if the infant inherits type A blood from the father, the mother's anti-A antibodies can cross the placenta and attack the infant's red blood cells, leading to hemolysis and potential complications, such as jaundice.

In contrast, other combinations have lower risks. For example, a mother with type A blood can generally have an infant with type B without severe issues, as the presence of anti-B antibodies is less prevalent or absent. Similarly, a mother with type AB is less likely to face serious ABO incompatibility issues, regardless of the infant's blood type. A mother with type B blood having an infant with type A also may show some level of incompatibility, but the mother's antibodies are not as aggressive as those found in a type O mother.