Management of Red Cell Alloimmunization in Pregnancy : Obstetrics & Gynecology (2024)

Rhesus alloimmunization in pregnancy and the associated disease in the pregnant patient's offspring—hemolytic disease of the fetus and newborn (HDFN)—have become rare entities in many countries because of the widespread adoption of antenatal and postpartum use of Rhesus immune globulin (RhIG). However, a recent U.S. study of more than 9 million prenatal screens between 2010 and 2021 found that anti-D antibody was present in 586 of 100,000 pregnancies.¹ A review of neonatal discharge diagnoses in the United States between 1996 and 2010 showed a rate of HDFN secondary to RhD disease of 44.4 cases per 100,000 births.² In countries with limited access to Rhesus immune prophylaxis, stillbirth related to HDFN still occurs in 14% of affected pregnancies, and 50% of newborn survivors either die in the neonatal period or develop cerebral injury.³

PATHOPHYSIOLOGY

When fetal red cells that express antigens dissimilar from the pregnant patient gain access to the maternal circulation, the maternal immune system can be activated. β-Lymphocyte clones that recognize the foreign red cell antigen are established. The initial maternal production of immunoglobulin M (IgM) is short lived and is followed by a rapid change to an immunoglobulin G (IgG) response (usually IgG₁ and IgG₃ subclass).⁴ Once there has been a secondary exposure to the offending red cell antigen, memory β-lymphocytes differentiate into plasma cells to produce antibody. The maternal antibody level typically will not be detected by indirect Coombs testing until 5–16 weeks later. The maternal IgG then will be actively transported to the fetal circulation by the neonatal Fc receptor in the placenta. After attaching to the fetal erythrocytes expressing the red cell antigen incompatible with the pregnant patient, the cells will be sequestered in the fetal spleen where they undergo extravascular hemolysis. Although the fetus can respond with increasing reticulocytes and erythroblasts, without intervention, severe anemia eventually occurs, leading to the development of hydrops fetalis, a collection of fluid in at least two serous compartments. Although ultrasonographic detection of hydrops once was used a diagnostic criterion for the need for fetal intervention, this is no longer the case with the advent of middle cerebral artery peak systolic velocity Doppler. The exact mechanism for the development of hydrops is poorly understood because it may be absent in the early second trimester even in the face of severe anemia.

PREVENTION OF ALLOIMMUNIZATION

Formulations of Rhesus Immune Globulin

All current RhIG products available in the United States (RhoGAM, Kedrion Biopharma, Inc; HyperRHO S/D, Grifols, S.A; Rhophlac, CSL Bering, AG; and WinRHO SDF, Kamada Pharmaceuticals, Inc) are polyclonal antibody products derived from human plasma. The last two products are purified by ion-exchange chromatography and can therefore be administered by either the intravenous or intramuscular route. All current products undergo micropore filtration to eliminate viral transmission. To date, no reported cases of viral infection related to RhIG administration have been reported in the United States, although an outbreak of hepatitis C related to RhIG was reported in Ireland in the 1970s.⁵

Indications and Recommendations for Rhesus Immune Globulin Administration

All pregnant patients should undergo an antibody screen at the first prenatal visit. If there is no evidence of anti-D alloimmunization in the RhD-negative pregnant person, patients are candidates for RhIG. Evidence is lacking for the use of RhIG in association with such clinical events as first-trimester bleeding, ectopic pregnancy, hydatidiform mole, genetic amniocentesis and chorion villus biopsy, blunt trauma to the abdomen, and external cephalic version. In general, the American College of Obstetricians and Gynecologists (ACOG) recommends consideration for RhIG in these situations.⁶

The RhD antigen has been demonstrated on fetal red blood cells (RBCs) as early as 38 days after conception.⁷ In 2022, the Society of Family Planning recommended that Rh testing and administration of RhIG not be undertaken in patients undergoing spontaneous abortion or induced abortion through medication or uterine aspiration before 12 weeks of gestation.⁸ A flow cytometry study in 506 patients undergoing first-trimester abortions was recently published in support of this policy.⁹ In February 2024, the Society for Maternal-Fetal Medicine released an official statement in support of ACOG's previous recommendation for the continued administration of RhIG for spontaneous and induced abortions before 12 weeks of gestation, citing a paucity of convincing data to suggest the safety of withholding prophylaxis.¹⁰

