Alloimmunization

Hemolytic disease of the fetus and newborn

1. What every clinician should know

Any fetus inheriting a blood group factor not possessed by its mother may stimulate an immune reaction in the mother. Usually associated with fetomaternal hemorrhage before birth or at delivery, fetal red cells cross into maternal circulation and are recognized as foreign by B lymphocytes, which lead to production of IgM and subsequently IgG antibodies.

Evidence of this response is evident 5 to 16 weeks after the sensitizing event. Memory B lymphocytes sensing the presence of the antigen (usually in a subsequent pregnancy, rarely late in pregnancy after an early sensitization) generate plasma cells that produce IgG antibodies. It is these IgG antibodies that cross into fetal circulation via active transport through the placenta and attack fetal erythrocytes. Sequestration of these cells in the fetal spleen leads to hemolysis and anemia.

The Rh (CDE) blood group typing nomenclature identifies three genetic loci with two major alleles (C, c, D, E, and E). There is no “d” identified. There are 30 other antigenic variants and more than 60 antired cell antibodies that have been associated with hemolytic disease of the fetus and newborn. RhD negative mothers with RhD positive fetuses are the most likely to develop an immune response to the fetal red blood cell antigens. Incidence of Rh incompatibility varies by race (15% of whites are Rh negative compared with 5% to 8% of African Americans, and 1% to 2% of Asians and Native Americans).

Widespread use of rhesus immune globulin (RhIG) has resulted in decreased incidence of alloimmunization related to the RhD antigen. In 2002, there were 6.7 cases of Rh alloimmunization per 1000 live births. Other antibodies, such as anti-Kell (26%) and anti-c (10%), are associated with severe hemolytic disease of the fetus and newborn. Anti-E and anti-Fya (Duffy) can result in clinically relevant anemia. There are no immune globulins available for antigens other than RhD. Most cases of alloimmunization due to “minor” (non-D) antigens are due to incompatible blood transfusion. These occur in 1.5% to 2.5% of obstetric patients.

Fetomaternal hemorrhage sufficient to cause alloimmunization occurs most commonly at delivery, in 15% to 50% of births. The volume of hemorrhage may be increased by cesarean delivery, multiple pregnancy, placenta previa or abruption, and manual removal of the placenta. Alloimmunization has been reported after early pregnancy complications and invasive procedures. Male fetuses may be at 13 times increased risk for hydrops fetalis and 3 times higher risk of perinatal mortality than female fetuses. Patients with ABO incompatibility are at decreased risk, as anti-A or anti-B antibodies will lyse fetal cells in the maternal circulation.

2. Diagnosis and differential diagnosis

Establishing the diagnosis

Red cell alloimmunization is detected during blood type screening in routine antepartum care. The indirect Coombs test can determine the maternal IgG response, reported as a titer (equivalent to dilution). Usually a critical value of 1:32 is considered a titer of clinical significance for anti-D antibody and most others (the critical value is 1:8 for anti-Kell antibodies.)

Paternal blood sampling should be performed to determine genotype. In cases of heterozygous paternal genotypes, cell-free fetal DNA testing can be used to determine fetal RhD genotype, and is available outside the USA for CcE and Kell (K1) antigens. Amniocentesis can be used to determine fetal antigen status for most clinically relevant antigens. Cell-free fetal DNA for fetal RhD genotyping may be used in the future to determine which patients should receive RhIG antepartum.

Differential diagnosis

Rh D typing does not detect all genetic variants such as “weak D” (previously known as “Du”). These patients have altered D protein or reduced expression RhD antigens, and will be reported as RhD negative. Some patients who have donated blood will have had additional Coombs testing performed and be aware of this RhD variant status, and may have been reported as RhD positive in those cases. These patients should still receive RhIG in pregnancy.

Anti-RhC, -RhE, and -Rhe may add to the hemolytic effect of anti-D, but are rarely clinically significant when occurring in isolation. In patients with equal anti-D and anti-C titers, anti-RhG may be present, and blood bank specialists should be consulted. Anti-k rarely is associated with need for intrauterine transfusion, but in those cases presents similarly to anti-K1 and should be treated similarly with a lower critical titer of 1:8. Other antibodies (anti-M, anti-N, anti-Duffy [FY a, b, and c], anti-Kidd [JKa and Jkb]) are associated only with mild hemolytic disease of the fetus and newborn.

