Hemolytic Disease of the Fetus and Newborn

Maternal antibody transfer to the fetal circulation

Small volumes of fetal red cells commonly cross into the maternal circulation, and the frequency of detectable transfer rises with gestational age. Amniocentesis, chorionic villus sampling, and abdominal trauma all increase the chance of a larger fetomaternal hemorrhage, although most episodes remain small.^{11 Bloch EM, et al. Technical Manual. 2026.} Because every fetal antigen foreign to the mother was inherited from the father, who contributes only one allele, the fetal antigen dose corresponds to heterozygous expression.

The immune response to D varies by exposure and patient. Large experimental transfusion exposures produced high sensitization rates, while modern clinical cohorts report lower rates and substantial variation.^{2,32 Avent ND, et al. The Rh blood group system. 2000.3 Ji Y, et al. Incidence of anti-D alloimmunization in D-negative individuals… 2022.} Without Rh immune globulin prophylaxis, a D-negative mother carrying a D-positive fetus has a roughly 13 to 16% risk of forming anti-D after delivery. Routine postpartum and antepartum prophylaxis reduce that risk markedly.^{44 American College of Obstetricians and… Practice Bulletin No. 2017.}

Only IgG crosses the placenta, through neonatal Fc receptor (FcRn)-mediated transport that is detectable from about 13 weeks, becomes substantial during the second trimester, and reaches its maximum near term.^{55 Palmeira P, et al. IgG placental transfer in healthy and pathological pregnancies. 2012.} All four IgG subclasses cross, but IgG1 and IgG3 are substantially more efficient at driving red-cell hemolysis than IgG2 and IgG4, so disease severity depends on the maternal subclass distribution as well as on antibody presence. Subclass alone does not predict severity.^{5,65 Palmeira P, et al. IgG placental transfer in healthy and pathological pregnancies. 2012.6 Velkova E. Correlation between the amount of anti-D antibodies and IgG… 2015.}

Antibody-coated fetal cells are cleared extravascularly by fetal splenic macrophages at a rate set by antibody titer, specificity, and fetal red-cell antigen density. The fetal marrow accelerates erythropoiesis and releases nucleated erythroblasts into the circulation, the finding that gave the disease its historical name, erythroblastosis fetalis. When marrow output can no longer keep pace, erythropoiesis expands into the liver and spleen, producing hepatosplenomegaly. Severe anemia combined with hypoproteinemia from reduced hepatic synthesis can progress to high-output cardiac failure with generalized edema, effusions, and ascites, a condition called hydrops fetalis. Hydrops has appeared as early as 18 to 20 weeks of gestation, and before modern intervention it was almost uniformly fatal.^{7,87 Moise KJ Jr. Considerations and approaches for early onset fetal anemia due… 2025.8 Hall V, et al. Hemolytic disease of the fetus and newborn. 2025.} Hemolysis continues after birth for as long as maternal IgG persists in the newborn's circulation. Because IgG has a half-life of about 25 days, hemolysis can continue for days to weeks even though no new maternal antibody is being delivered.^{55 Palmeira P, et al. IgG placental transfer in healthy and pathological pregnancies. 2012.}

Bilirubin handling differs before and after birth. Hemoglobin released by red-cell destruction is converted to unconjugated bilirubin, which crosses the placenta and is conjugated and excreted by the maternal liver, so elevated fetal bilirubin alone does not cause fetal disease. After delivery this placental route disappears. The newborn liver, especially in a premature infant, conjugates bilirubin poorly, so unconjugated bilirubin accumulates and, if untreated, can cross into the central nervous system as kernicterus.^{1,91 Bloch EM, et al. Technical Manual. 2026.9 Kemper AR, et al. Clinical practice guideline revision. 2022.}

ABO incompatibility between mother and fetus is partially protective. The presumed mechanism is that when a group O mother carries a group A or B fetus, maternal anti-A and anti-B hemolyze ABO-incompatible D-positive fetal cells before the immune system recognizes D, and both the rate of detectable fetomaternal hemorrhage and the rate of D immunization fall.^{44 American College of Obstetricians and… Practice Bulletin No. 2017.} ABO incompatibility reduces risk but never replaces prophylaxis.

