What Rh blood types must the mother and fetus have before HDN can occur?

Hemolytic Disease of the Newborn (HDN) typically occurs when an Rh-negative mother carries an Rh-positive fetus. In this scenario, if the mother is Rh-negative and the fetus inherits the Rh-positive factor from the father, the mother's immune system may produce antibodies against the fetus's Rh-positive red blood cells, leading to HDN. Therefore, for HDN to occur, the mother must be Rh-negative and the fetus must be Rh-positive.