Final answer:
Rh incompatibility issues typically don't occur during a first pregnancy with an Rh-negative mother and Rh-positive fetus, because it takes time for the mother's immune system to develop antibodies against the fetus's Rh-positive cells. Subsequent pregnancies carry a risk for HDN unless treated with RhoGAM.
Step-by-step explanation:
Problems with Rh incompatibility usually do not arise during a first pregnancy with an Rh-negative mother and an Rh-positive fetus. This is because the process of sensitization, where the mother's immune system is exposed to the baby's Rh-positive cells, typically occurs late in the pregnancy or during birth, which does not allow enough time for the mother to produce a significant number of anti-Rh antibodies. Hence, the first Rh-positive baby is usually not affected. However, if the mother conceives another Rh-positive baby, the preformed antibodies can cross the placenta and cause hemolytic disease of the newborn (HDN).