In ABO HDN, the mother is usually which ABO type?

• A. Type A
• B. Type B
• C. Type O
• D. Type AB

Answer: (C)

ABO hemolytic disease of the newborn happens when maternal antibodies against ABO antigens cross the placenta and attack fetal red cells that express those antigens. The typical scenario is a type O mother, who naturally has anti-A and anti-B antibodies. These antibodies are predominantly IgG, which can cross the placenta during pregnancy. If the fetus has A or B antigens (often a type A, B, or AB baby), the maternal IgG anti-A and/or anti-B bind to the fetal red cells and cause hemolysis, leading to jaundice and anemia in the newborn. In contrast, antibodies from type A or type B mothers are mainly IgM, which do not cross the placenta efficiently, so ABO HDN is less likely. Therefore, the mother is usually type O.