Although postpartum administration of RhIG has been routine practice in industrialized countries for the past five decades, antenatal administration has not been universally accepted outside of North America until more recently. In a Cochrane meta-analysis, McBain et al¹¹ found that 100 micrograms of RhIG given at 28 and 34 weeks of gestation reduced the risk of RhD alloimmunization during or immediately after the first pregnancy from about 1% to about 0.2%.

In the United States, RhD-negative patients who are not alloimmunized should receive 300 micrograms of immune globulin at 28 weeks of gestation.⁶ A repeat antibody screen at that gestation is also recommended by ACOG. Although severe HDFN in the case of a negative early antibody screen is a rare event, this practice provides the opportunity of detecting the potentially salvageable anemic fetus late in pregnancy. A maternal blood sample can be drawn at the same office visit as the RhIG injection because the peak anti-D titer will not occur for 2 to 7 days.¹² After the administration of antenatal RhIG, a large percentage of patients will have evidence of anti-D at low titer (2–4) at term.

In ongoing pregnancies in which RhIG is administered in the first or second trimester, a repeat dose should still be given at 28 weeks of gestation. Alternatively, if the antenatal dose was given in the late second trimester (eg, 22 weeks for suspected placental abruption), the dose should be repeated 12 weeks later (at 34 weeks of gestation in the example scenario).

A standard dose of 300 micrograms of RhIG has proved adequate to prevent sensitization caused by a fetomaternal hemorrhage of 30 mL fetal whole blood. Current recommendations in North America indicate that this dose should be administered within 72 hours of delivery if umbilical cord blood typing reveals an RhD-positive neonate or if fetal antigen status is unknown. Rhesus immune globulin still should be considered even up to 28 days after delivery if the 72-hour window has been missed. If RhIG has been given for prophylaxis for an antenatal event such as an external cephalic version, a repeat dose is unnecessary unless more than 21 days have elapsed.

Approximately 3 in 1,000 deliveries will be associated with an excessive fetomaternal hemorrhage. One case–control study found that cesarean delivery, assisted vagin*l delivery, postterm, blood cell transfusions, and maternal age were risk factors; however, these were present in only 43% of cases of excess fetomaternal hemorrhage.¹³ The Association for the Advancement of Blood and Biotherapies standards indicate that testing for excess fetomaternal hemorrhage should be routinely undertaken after the delivery of an RhD-positive fetus to an RhD-negative patient. Typically, a qualitative test called the rosette test is done to screen for excessive fetomaternal hemorrhage and reported as positive or negative. A negative result warrants administration of a standard 300-microgram dose of RhIG. If the rosette test is positive, a qualitative technique such as Kleihauer–Betke stain or fetal hemoglobin flow cytometry is undertaken. Both qualitative techniques report the percent of fetal cells, which can then be multiplied by the estimated whole blood volume of the mother (a proposed volume of 5,000 mL is used to estimate the maternal blood volume) to calculate the total volume of circulating fetal blood. This is then divided by 30 mL to determine the number of vials of RhIG to administer. If the calculation results in a fraction of a unit of 0.5 or greater, the number of vials required is rounded up to the higher whole integer. Another vial is usually added to the calculation to ensure that sufficient RhIG is administered.

In cases of a large-volume fetomaternal hemorrhage that may accompany third-trimester fetal death or after the transfusion of a mismatched unit of RhD-positive red cells, calculations may indicate the need for a large volume of RhIG. No more than five units of RhIG should be administered by the intramuscular route in one 24-hour period. Alternatively, the calculated total amount can be administered intravenously in divided doses of 3,000 international units or 600 micrograms every 8 hours with either Rhophlac or WinRHO SDF. The possibility of a conception with a new partner after a postpartum sterilization procedure suggests that RhIG should still be considered. In addition, alloimmunization to RhD would limit the availability of blood products if the patient requires a red cell transfusion later in her life.