Other commonly encountered antibodies, Lewis and I antibodies are cold agglutinins that do not cause erythroblastosis fetalis because they are predominantly of the IgM type, which do not cross the placenta.

Once RhIG is employed, the anti-D antibody screen may remain positive for up to 6 months. A positive anti-D after this time is likely the result of sensitization.

3. Management

Antepartum – All pregnant women should have antibody screening upon beginning prenatal care. In RhD negative women without evidence of alloimmunization, 300 micrograms of Rhesus immune globulin (RhIG) should be administered at 28 weeks gestation. RhIG should be protective against sensitization for 12 weeks. Repeating the antibody screen at this time is recommended, although sensitization during the pregnancy is rare (0.18%). Some clinicians recommend repeat RhIG dosing at 40 weeks if the patient has not delivered.

In women with vaginal bleeding early in pregnancy, RhIG should be administered. Other events that should prompt dosing of RhIG are abortion, ectopic pregnancy, molar pregnancy, chorionic villus sampling, amniocentesis, blunt trauma to the abdomen, fetal death, bleeding from placenta previa, or external cephalic version. Prior to 12 weeks, a smaller dose of 50 micrograms is effective, but if that is not available, the 300 microgram dose is safe and should be administered. RhIG should be administered within 72 hours, but if not given during that, it should be given up to 13 days following the exposure, as it may still offer partial protection against sensitization.

In patients with positive antibodies on initial blood type screening, the antibody should be identified and testing of the fetus should be done with amniocentesis. Antibody titers should be performed every month until 24 weeks, then every 2 weeks. In patients with titers below the critical value, repeat titers should be performed monthly.

Sonographic measurement of peak systolic velocity of the fetal middle cerebral artery (MCA) can detect fetal anemia, using a threshold value of 1.5 multiples of the median (MoM) to predict moderate-severe fetal hemoglobin. This can begin at 16 to 18 weeks gestation and continue weekly. Use of MCA Dopplers has largely supplanted use of amniotic fluid spectrometry (delta-OD 450 analysis).

For fetuses with elevated MCA Dopplers, umbilical cord blood sampling (a.k.a. percutaneous umbilical blood sampling (PUBS)) can be used to confirm anemia. Blood should be sent for blood type, hematocrit, direct Coombs test, reticulocyte count, and total bilirubin). Cordocentesis can be used to administer blood transfusions (using freshly donated, CMV-negative type O, RhD-negative blood that has been leukoreduced and irradiated) to severely anemic fetuses (Hct <30%) to prevent sequelae of hemolytic disease of the fetus and newborn. In severely sensitized pregnancies requiring multiple procedures, the last transfusion should be performed at 30 to 32 weeks, with a plan for delivery at 32 to 34 weeks after antenatal corticosteroid administration.

Antenatal testing should begin by 32 weeks. In the 7 to 10 days prior to delivery, oral phenobarbital may be considered to improve hepatic maturity and enhance conjugation of bilirubin.

Intrapartum – Optimal timing of delivery is controversial in pregnancies, with milder forms of anemia and pregnancies requiring transfusion, but it is reasonable to proceed with induction of labor at 37 to 38 weeks gestation, or earlier if fetal lung maturity is documented.

Postpartum – In order to prevent sensitization, Rh negative women should receive a 300 microgram dose of RhIG after delivery of an Rh positive fetus, with additional dosing based on quantity of fetal cells detected in maternal serum. If a fetomaternal hemorrhage greater than 30 mL is suspected based on a rosette test, Kleihauer-Betke stain, or flow cytometry, additional doses should be given based on the amount of fetomaternal hemorrhage. Risk factors identify only 50% of cases of excessive fetomaternal hemorrhage, and laboratory testing should be done with all patients. If the RhIG is not administered during the recommended immediate postpartum period, it should still be given up to 28 days postpartum. Women with pregnancies affected by alloimmunization should be counseled that future pregnancies are generally more severely affected.

