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.
The basics of HDFN
Hemolytic disease of the fetus and newborn (HDFN) occurs because of a mismatch, or incompatibility, between a mother’s and her baby’s blood type (A, B, AB, or O) and/or Rhesus (Rh) factor during pregnancy. The major antigens found on human RBCs are the O, A and B antigens.
RBCs can sometimes have another antigen—a protein called the Rh(D) antigen. 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.
What causes fetal anemia?
In HDFN, a baby’s RBCs break down quickly—a process known as hemolysis. Certain maternal antibodies can pass through the placenta and attack the fetal RBCs, which can then lead to the development of fetal anemia.
Certain tests can be performed prior to a baby’s birth to check for the presence of HDFN.
Read more about HDFN symptoms and risks
How can fetal anemia be detected?
Fetal anemia is monitored with the use of a noninvasive technique called middle cerebral artery (MCA) Doppler ultrasound measurement, which is performed every two weeks to evaluate the risk for fetal anemia in a fetus. This procedure is used in mothers with pregnancies that have reached between 16 and 24 weeks.
MCA Doppler ultrasound is also used when a critical antibody titer has been achieved. The titer is based on the mother’s history of prior pregnancies that have been impacted by HDFN and fetal anemia.
The prediction of fetal anemia with the use of MCA Doppler ultrasound has revolutionized the assessment of Rh-sensitized pregnancies.
Why is MCA Doppler ultrasound used to diagnose fetal anemia?
When a fetus develops anemia, an increase in stroke volume, along with a rise in cardiac output, can be observed. This situation is associated with the fetus developing a hyperdynamic circulation. In such situations, blood flow becomes redistributed to vital end organs, such as the heart, brain, and adrenal glands. Changes in vascular resistance, along with a decrease in blood viscosity, or thickness, are reported as well.
The value of Doppler ultrasound in evaluating fetal anemia is based on the premise that if the cross-sectional area of a blood vessel stays constant, the blood velocity, or rate, will be directly proportional to the blood flow. In fact, decreased blood viscosity is linked to increased blood flow. Such changes can be observed on MCA Doppler ultrasound.
The MCA is the vessel selected for Doppler ultrasound assessment because of its sensitivity to the effects of hypoxia, or insufficient amounts of oxygen, and the fact that is readily accessible.
The severity of fetal anemia can vary
In neonates with HDFN who are anemic, the level of severity of their anemia can range from mild to severe. In fact, severe cases of anemia can lead to the development of edema, ascites (excessive amounts of abdominal fluid), heart failure, hydrops fetalis, and even death.
Infants with HDFN can present with hyperbilirubinemia in the acute phase. For weeks to months after birth, these children can develop hyporegenerative anemia, in which hemoglobin levels and reticulocyte counts—a measure of the number of immature RBCs (reticulocytes) in the bone marrow—are low.