Rhesus immune globulin has not been shown to be effective once alloimmunization to the RhD antigen has occurred.

THE CONFUSING “WEAK D” RESULT

Originally called Du positive, a weak D blood type result can be expected in 1% of White people, 2.6% of Black people, and 2.7% of people of Hispanic origin.⁶ Research has shown that the individuals with a weak D phenotype can belong to one of two groups; some of these patients have intact D antigens that are expressed in reduced numbers on the surface of the red cells. Genotyping reveals that individuals with weak D types 1, 2, 3, and 4.1 are not at risk of becoming alloimmunized and can be treated as Rh positive. Other individuals with a weak D phenotype (called D variants or partial D) express D antigens on their red cells that are missing parts of the antigen. Thus, this latter group can become alloimmunized to the portion of the RhD antigen that is missing. Although the blood-banking community has recommended genotyping in cases of weak D to decide who is a candidate for RhIG, this has not been adopted by ACOG.¹⁴ Therefore, all individuals with a weak D phenotype should currently be treated as Rh negative and receive appropriate RhIG.⁶

EMERGING ISSUES

Use of Whole Blood in Trauma Resuscitation: A New Potential Source of RhD Alloimmunization

The use of O-positive, low-titer whole blood to manage trauma associated with acute blood loss is currently being studied in several large trials encompassing multiple clinical sites. Because O, RhD-negative blood is in short supply, O, RhD-positive whole blood with low titers to anti-A and anti-B has been studied for use in hospital emergency rooms, ambulances, and life-flight helicopters. Because of suspected immune suppression secondary to trauma, it has been estimated that approximately 17% of RhD-negative women of reproductive age and RhD-negative children exposed to O-positive, low-titer whole blood units will develop Rh alloimmunization.¹⁵ Patients in this category should have an antibody screen performed at 6 months after the transfusion to detect any level of alloimmunization. Use of RhIG in the setting of large-volume RhD-positive transfusion is not well studied for prevention of alloimmunization. In addition, RhIG in this setting can cause massive and overwhelming hemolysis. Therefore, it is not currently advised or standard of care to administer large doses of RhIG after large-volume RhD-positive transfusion because the clinical utility is unestablished and risks are known.

Shortage of RhoGAM in the United States

In January 2024, Kedrion Biopharma, Inc announced a shortage of availability of RhoGAM that was expected to continue through the remainder of year.¹⁶ Many countries around the world use cell-free DNA to determine which RhD-negative nonalloimmunized patients are candidates for antenatal RhIG based on the RHD genotype of the fetus (see the Determination of the Paternal–Fetal Genotype section).¹⁷ In up to 40% of cases, the fetus will be determined to be RHD negative, and RhIG is not administered. On the basis of this long-standing practice in countries outside of the United States and the current availability of accurate assays, an ACOG advisory was issued in April 2024 suggesting that cell-free DNA to determine fetal RHD status could be considered to prioritize the use of RhIG and to conserve the supply.¹⁸

DIAGNOSTIC TOOLS

Antibody Titer

Alloimmunization in pregnancy is first identified when an antibody screen obtained at the first prenatal visit returns positive. The patient's serum is mixed with a panel of indicator RBCs to identify the antibody. If the identified antibody has been associated with HDFN (Table 1), the positive antibody screen should reflex to a titer, although in some cases, this requires a separate order. Often, values for maternal titers are reported as the final serum dilution of the indirect Coombs test (eg, 1:32). However, blood-banking convention dictates that the values should be reported as a reciprocal value of the dilution (eg, 1:16 is equivalent to a titer of 16).

