Hemolytic disease of the fetus and newborn (HDFN), also known as alloimmune HDFN or erythroblastosis fetalis, is caused by the destruction of red blood cells (RBCs) of the neonate or fetus by maternal immunoglobulin G (IgG) antibodies. The formation of maternal antibodies in response to a fetal antigen is called isoimmunization. These antibodies form when fetal erythrocytes that express certain RBC antigens that are not expressed in the mother cross the placenta and gain access to maternal blood. This antibody response may be sufficient enough to destroy fetal red cells leading to hemolysis, the release of bilirubin, and anemia. The severity of the illness in the fetus depends on various factors including the amount and strength of antibody produced by the mother, the gestational age of the fetus, and the ability of the fetus to replenish the destroyed RBCs and clear bilirubin.
Numerous blood group systems have been implicated in HDFN including, ABO, Rhesus (Rh), Kell, Duffy, Kidd, MNS, Diego.P, Lutheran, and Xg.  Rhesus and ABO are by far the most common. ABO incompatibility generally occurs in a group O mother with a group A or B baby, but ABO incompatibility causes less severe hemolytic disease of the newborn than does Rh(D) incompatibility. Affected infants are usually asymptomatic at birth with absent or mild anemia and develop neonatal jaundice, which is usually successfully treated with phototherapy. Because Rhesus factor incompatibility is more severe, it will be the focus of the rest of this discussion.
The introduction of postnatal immunoprophylaxis in 1970 reduced the incidence of maternal RhD alloimmunization from 14% to 1% or 2%. Subsequently, antenatal immunoprophylaxis was also started which further reduced RhD alloimmunization to 0.1%. In the western world, ABO incompatibility is now the single largest cause of HDFN.
The incidence of Rh incompatibility varies by race and ethnicity. Approximately 15% of whites are Rh negative, compared with only 5% to 8% of African Americans and 1% to 2% of Asians and Native Americans. Among whites, an Rh-negative woman has an approximate 85% chance of mating with an Rh-positive man, 60% of whom are heterozygous and 40% of whom are homozygous at the D locus.
Following maternal exposure to RhD-positive blood, B-lymphocyte clones that recognize the RBC antigen are established. The primary maternal immune response is the production of IgM isotype. It is important to note that this primary maternal immune response is dose-dependent. It occurs in about 15% of pregnancies with 1 mL of Rh-positive cells and 70% after 250 mL of Rh-positive cell exposure. Maternal IgG response occurs later in subsequent pregnancies. The secondary immune response follows repeat exposure to as little as 0.03 mL of Rh-positive cells. Maternal anti-D (IgG) antibodies cross the placenta and attach to Rh antigens on fetal RBCs. RBC destruction occurs by lysis of antibody-coated RBCs by macrophage lysosomal enzymes. The fetus initially responds to the subsequent anemia and tissue hypoxia through reticulocytosis, and a rise in umbilical artery lactate indicates severe fetal anemia. Erythroblastosis fetalis results when RBC destruction exceeds production.
Mild to Moderate Disease: Less severely affected infants typically present with the self-limited hemolytic disease which is manifested as hyperbilirubinemia within the first 24 hours of life.
Hydrops Fetalis: Infants with severe, life-threatening anemia (e.g., hydrops fetalis) present with skin edema, pleural or pericardial effusion, or ascites. Infants with RhD and some minor blood group incompatibilities, such as Kell, are at risk for hydrops fetalis, especially pregnancies without antenatal care.
For patients who are Rh-negative and also have a negative antibody screen, it is important to try and prevent them from becoming sensitized during the pregnancy course. Possible reasons a patient may become exposed to fetal blood and thus sensitized include miscarriage, amniocentesis, vaginal bleeding, placental abruption, and abdominal trauma. If any of these instances occur, RhoGAM (anti-D immunoglobulin or Rhesus factor IgG) should be administered.
If the antibody screen for Rh comes back positive during the initial prenatal visit, the titer is checked as well. Antibody titers of 1:16 and greater have been associated with fetal hydrops. If paternity is not in question, blood type can be performed on the father of the baby to determine whether the fetus is at risk. However, because approximately 5% of all pregnancies have unknown or incorrect paternity, the safest course is to treat all pregnancies as if the fetus is at risk. 
Throughout pregnancy, the antibody titer is followed approximately every 4 weeks. As long as it remains less than 1:16, the pregnancy can be managed expectantly. However, if it exceeds 1:16, serial amniocentesis should be started as early as 16 to 20 weeks. At the first amniocentesis, fetal cells can be collected and analyzed for the Rh antigen to determine fetal Rh status. If negative, the pregnancy can be followed expectantly. However, if the fetus is Rh positive, fetal anemia is screened for, using fetal middle cerebral artery (MCA) Doppler measurements. It was demonstrated more than a decade ago that in anemic fetuses there is greater blood flow to the brain, thus the MCA Doppler measures peak systolic velocity (PSV). In fetuses with greater PSV measurements, concern for fetal anemia merits more invasive testing and potential treatment.
Historically, prior to the use of MCA Doppler, the evaluation of Rh-positive fetus in an Rh-negative woman with positive titers 1:16 or greater was done with serial amniocenteses to assess the amniotic fluid by a spectrophotometer.
