Final answer:
Rh incompatibility leading to HDN is most likely to occur in an Rh-negative mother with a subsequent Rh-positive pregnancy. Sensitization occurs after exposure to Rh-positive fetal cells in a first pregnancy, with the risk mitigated by administering RhoGAM treatment.
Step-by-step explanation:
A potentially fatal Rh incompatibility is most likely to occur in an Rh-negative mother during a subsequent pregnancy with an Rh-positive fetus after being sensitized by a first Rh-positive pregnancy. This condition can lead to hemolytic disease of the newborn (HDN), which can be severe and possibly fatal without treatment.
During the first pregnancy, an Rh-negative mother is usually not sensitized because the baby's Rh-positive cells rarely cross the placenta. However, during or immediately after the birth, the mother may be exposed to the baby's Rh-positive cells, leading to the production of anti-Rh antibodies by the mother's immune system. If the mother becomes pregnant again with an Rh-positive baby, these antibodies may cross the placenta and attack the fetal red blood cells, causing HDN. To prevent this, RhoGAM, an immune globulin, is administered to the mother during and after the first pregnancy involving an Rh-positive fetus. This treatment helps to prevent the formation of anti-Rh antibodies, thereby protecting subsequent pregnancies from the threat of HDN.