Conflicting results between laboratories are not uncommon because many variations exist between processing and testing procedures and the hemagglutination method used has limitations in precision.¹⁹ Although in the United States there is a push to standardize laboratory technique, some laboratories may still use significantly more sensitive techniques to perform a titer such as gel column agglutination and enzyme treatments. These methods cause a marked elevation in titer compared with the use of nonenzymatic-treated cells and tube methodologies. The clinician should be aware that gel microcolumn assays will result in higher titers than conventional tube testing. In one study, the mean titer was 3.4-fold higher with gel technology.²⁰

Variations in laboratory practices should be taken into consideration when serial titers are run to see whether there has been an increase. For these reasons, serial titers should be run in parallel with stored sera from the previous testing. In the same laboratory, the titer should not vary by more than one dilution if the two samples are run in parallel. Thus, an initial titer of 8 that returns at 16 does not represent a true increase in the amount of antibody in the maternal circulation, whereas a value of 32 would suggest an increase.

The application of a critical titer is used in the first pregnancy when an anti–red cell antibody is identified and may be useful in subsequent pregnancies, depending on fetal antigen status. A critical titer is defined as the anti–red cell titer associated with a significant risk for fetal anemia. When a critical titer is reached, further fetal surveillance is warranted (Fig. 1). This value will vary with institution and methodology; however, a value of 16 is typically used for anti-D and other red cell antibodies. Anti-c and anti-K1 (Kell) are exceptions for which a critical value of 4 or greater should be used until further research is available.^21,22 If below the critical value, maternal titers are repeated at monthly intervals until 24 weeks of gestation, when the frequency in increased to every 2 weeks. A titer still should be obtained early in subsequent alloimmunized pregnancies. In these situations, the clinical course of the HDFN in the preceding pregnancy is more important in guiding management. However, a markedly elevated titer may warrant immunomodulation in the late first trimester (see below).

Determination of the Paternal–Fetal Genotype

Determining paternal zygosity has been proposed as the first step in the management of red cell alloimmunization in pregnancy (Fig. 2).²³ However, several issues may come into play, including nonpaternity, unavailability of the partner, and lack of insurance coverage for testing of the patient's partner. If paternity is ensured, genotype testing for RHD and serologic testing for other red cell antigens should be undertaken to determine zygosity (Table 2). False negatives can occur with the Duffy system, so paternal genotype testing is recommended. In the case of a hom*ozygous partner, one can assume that the fetus carries the putative antigen. With paternal heterozygosity or in other circ*mstances in which the partner cannot be tested, cell-free DNA testing is now available at 10–12 weeks of gestation in the United States for fetal RhD, C, c, E, K and Fy^a red cell antigens.²⁴ This cell-free DNA assay has been evaluated with 1,077 laboratory-based samples; 600 were positive and 461 were negative for the six red cell antigens, yielding a sensitivity of 100% and specificity of 100%. In a second study, this same assay was compared with neonatal genotyping using buccal smears in 156 patients.²⁵ In the 456 antigen calls (145 antigen-positive and 320 antigen-negative cases), there was 100% concordance. More recently, a second assay for fetal RHD genotyping was developed as part of a commercial²⁶ noninvasive prenatal testing in the United States. In 655 RhD-negative patients, cell-free DNA results were compared with neonatal serology and revealed an overall sensitivity of 100%. Two false-positive results occurred in which the fetus was RHD positive by cell-free DNA but RHD negative by cord serology at birth. These data have led ACOG to recommend cell-free DNA for fetal RHD testing among alloimmunized patients with potentially at-risk pregnancies who decline amniocentesis.²⁷

Middle Cerebral Artery for the Detection of Fetal Anemia

The use of pulsed Doppler to determine the peak systolic velocity in the fetal middle cerebral artery as a screening test for fetal anemia is now the standard of care in alloimmunized pregnancies.²⁸ The middle cerebral artery peak systolic velocity determination should be performed by an experienced sonologist because technical issues such as angle of insonation and measurement during fetal quiescence are paramount for accurate determinations.