Rho(D) immune globulin is a preparation of human IgG containing antibodies against the Rho(D) antigen of the red cell. Rho(D) immune globulin is used for prevention of Rh hemolytic disease of the newborn. Administration of Rho(D) immune globulin to Rho(D) negative mothers at the time of antigen exposure, such as the birth of a Rho(D) positive child, blocks the primary immune response to the foreign cells. Therefore, maternal antibodies to Rh-positive cells are not produced in subsequent pregnancies, and hemolytic disease of the neonate is averted.
RhoGAM should be administered at 28 weeks since it has a half-life of about 12 weeks and covers the mother until term or 40 weeks, and postpartum if the neonate is Rh positive. A standard dose of RhoGAM (0.3 mg) will eradicate 15 mL of fetal RBCs. This dose is adequate for a routine pregnancy. In cases of antepartum bleeding, abdominal trauma, amniocentesis, or placental abruption where more blood is transferred from the fetus to the mother than normal, the standard 0.3 mg dose of RhoGAM may not be sufficient. A Kleihauer-Betke test that determines the amount of fetal RBCs in the maternal circulation should be performed. If the amount of fetal RBCs is more than can be eliminated by the single RhoGAM dose, additional dosages must be given.
Cordocentesis and measurement of fetal hemoglobin are used to assess the severity of anemia when MCA dopplers are elevated. Fetal hemoglobin is two standard deviations below the mean value for gestational age. A hemoglobin level of more than 7 g/dL below the normal mean for gestational age or hydrops (actual hemoglobin level of less than 5 g/dL). A hematocrit of less than 30% also can be used as the threshold for fetal transfusion.
Causes of elevated unconjugated bilirubin are vast. The most common cause is physiologic jaundice. Physiologic jaundice presents around day 2 or 3 with a serum bilirubin of less than 12 mg/dL, mainly unconjugated. It commonly disappears by the end of the first week and happens in 60% of term and 80% of preterm infants because of limited ability to conjugate bilirubin. Risk factors include maternal diabetes, polycythemia, cephalohematoma, prematurity, male, Asian, Down syndrome, delayed bowel movement or upper gastrointestinal obstruction, hypothyroid, and a sibling with physiologic jaundice.
Jaundice in the first 24 hours after birth is not physiologic jaundice and needs further evaluation. Early-onset breastfeeding jaundice is the most common cause of pathologic unconjugated hyperbilirubinemia. Breastfeeding can potentiate physiologic jaundice in the first week of life because of caloric deprivation, leading to an increase in enterohepatic circulation and thus a decrease in bilirubin reabsorption via the gut. Successful breastfeeding every 2 to 3 hours, while monitoring stool and urine output to determine if the infant is feeding adequately, significantly decreases the risk of hyperbilirubinemia. 
Breast milk jaundice occurs after the first week of life and is secondary to breast milk’s ability to inhibit 2,3 UDP glucuronyltransferase, the enzyme responsible for conjugating bilirubin.
Genetic causes of unconjugated hyperbilirubinemia include Gilbert syndrome which presents with jaundice later in life following mild illnesses, fasting, or physical stress. Gilbert syndrome is due to a UDP glucuronosyltransferase defect. Crigler-Najjar is due to an absence or decrease in UDP glucuronosyltransferase.
Other causes due to an increase in bilirubin production, similar to similar to Rh/ABO incompatibility include enzyme defects (glucose-6-phosphate deficiency and pyruvate kinase deficiency), structural defects (spherocytosis and elliptocytosis), birth trauma (cephalohematoma and excessive bruising), and polycythemia.
The work-up for indirect hyperbilirubinemia includes CBC, reticulocyte count, blood smear, serum haptoglobin, direct and indirect Coombs test, hemoglobin electrophoresis, red cell enzyme assay, and spherocytosis test.
The prognosis of this disease has significantly improved over the past few years thanks to the developments of tools and noninvasive testament, which when performed antenatally will prompt early recognition and treatment.
Anemia can lead to a high-output cardiac failure/myocardial ischemia. As the cardiac system attempts unsuccessfully to keep pace with the oxygen delivery demands, the myocardium becomes dysfunctional, resulting in effusions, edema, and ascites due to hydrostatic pressure increases. This combination of fluid accumulation in at least two extravascular compartments (pleural effusion, ascites, pericardial effusion, or subcutaneous edema) is referred to as hydrops fetalis.
Unconjugated bilirubin is lipid soluble, thus giving it the ability to cross the blood-brain barrier (BBB) and cause kernicterus. The risk of kernicterus is higher with indirect bilirubin levels greater than 20 or rising levels despite phototherapy. Kernicterus can present with lethargy and poor feeding, followed by a toxic appearance with respiratory distress and decreased deep tendon reflexes. Kernicterus may resemble sepsis, asphyxia, hypoglycemia, and intracranial hemorrhage. The risk of kernicterus is increased with acidosis and sepsis, which increases BBB permeability. The risk is also increased with hypoalbuminemia, which leads to a reduced ability to transport unconjugated bilirubin to the liver. Finally, the risk is made worse by drugs that displace bilirubin from albumin, including ceftriaxone. To prevent kernicterus, phototherapy is done on at-risk infants with elevated bilirubin.
The prognosis of this disease has significantly improved due to a multidisciplinary approach toward diagnosis and treatment. The team of nurses and physicians now have the tools and noninvasive testament, which when performed antenatally will prompt early recognition and treatment and improve the care of the patient. [Level V]
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