The most common severe scenario is that an Rh (D antigen) - negative mother becomes sensitized to the D antigen during a previous pregnancy with an Rh-positive fetus by fetal-to-maternal passage of blood; a later pregnancy with an Rh-positive fetus may then prompt an anamnestic maternal IgG response when the mother is re-exposed to fetal blood during this later pregnancy that may result in fetal and neonatal hemolysis.