Measurements can be initiated as early as 15 weeks of gestation and repeated weekly, especially in cases of Kell alloimmunization (in which the onset of fetal anemia can occur rapidly), high-titer cases with other antibodies, or cases with a history of significant early-onset HDFN. In other cases in which the maternal titer is just above a critical threshold, testing can be started as early as 16 weeks of gestation because intraperitoneal intrauterine blood transfusions can be used at this gestation if needed. Detti et al²⁹ recommended weekly middle cerebral artery peak systolic velocity assessment for three measurements followed by assessment of the slope between the values. A middle cerebral artery peak systolic velocity value below 4.7 cm/second/week was associated with the development of only mild fetal anemia. In these cases, with a gradual increase in serial values, middle cerebral artery peak systolic velocity measurements can be repeated at up to 2-week intervals. Because the normal middle cerebral artery peak systolic velocity increases with advancing gestational age, data should be adjusted for gestational age. Web-based calculators such as the those found at https://portal.medicinafetalbarcelona.org/calc-en/ for gestations less than 18 weeks and www.perinatology.com for gestations of 18 weeks or more can be used to convert the actual middle cerebral artery peak systolic velocity to multiples of the median. A value above 1.5 multiples of the median for gestational age can detect virtually all cases of moderate to severe anemia with a false-positive rate of 12%.²⁸

Administration of maternal corticosteroids has been reported to cause a transient decrease in the middle cerebral artery peak systolic velocity.³⁰ Thus, antenatal steroids should be withheld when a patient with an elevated middle cerebral artery peak systolic velocity is referred to a center for intrauterine transfusion so that an accurate preoperative assessment can be undertaken before steroids are administered.

CLINICAL MANAGEMENT OF SUSPECTED FETAL ANEMIA

First Affected Pregnancy (Pregnancy in Which Anti–Red Cell Antibody Is First Detected)

Once a critical titer is reached in the setting of evidence of an antigen-positive fetus, serial middle cerebral artery peak systolic velocity Dopplers are undertaken at weekly intervals starting at 16 weeks of gestation. A middle cerebral artery peak systolic velocity greater than 1.5 multiples of the median is an indication for cordocentesis for fetal hematocrit determination and intrauterine transfusion as needed. Antenatal testing (nonstress test or biophysical profile) should be initiated after 32 weeks of gestation. Induction by 37–38 weeks of gestation should be considered.

Previously Affected Fetus or Neonate

Ideally, a preconceptual visit should be arranged with a maternal–fetal medicine specialist. If the patient has a new partner and there is a history of a red cell alloimmunization in a previous pregnancy, paternal red cell typing and zygosity testing are indicated. A prepregnancy antibody titer will aid in counseling to evaluate whether the patient may be a candidate for immunomodulation in pregnancy (see below). In the case of heterozygous paternal genotype, uncertain paternity, or urgent need for fetal antigen determination, cell-free DNA to determine the fetal genotype can be used early in the pregnancy (Fig. 2). Patients treated with intrauterine transfusions in previous pregnancies and those experiencing perinatal loss should be referred to a perinatal center with experience in the management of severely alloimmunized pregnancies. One respective study found that intrauterine transfusions are typically required in more than 86% of previously treated pregnancies; procedures are usually needed 3 weeks earlier in gestation.³¹ In the at-risk pregnancy, weekly middle cerebral artery peak systolic velocity measurements should be imitated as early as 15 weeks of gestation.

The pregnant patient with a history of early second-trimester onset of severe HDFN in a previous pregnancy is especially challenging. Conception through artificial insemination with RhD-negative donor sem*n, surrogate pregnancy, and preimplantation genetic testing for monogenic traits in the case of a heterozygous paternal genotype should be presented as options to the couple.³² In the case of pregnancy, immunomodulation through plasmapheresis with or without intravenous immune globulin (IVIG) has been reported to result in improved outcomes. One retrospective study of 24 patients with a history of severe hemolytic disease found that if treatment with IVIG was initiated before 13 weeks of gestation, fetal anemia developed 25 days later, and its onset before 20 weeks of gestation was reduced by almost one-third compared with the previous pregnancy with early-onset HDFN.³³

Intrauterine Transfusion

The mainstay of therapy for severe anemia is the intravascular transfusion of compatible donor red cells into the fetal circulation. Intravascular transfusions performed before 20 weeks of gestation are associated with a 10-fold increase in procedure-related fetal loss because of the technical challenges in the size of the umbilical vessels.³⁴ In these cases, serial intraperitoneal transfusions have been used as a bridging technique until a more advanced gestational age is achieved.³⁵

The red cells for intrauterine transfusion are typically blood type O, RhD negative, cytomegalovirus safe, leukoreduced, fresh (fewer than 10 days from collection), and crossmatched to maternal serum. Units are packed to a hematocrit of 75–85% to prevent volume overload in the fetus, followed by 25 Gy gamma irradiation to prevent fetal transfusion associated graft-versus-host disease.

