Hemolytic disease of the newborn occurs during which pregnancy and with what Rh statuses?

• A. First pregnancy; mother Rh positive and baby Rh negative
• B. Second pregnancy; mother Rh negative and baby Rh positive; same ABO
• C. Third pregnancy; mother Rh negative and baby Rh positive; opposite ABO
• D. Second pregnancy; mother Rh positive and baby Rh negative

Answer: (B)

Hemolytic disease of the newborn from Rh incompatibility happens when a mother who is Rh-negative has been sensitized to the D antigen after a prior exposure to Rh-positive fetal cells. Those anti-D antibodies are IgG and can cross the placenta during a later pregnancy. If the next fetus is Rh-positive, the antibodies attack the fetal red cells, causing hemolysis, fetal anemia, and potential jaundice or hydrops after birth. The classic scenario is a second pregnancy in an Rh-negative mother with an Rh-positive baby, because sensitization typically occurs after the first pregnancy and the antibodies affect a subsequent Rh-positive fetus. ABO compatibility can influence HDN as well, but it’s not the determinant here; Rh sensitization is the key factor.