4. Complications

Complications as a consequence of the condition

Anemia in the fetus leads to increased production of fetal reticulocytes from the bone marrow and erythroblasts from the fetal liver. There is decreased albumin synthesis by the liver, and increased cardiac output in an attempt to increase oxygen delivery to tissues. Anemia leads to hyperbilirubinemia and erythroblastosis fetalis. Both the fetal liver and spleen can become enlarged. Tissue hypoxia from edema may increase capillary permeability contributing to the evolution of hydrops fetalis, the end-stage of the condition. Hydrops fetalis (defined as fluid collections in two compartments: ascites, pleural effusion, scalp edema) occurs with hemoglobin deficits of 7 g/dL or greater compared to the mean hemoglobin value for the gestational age. Early detection of anemia and administration of intrauterine transfusion can prevent these sequelae.

Complications as a consequence of management

Amniocentesis and cordocentesis are invasive procedures that bear with them risks of fetal loss. Cordocentesis at the umbilical insertion site or midsegment of the cord is associated with fetal bradycardia, and so sampling and intrauterine transfusion should be performed at the placental insertion of the cord or the intrahepatic vein. Absorption of transfused cells is limited when intraperitoneal infusion of blood transfusion is employed. Fetal loss rate is 1% to 2% per procedure, with an approximately 5% overall fetal loss rate. With transfusions greater than 32 weeks gestation, the rate of emergency cesarean due to fetal distress after cordocentesis is approximately 5%.

5. Prognosis and outcome

Maternal and fetal/neonatal outcomes

Overall survival of fetuses with alloimmunization who require intrauterine transfusion has been reported at 89% to 92%. Each procedure carries a 1% to 2% rate of fetal loss. Pregnancies complicated by hydrops fetalis may have a decreased survival of 78%. Short-term studies have shown normal neurologic development in 94% of surviving infants up to age 2 years. Elevated bilirubin levels may be associated with hearing loss in the neonate. Fetuses affected by hydrops are at risk for severe neurologic morbidity (cerebral palsy, deafness, developmental delay). Some neonates will require exchange transfusions or phototherapy.

Long-term health impact

A patient’s first pregnancy complicated by alloimmunization usually involves minimal fetal or neonatal disease, but subsequent pregnancies are commonly associated with worsening disease.

In patients with previously affected pregnancies, maternal titers should not be used to guide the need for fetal surveillance. Sensitized women should not be given RhIG prophylaxis. In those pregnancies, sonographic evaluation of MCA peak systolic velocity to detect anemia should begin at 16 to 18 weeks gestation.

Women with prior pregnancies affected by hemolytic disease of the fetus or newborn should be managed by centers with experience managing severe alloimmunization in pregnancy. Maternal titers may not be predictive of clinical sequelae in these cases. In patients with loss in the early second trimester due to hemolytic disease of the fetus, options may be limited as cordocentesis and intrauterine transfusion are particularly limited in a pregnancy less than 22 weeks gestation. These patients should be offered intrauterine insemination with RhD negative donor sperm, preimplantation genetic diagnosis (for paternal heterozygous genotype), or surrogate pregnancy.

6. What is the evidence for specific management and treatment recommendations

(ACOG guidelines for management of alloimmunization provide practical guidance for clinicians managing pregnancies impacted by this condition.)

(ACOG guidelines for prevention of Rh D alloimmunization revisit the pathophysiology behind the condition and the importance of prevention, and provide practical guidance.)

Creasy, RK, Resnick, R, Iams, JD, Lockwood, CJ, Moore, TR, Greene, MF. “Creasy & Resnick's maternal-fetal medicine: Principles and practice”. 2014. (This text reviews the pathophysiology behind the condition and the mechanisms of action of treatments for theprevention and treatment of alloimmunization.)

Gabbe, S, Niebyl, J, Simpson, J. “Obstetrics: normal and problem pregnancies”. 2007. (This text reviews the pathophysiology behind the condition and the mechanisms of action of treatments for the prevention and treatment of alloimmunization.)

Moise, KJ, Argoti, PS. “Management and prevention of red cell alloimmunization in pregnancy: a systematic review”. Obstet Gynecol. vol. 120. 2012. pp. 1132-9. (This review article collates the scientific evidence behind our current understanding of red cell alloimmunization.)

Schenone, MH, Mari, G. “The MCA Doppler and its role in the evaluation of fetal anemia and fetal growth restriction”. Clin Perinatol. vol. 38. 2011. pp. 83-102, vi. (This article reviews in greater detail the use of ultrasonographic Doppler measurements to evaluate anemia, which is an important part of the management of alloimmunization in pregnancy.)

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