Final answer:
Measurement of bilirubin in the amniotic fluid is most indicative of the severity of Rh hemolytic disease, reflecting the level of fetal red blood cell destruction by maternal antibodies.
Step-by-step explanation:
The measurement of bilirubin in the amniotic fluid is best indicative of the severity of Rh hemolytic disease. In the context of Rh incompatibility, bilirubin is a byproduct of fetal hemoglobin degradation resulting from the destruction of fetal red blood cells by maternal anti-Rh antibodies. Elevated levels of bilirubin in the amniotic fluid can indicate severe hemolysis and help assess the extent of the disease, determining the need for potential interventions such as early delivery or intrauterine transfusions.