206 Obstetrics and Gynecology Board Review •••
❍ What percentage of Rh-positive individuals are heterozygous?
Among whites, approximately 60% of Rh-positive individuals are heterozygous at the D locus.
❍ What must occur for Rh alloimmunization to develop?
A patient must be Rh-negative. She must be exposed to RhD antigen. She must be able to produce antibody to the
RhD antigen.
❍ How does Rh alloimmunization develop during pregnancy?
An Rh-negative mother must be exposed to a sufficient amount of Rh-positive erythrocytes. The most common
source is transplacental fetomaternal hemorrhage. Risk of fetomaternal hemorrhage increases with the duration
of pregnancy. Other, less common causes of maternal Rh alloimmunization include transfusion with Rh-positive
blood and injection with needles contaminated with Rh-positive blood.
❍ What is hemolytic disease of the fetus and newborn (HDFN)?
HDFN occurs when maternal IgG antibodies to red cell antigens cross the placenta and bind the corresponding
antigens on fetal red blood cells. This causes hemolysis of the fetal red blood cells, resulting in fetal anemia and
occasionally fetal hydrops. HDFN has previously been known by the names hemolytic disease of the newborn
(HDN) and erythroblastosis fetalis. Historically, the most common cause of HDFN was RhD alloimmunization.
This has changed with routine use of anti-D immune globulin prophylaxis.
❍ Why are most first pregnancies unaffected by HDFN?
The mother’s antibody response mounts slowly (over 5–16 weeks). Exposure during pregnancy is mostly likely
to occur after 28 weeks’ gestation, meaning that a first child will likely be delivered before he or she is affected.
In addition, transplacental fetomaternal hemorrhage is most common at delivery.
❍ Does a maternal antibody response occur in all cases of Rh incompatible pregnancies?
No. Rh alloimmunization only occurs in approximately 15% of Rh-negative women who do not receive any
anti-D immune globulin prophylaxis.
❍ What is the primary factor influencing severity of fetal anemia in Rh disease?
Antibody concentration.
❍ How do fetal cells enter maternal circulation?
Fetomaternal hemorrhage and transplacental passage of fetal red blood cells into the maternal circulation.
❍ When is fetomaternal hemorrhage most common?
Fetomaternal hemorrhage is most common at the time of delivery.
❍ What percentage of fetomaternal hemorrhages at the time of delivery are thought to be sufficient to cause
alloimmunization?
15% to 20%.
❍ An estimated fetomaternal hemorrhage of >30 mL occurs in what percentage of cases?
Approximately 1%.