Disease-associated antibody specificities

Anti-D can cause severe hemolytic disease of the fetus and newborn (HDFN), and group O D-negative red-cell inventory is limited. AABB Association Bulletin 19-02, revised in 2026, recommends reserving group O D-negative red cells for specified cohorts of patients with childbearing potential. A transfusion is not withheld during life-threatening bleeding. Emergency release, shortages, and local inventory are managed under the transfusion service's approved policy.^{1010 AABB. Association Bulletin #19-02. 2019.}

Maternal testing through pregnancy

Baseline and repeat screening

ABO and D typing plus an antibody screen are performed early in pregnancy, together with a history of prior pregnancies, their outcomes, and transfusions. Previous severe hemolytic disease places the current pregnancy at high risk regardless of the present antibody level. The screen uses a validated indirect antiglobulin method capable of detecting clinically significant antibodies. The laboratory's approved cells, method, and enhancement system define its sensitivity.^{1414 Regan F, et al. Guideline for the investigation and management of red cell… 2025.}

Repeat testing follows obstetric and laboratory policy. A common schedule repeats ABO and D typing and the antibody screen at 28 weeks, with the sample from an eligible D-negative patient collected before antenatal Rh immune globulin. Additional testing may be needed after transfusion, a sensitizing event, or a previous unexpected antibody.^{1414 Regan F, et al. Guideline for the investigation and management of red cell… 2025.}

Antibody identification in pregnancy

Cold-reactive IgM specificities such as anti-I, anti-IH, anti-Le^a, anti-Le^b, and anti-P1 usually need no HDFN follow-up when they behave as expected. Lewis antibodies are common in pregnancy and are not recognized causes of disease. Anti-M and anti-N may be IgM, IgG, or a mixture. Sulfhydryl reagents reduce disulfide bonds and disrupt IgM while preserving IgG reactivity, which helps determine whether an IgG component is present.^{11 Bloch EM, et al. Technical Manual. 2026.}

Passive anti-D can remain detectable for months after Rh immune globulin, typically fading within 3 to 6 months, and no single laboratory test cleanly separates passive from immune anti-D. Interpret any anti-D result with the pre-Rh immune globulin antibody screen, the dose and timing of prior Rh immune globulin, the test method, and the serial trend. Reaction strength alone is unreliable, and prophylaxis continues when immune anti-D has not been established. An apparent anti-C plus anti-D pattern may instead be anti-G or anti-C plus anti-G. Send this pattern to a reference laboratory for resolution, because a patient with anti-G and no immune anti-D remains eligible for Rh immune globulin.^{1414 Regan F, et al. Guideline for the investigation and management of red cell… 2025.} Beyond anti-D, the specificities most often clinically significant in pregnancy are anti-K, anti-E, anti-c, anti-C, and anti-Fy^a.^{11 Bloch EM, et al. Technical Manual. 2026.}

Paternal and fetal genotyping

Testing the father's phenotype, or his DNA-based genotype and zygosity for the corresponding antigen, can narrow the workup substantially. Only about 9% of people of European ancestry carry K. The antigen is less common in some populations, around 2% in people of Black African ancestry, but it reaches up to 25% in some Arab populations. When paternity is certain, a K-negative father rules out fetal K expression and ends the concern entirely. Uncertain paternity removes that reassurance.^{1515 Dean L. The Kell blood group. 2005.} When the father is heterozygous or his testing is unavailable, fetal antigen status can be determined invasively by chorionic villus sampling at about 10 to 13 weeks or by amniocentesis from about 15 weeks. In an alloimmunized pregnancy, amniocentesis is preferred over chorionic villus sampling because it carries a lower risk of worsening fetomaternal hemorrhage.^{1616 Rink BD, et al. Society for Maternal-Fetal Medicine Statement. 2026.}

Testing cell-free fetal DNA in maternal plasma avoids an invasive procedure. Available assays, validated antigens, and eligible populations vary. Published United States validation includes D, C, c, E, K, and Fy^a. SMFM's 2026 statement keeps amniocentesis as the standard fetal antigen test when an alloimmunized patient's partner is heterozygous or unavailable. Cell-free DNA may be offered after counseling when the patient declines diagnostic testing or empiric monitoring. A presumed antigen-negative result does not by itself establish that surveillance can stop. SMFM does not recommend this approach for a prior pregnancy affected by HDFN, a clinically significant non-D alloantibody, or a known weak-D or other nondeletion RHD variant. Routine fetal D screening is not recommended for a nonalloimmunized D-negative patient when Rh immune globulin is readily available, although it may be considered with counseling and local policy during a shortage.^{1616 Rink BD, et al. Society for Maternal-Fetal Medicine Statement. 2026.}

Titration and the critical titer

Titer follows the relative concentration of an antibody capable of crossing the placenta. Test serial samples by the same validated method, with the stored previous sample tested in parallel when possible. Results from different methods or laboratories are not interchangeable, so each laboratory defines a significant change and communicates the method used.^{1414 Regan F, et al. Guideline for the investigation and management of red cell… 2025.}

