What is HDFN?
Hemolytic disease of the fetus and newborn (HDFN) is an immune-mediated red blood cell (RBC) disorder that occurs when a baby’s RBCs break down quickly, which is called hemolysis. HDFN is caused by a mismatch between a mother’s and her baby’s blood type (A, B, AB, or O) or Rhesus (Rh) factor (Rh-positive or Rh-negative) during pregnancy. Numerous antibodies to RBC antigens can be linked to HDFN, such as those from the ABO and Rh blood group systems.
Hemolytic disease of the fetus and newborn (HDFN) is a serious medical disorder that occurs when there is a mismatch, or incompatibility, between a mother’s and her fetus’s blood type (A, B, AB, or O) and/or Rhesus (Rh) factor during pregnancy. This incompatibility causes the maternal antibodies to attack the red blood cells (RBCs) of the fetus or newborn.
The major antigens found on human RBCs are the O, A, and B antigens. However, RBCs can sometimes have another antigen—a protein called the Rh(D) antigen.
There are two main causes of HDFN—ABO incompatibility and Rh incompatibility.
How can the blood type of the mother and the baby differ?
The RhD antigen is either present or absent on the surface of human RBCs. In Rh-positive individuals, the Rh(D) antigen is present on their RBCs, and their blood type is RhD-positive. In Rh-negative individuals, the Rh(D) antigen is not present on their RBCs and their blood type is RhD-negative.
Most often, the mother is blood type O and her baby is blood type A, B, or AB. Less frequently, however, the mother can be Rh-negative and her baby is Rh-positive.
Why does HDFN develop at a particular time?
Women who have blood type O have naturally occurring anti-A and anti-B antibodies. Most of these antibodies are of the immunoglobulin M (IgM) class, but some can be of the immunoglobulin G (IgG) class. Although IgM maternal antibodies cannot pass through the placenta and attack fetal RBCs, the IgG antibodies—which are considered anti-A and anti-B antibodies—are able to cross the placenta and affect the developing fetus.
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What are the main differences between ABO incompatibility and Rh incompatibility?
ABO incompatibility usually is less severe than Rh incompatibility, because fewer group A or group B antigens, compared with Rh antigens, are found on neonatal RBCs. This permits A cells or B cells that are sensitized to survive longer in an infant’s circulation compared with anti-Rh antibodies.
In Rh incompatibility, maternal IgG antibodies form following exposure to fetal Rh-positive blood during birth or with pregnancy-linked complications, such as fetomaternal hemorrhage. Although initial pregnancies are usually not impacted, later pregnancies are at risk for developing fetal hemolysis (destruction of RBCs) or, in severe cases, intrauterine hydrops fetalis.
What is hydrops fetalis?
Hydrops fetalis is a symptom of an underlying medical condition that affects an unborn baby and can occur with HDFN. In this condition, large amounts of fluid accumulate in the baby’s tissues or organs. If it remains untreated, this abundance of fluid can place stress on the fetal heart and other vital organs, thus causing the life of the fetus to be at risk.
In contrast, ABO incompatibility may result in fetal hemolysis in a mother’s initial pregnancy because of preexisting antibodies.
Comparing ABO vs RH HDFN
Typically, ABO HDFN either produces no symptoms, or is associated only with mild anemia or hyperbilirubinemia. ABO HDFN is reported in some infants who are type A or type B and are born to mothers who are type O.
ABO HDFN is a milder disease than Rh HDFN, because fewer group A and group B antigens exist on neonatal RBCs compared with Rh antigens.
Because the A and the B antigens are not well developed on fetal and newborn RBCs, this decreases the amount of maternal antibody that is directed against fetal RBCs.
How is Rh HDFN treated?
Prior to producing any anti-Rh antibodies, women who are Rh-negative first must be sensitized against the Rh antigen. The typical way in which a pregnant woman becomes sensitized is via a prior pregnancy, an unrecognized miscarriage or a blood transfusion that is not compatible with their own blood.
All women who are Rh-negative undergo antibody testing at the beginning of their pregnancy and then at 28 weeks’ gestation. If a woman continues to exhibit Rh-negative results, she receives passive immunization with anti-Rh globulin—a drug known as Rh immunoglobulin, or RhoGAM—to prevent her from becoming sensitized later on in her pregnancy.
Following delivery, a woman will receive an additional dose of RhoGAM, in an effort to extend her protection against developing sensitization.