There is considerable variation between centers in the specifics of the intrauterine transfusion procedure. In general, ultrasound guidance is used to target the umbilical vein either at the site of the placenta insertion or in its intrahepatic location. Hematologic parameters are assessed at first entry. At the time of the initial intrauterine transfusion, confirmation of the putative fetal antigen and a direct Coombs should be done. Then, an intravenous paralytic agent is given, followed by a calculated amount of donor blood based on formulas using the estimated fetal weight. Repeat procedures are required with the interval between intrauterine transfusions determined either by empiric timing or through the use of middle cerebral artery peak systolic velocity measurements.³⁶

Short-Term Outcome

Experienced referral centers have reported excellent rates of survival with intravascular intrauterine transfusions. In the largest series reported to date of 1,678 procedures in 589 fetuses treated at a national referral center in the Netherlands between 1988 and 2015, the overall perinatal survival improved from 88.6% in the first 12 years to 97% in the latter period.³⁷ Procedure-related complications per fetus decreased from 9.8% to 3.3%. A study of the effect of clinical experience on perinatal outcomes found that 30–50 procedures are needed during initial training to achieve an optimal outcome.³⁸ This same study suggested that a minimum of 10 procedures annually are necessary to maintain proficiency.

Neonatal Care

Initial care for neonates with HDFN should emphasize the treatment of hyperbilirubinemia (Fig. 3). Obtaining a cord hemoglobin, total bilirubin, and direct Coombs at the time of delivery can give the neonatologists valuable information. The American Academy of Pediatrics has updated their guidelines for the management of jaundice in an effort to prevent bilirubin-induced neurologic damage (formerly called kernicterus).³⁹ Intense phototherapy is the mainstay of treatment with IVIG and ultimately exchange transfusion used for rising levels.

Neonates with HDFN related to anti-Kell require less phototherapy and fewer exchange transfusions because of lower elevations of bilirubin; anti-Kell seems to be a more hypoproliferative form of anemia rather than hemolytic.⁴⁰

In the case of HDFN requiring serial intrauterine transfusions, suppression of fetal erythropoiesis is not uncommon. These neonates are born with a virtual absence of reticulocytes, with their red cell mass composed almost entirely of donor red cells. However, as a result of the extreme suppression of erythropoiesis, they are more likely to need transfusions and experience “late anemia.”

Therefore, it would appear prudent to have these children followed up by a pediatric hematologist with weekly reticulocyte counts and hematocrits for 1 to 3 months until a rising reticulocyte count is noted for at least 2 consecutive weeks (Fig. 3). Recently, a small randomized trial of weekly subcutaneous erythropoietin injections indicated a decreased need for these “top-up transfusions.”⁴¹

Long-Term Outcome

The LOTUS (Long-term Follow-Up After Intra-Uterine Transfusions) study reported the neurodevelopmental outcomes of 281 children with HDFN who were treated with intrauterine transfusions.^42–44 Median age at follow-up was 8.2 years (range 2–17 years). Cerebral palsy was detected in 2.1%, severe developmental delay in 3.1%, and bilateral deafness in 1.0% of the study population, compared with an incidence of cerebral palsy in the general Dutch population of 0.2–0.7% and a rate of severe neurodevelopmental delay of 2.3%. In a multivariate regression analysis including only preoperative risk factors, only severe hydrops fetalis was independently associated with neurodevelopmental impairment (odds ratio 11.2, 95% CI, 1.7–92.7).