A critical titer is the method- and laboratory-validated titer at which the obstetric service begins fetal-anemia surveillance. It is not a universal number, and no single titer distinguishes passive from immune anti-D. The 2025 Moise et al. guideline recommends thresholds of 16 or greater for most antibodies and 4 or greater for anti-K on very-low-certainty evidence, while a previous affected fetus or neonate warrants surveillance regardless of the current titer. The laboratory reports anti-K to the obstetric service promptly, because its relationship with fetal anemia is less predictable than that of many other antibodies.^{14,1714 Regan F, et al. Guideline for the investigation and management of red cell… 2025.17 Moise KJ Jr, et al. A clinical practice guideline for the management of pregnancy… 2025.} Middle cerebral artery peak systolic velocity (MCA-PSV) above 1.5 multiples of the median (MoM) is a screening threshold for moderate-to-severe fetal anemia. Amniotic-fluid bilirubin analysis by delta OD450 (the optical density difference at 450 nm) is a historical method and is not used to diagnose fetal anemia.^{1818 Mari G, et al. SMFM Clinical Guideline #8. 2015.}

Fetal assessment and transfusion-service support

Fetal anemia can raise middle cerebral artery flow velocity. The maternal-fetal medicine service interprets Doppler findings against gestational-age reference values and the pregnancy history. Moise et al. recommend weekly MCA-PSV surveillance by 16 weeks when the fetus is antigen positive or its antigen status is unknown and the critical titer has been reached. This is an expert-opinion recommendation supported by very-low-certainty evidence. The laboratory supplies the antibody specificity, method, titer trend, and previous affected-pregnancy history to that service. It does not set the surveillance schedule.^{17,1817 Moise KJ Jr, et al. A clinical practice guideline for the management of pregnancy… 2025.18 Mari G, et al. SMFM Clinical Guideline #8. 2015.} MCA-PSV replaced amniotic-fluid delta OD450 as the primary noninvasive monitoring tool, and delta OD450 survives only as a historical method.^{1818 Mari G, et al. SMFM Clinical Guideline #8. 2015.}

Cordocentesis yields fetal blood for hemoglobin, hematocrit, bilirubin, blood type, direct antiglobulin testing, and antigen phenotype or genotype. A repeatedly elevated MCA-PSV above 1.5 MoM or fetal hydrops should be communicated promptly to the maternal-fetal medicine service. That service decides whether fetal blood sampling or intrauterine transfusion is needed.^{1818 Mari G, et al. SMFM Clinical Guideline #8. 2015.}

When intrauterine transfusion is ordered, the transfusion service prepares red cells and follows the maternal antibody history, fetal testing, and posttransfusion hematocrit. The fetal-therapy service sets the target hematocrit and timing of later procedures. Preserve the transfusion history for interpretation of postnatal anemia and later compatibility testing, because transfused cells suppress fetal erythropoiesis.^{1717 Moise KJ Jr, et al. A clinical practice guideline for the management of pregnancy… 2025.} Cordocentesis, intrauterine transfusion, and amniocentesis can increase fetomaternal hemorrhage. The laboratory therefore treats an invasive fetal procedure as important context for maternal antibody testing, hemorrhage quantitation, and Rh immune globulin support under local policy.^{11 Bloch EM, et al. Technical Manual. 2026.}

For intrauterine transfusion, donor red cells are crossmatch-compatible with maternal plasma, ABO compatible with the mother and fetus, antigen-negative for the maternal antibody, Hb S-negative, irradiated, and leukocyte-reduced or otherwise CMV-risk-reduced according to policy. A hematocrit of about 75% to 80% corrects severe anemia in a small volume.^{19,2019 Vikraman SK, et al. SFM Fetal Therapy Practice Guidelines. 2023.20 New HV, et al. Guidelines on transfusion for fetuses, neonates and older… 2016.}

Neonatal evaluation

Serologic testing

Newborn ABO antigens are not fully developed, so forward typing may react more weakly than in older children, and reverse grouping cannot confirm the ABO group because newborns lack their own isoagglutinins. Any anti-A or anti-B in neonatal serum is maternal in origin.^{2020 New HV, et al. Guidelines on transfusion for fetuses, neonates and older… 2016.} D typing can occasionally yield a false-negative result called blocked D, when the infant's red cells are so heavily coated with maternal anti-D that reagent anti-D cannot bind additional sites. An eluate may identify the bound antibody as anti-D, and the infant's true D status is determined by a validated antibody-removal and retyping method or by molecular testing.^{2121 Wang H, et al. A case of a newborn with blocked RhD antigen and hemolytic… 2017.}

