Final answer:
The baseline risk of isoimmunization in an Rh-negative mother not receiving RhoGAM when having an Rh-positive fetus is about 13-14%. This can lead to hemolytic disease of the newborn (HDN). RhoGAM significantly reduces this risk by preventing the production of anti-Rh antibodies.
Step-by-step explanation:
Baseline Risk of Isoimmunization without RhoGAM
When a female patient who is Rh-negative (Rh−) does not receive RhoGAM and is pregnant with an Rh-positive (Rh*) fetus, the baseline risk of isoimmunization, leading to hemolytic disease of the newborn (HDN), is approximately 13-14 percent. This is because the mother may produce anti-Rh antibodies after being exposed to the baby's Rh-positive cells, either during delivery or due to transplacental hemorrhage. Without the prophylactic intervention of RhoGAM, these antibodies can cross the placenta in subsequent pregnancies and potentially destroy the fetal red blood cells (RBCs), leading to HDN. However, with the introduction of RhoGAM, which prevents the mother's immune system from mounting a primary anti-Rh antibody response, the incidence of HDN has dramatically decreased to about 0.1 percent in the United States since its introduction in 1968.