NON-RHD ANTIBODIES

Antibodies Not Associated With Hemolytic Disease of the Fetus and Newborn

Antibodies to the red cell antigens Lewis, I, and P are often encountered through antibody screening during prenatal care. Because these antibodies are typically of the IgM class, they are not associated with HDFN. Cold agglutinins and warm autoantibodies are antibodies that bind to self-RBC epitopes. Cold agglutinins are of the IgM class and bind at cooler temperatures; warm autoantibodies are of IgG class and bind at warmer temperatures. These autoantibodies can be found in otherwise healthy individuals and only rarely cause clinical disease such as autoimmune hemolytic anemia. Cold agglutinins are not associated with hemolytic disease of the fetus.⁴³ Warm autoantibodies have rarely been associated with HDFN, but only in mothers experiencing severe clinical disease themselves. For reasons yet unknown, warm autoantibodies typically do not cross the placenta and attach to fetal RBCs.⁴⁴ Thus, other than interfering with laboratory testing, warm and cold autoantibodies are of limited significance.

Antibodies Associated With Hemolytic Disease of the Fetus and Newborn

More than 60 anti–red cell antibodies have been associated with HDFN (Table 1). In the U.S. population, a study between 2010 and 2021 found a 1.5% incidence of red cell antibodies at the initial prenatal assessment.¹ A Swedish study divided these antibodies into three categories (Table 3).⁴⁵ Anti-K and anti-c were the only antibodies included with anti-D in the high-risk category for severe HDFN. This was confirmed by data from a large series of patients from a national referral center in the Netherlands that required intrauterine transfusions for the treatment of severe fetal anemia.⁴⁶ After anti-D, Kell (K1) was the next most common antibody involving severe fetal disease (10% of cases), followed by anti-c (3.5% of cases).

Anti-E

Anti-E in low titer is frequently found during antibody screens in pregnancy because its formation can be related to exposure to naturally occurring substances in the environment. With the advent of gel antibody screens, the detection of anti-E is becoming more common. As stated, it is rarely associated with severe HDFN.

Anti-c

Anti-c is an unappreciated alloantibody of the Rhesus antigen family and should be considered virulent or more virulent than anti-D in causing severe HDFN. A retrospective study of 121 pregnancies suggested that a titer of 4 or more or 7.5 or more international units/mL was associated with HDFN.²¹

Anti-Kell (K1)

The K1 antigen is 1 of 25 antigens in the Kell system. However, antibody to this antigen is the leading cause of HDFN related to Kell. K1 is found on the red cell of 9% of White individuals and 2% of Black individuals, with virtually all antigen-positive individuals being heterozygous (Table 3). These gene frequencies calculate to yield an approximately 5% risk of an affected fetus in Kell alloimmunized pregnancies if paternal antigen status and zygosity are unknown. Because the majority of cases of alloimmunization are the result of transfusion (blood is not routinely crossmatched in the United States for Kell), the first step in the management of these patients should be determining the paternal Kell type. The Kell antibody is noted to cause fetal anemia by two distinct mechanisms: fetal splenic sequestration of sensitized red cells and suppression of fetal erythropoiesis.⁴⁷ For this reason, a lower critical titer of 4 is considered a very conservative threshold.²²

Anti-M and Anti-N

Anti-M and anti-N are naturally occurring IgM antibodies that react at cooler temperatures and therefore are not typically associated with clinical disease. In a North American series of almost 400 cases detected during first-trimester screening, there were no conversion to an IgG response and no association with HDFN.⁴⁸ However, severe HDFN has been reported in the Chinese and Asian literature. Li et al⁴⁹ reported a series of 8 fetuses affected by severe HDFN secondary to anti-M. Two fetuses died secondary to hydrops, and four others required serial intrauterine transfusions. The authors found that anti-M caused a hypoproliferative fetal anemia when the initial maternal IgM response changed to IgG.

Anti-Duffy

The Duffy blood group is a highly immunogenic glycoprotein complex consisting of multiple antigens. The two most common antigens associated with antibodies are Fy^a and Fy^b. Anti-Fy^b antibodies have not been associated with HDFN. Anti-Fy^a is usually associated only with neonatal jaundice, although a rare case of severe fetal disease can occur with an elevated titer.

Anti-Kidd

The Kidd system antigen system consists of three antigens: Jk^a, Jk^b, and Jk3. Antibodies to these antigens are usually associated with only mild HDFN.

Overall Clinical Management

In general, non–anti-RhD antibodies are managed much the same way as anti-D antibodies. Paternal typing can be undertaken through serologic methods because these antigen systems are the result of codominant alleles. Cell-free DNA testing to determine fetal antigen status is available in the United States for K1, c, C, E, and Fy^a.²⁴ Most authorities recommend the use of a critical titer of 16 for non-RhD antibodies. The two exceptions are a titer of 4 for both anti-c and anti-K1. The middle cerebral artery peak systolic velocity appears to remain valid for the detection of fetal anemia.

FUTURE TREATMENT

Nipocalimab is a humanized monoclonal antibody designed to block the neonatal Fc receptor. Its mechanisms of action include decreasing the circulating pool of IgG (thus lowering the maternal titer) and effecting a nonspecific placental blockade of the transfer of IgG. In a phase II clinical trial of 13 patients with early-onset HDFN (history of fetal loss or the need for intrauterine transfusions before 24 weeks of gestation), 54% of patients achieved the primary end point of fetal survival to 32 weeks of gestation without the need for intrauterine transfusions (ClinicalTrials.gov NCT03842189).⁵⁰ Compared with the outcome in their antecedent pregnancy, overall perinatal survival occurred in 5 of 13 pregnancies compared with 12 of 13 pregnancies with nipocalimab treatment. Other secondary outcome parameters, including gestational age at delivery, gestational age at first intrauterine transfusion, number of intrauterine transfusions, and incidence of hydrops, were improved with treatment. These results have led to the initiation of a phase III, randomized, placebo-controlled clinical trial (ClinicalTrials.gov NCT03842189).

CONCLUSION

The treatment of red cell alloimmunization in pregnancy continues to evolve with the advent of cell-free DNA to determine which fetus is at risk of developing anemia and the use of serial middle cerebral artery peak systolic velocity Doppler to decide when intrauterine transfusions are needed. Maternal immunomodulation may one day replace invasive fetal therapy except in the most extreme cases of fetal hydrops.

CME FOR THE CLINICAL EXPERT SERIES

Learning Objectives for “Management of Red Cell Alloimmunization in Pregnancy”

After completing this continuing education activity, you will be able to:

• List the anti–red cell antibodies typically encountered in practice;
• Outline the methods used to determine fetal red cell antigen status;
• Discuss ways to determine whether the pregnancy is at risk of complications from the red cell antibodies; and
• Implement practice changes to optimize care for patients who demonstrated red cell alloimmunization.

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Obstetrics & Gynecology includes CME-certified content that is designed to meet the educational needs of its readers. This article is certified for 2AMA PRA Category 1 Credits.™ This activity is available for credit through October 31, 2027.

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To earn CME credit, you must read the article in Obstetrics & Gynecology and complete the quiz, answering at least 70 percent of the questions correctly. For more information on this CME educational offering, visit the Lippincott CMEConnection portal at https://cme.lww.com/browse/sources/196 to register and to complete the CME activity online. ACOG Fellows will receive 50% off by using coupon code, ONG50.

Hardware/software requirements are a desktop or laptop computer (Mac or PC) and an Internet browser. This activity is available for credit through October 31, 2027. To receive proper credits for this activity, each participant will need to make sure that the information on their profile for the CME platform (where this activity is located) is updated with 1) their date of birth (month and day only) and 2) their ACOG ID. In addition, participants should select that they are board-certified in obstetrics and gynecology.

The privacy policies for the Obstetrics & Gynecology website and the Lippincott CMEConnection portal are available at http://www.greenjournal.org and https://cme.lww.com/browse/sources/196, respectively.

Contact Information

Questions related to transcripts may be directed to [emailprotected]. For other queries, please contact the Obstetrics & Gynecology Editorial Office at [emailprotected]. For queries related to the CME test online, please contact [emailprotected] or 1-800-787-8985.

REFERENCES

Supplemental Digital Content

• AOG_144_4_2024_07_31_TPRMOISE_24-872_SDC1.pdf; [PDF] (530 KB)