The DAT with anti-IgG is one part of the neonatal evaluation. It identifies antibody coating. It does not prove hemolysis or measure disease severity. A negative DAT does not by itself exclude immune hemolysis, including ABO HDFN, and a positive DAT can occur without clinical hemolysis, for example after maternal antenatal Rh immune globulin.^{1,9,221 Bloch EM, et al. Technical Manual. 2026.9 Kemper AR, et al. Clinical practice guideline revision. 2022.22 Bahr TM, et al. Neonatal and obstetrical outcomes of pregnancies complicated by… 2024.} Routine elution on every DAT-positive infant is unnecessary. Elution is valuable when clinically important hemolysis is present and the antibody specificity is uncertain, including suspected ABO HDFN or a blocked-D typing result.^{2020 New HV, et al. Guidelines on transfusion for fetuses, neonates and older… 2016.}

Cord blood hemoglobin provides a useful baseline without removing additional blood from the infant. Interpret it by gestational and postnatal age, then follow the clinical and laboratory trend. Transfusion decisions also weigh symptoms, respiratory support, the cause of anemia, and the applicable neonatal protocol.^{23,2423 Jopling J, et al. Reference ranges for hematocrit and blood hemoglobin… 2009.24 Kaufman DA, et al. Postnatal cord blood sampling. 2025.}

Component selection

Group O red cells are commonly used for intrauterine and exchange transfusion when compatible with the mother and infant, and red cells for exchange may be resuspended in compatible plasma to limit the ABO antibody transfused. For routine neonatal transfusion, select units that are ABO and D compatible with the mother and infant, lack the antigen corresponding to every clinically significant maternal antibody, and are IAT crossmatch-compatible with maternal plasma when it is available. Prematurity alone is not an indication for irradiation. Exchange and routine neonatal transfusions follow different component specifications in the laboratory's policy.^{2020 New HV, et al. Guidelines on transfusion for fetuses, neonates and older… 2016.}

Laboratory findings and clinical handoff

The laboratory reports bilirubin with collection time and age after birth so the neonatal service can assess the value and rate of rise against an age-specific treatment threshold. Hemoglobin or hematocrit, reticulocyte count, DAT, antibody identification, and blood film findings help establish the cause and severity of hemolysis. Phototherapy, intravenous immune globulin, and exchange transfusion are clinical decisions made under the applicable neonatal guideline.^{9,259 Kemper AR, et al. Clinical practice guideline revision. 2022.25 Bhutani VK, et al. Phototherapy to prevent severe neonatal hyperbilirubinemia in… 2024.} If exchange transfusion is ordered, the transfusion service supplies a compatible component that meets the neonatal specifications in local policy and communicates any compatibility limitation or delay immediately.

Rh immune globulin prophylaxis

Rh immune globulin supplies passive anti-D to prevent active maternal immunization to D. Proposed mechanisms include rapid removal of D-positive fetal cells from the maternal circulation and suppression of D-specific B-cell activation. No single mechanism fully explains the protective effect.^{2626 Brinc D, et al. Mechanisms of anti-D action in the prevention of hemolytic… 2009.}

Without prophylaxis, a D-negative mother carrying a D-positive fetus has a roughly 13 to 16% chance of forming anti-D after delivery. Postpartum prophylaxis lowers the risk to about 0.5 to 1.8%, and adding routine antepartum prophylaxis lowers it further to about 0.14 to 0.2%. Even a small fetal-cell exposure can immunize a susceptible patient, and the dose needed for an exposure cannot be inferred from whether anti-D is detectable.^{4,274 American College of Obstetricians and… Practice Bulletin No. 2017.27 AABB. Association Bulletin #24-02. 2024.}

Clinical context for laboratory support

For an unsensitized D-negative patient, or a patient with a D variant managed as being at risk for anti-D, the obstetric service determines prophylaxis from gestational age, event type, immune status, product labeling, and local policy. The laboratory collects the antibody-screen specimen before routine prophylaxis when possible. Invasive fetal procedures, bleeding, trauma, external cephalic version, pregnancy loss, delivery, and accidental D-positive red-cell exposure are events that may trigger D testing, antibody investigation, or fetomaternal hemorrhage quantitation.^{27,2827 AABB. Association Bulletin #24-02. 2024.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.}

For an eligible mother whose infant is D-positive or whose D status cannot be established, the postpartum target is administration as soon as possible and within 72 hours. The transfusion service reports eligibility and hemorrhage-testing results promptly. If the target is missed, the obstetric service reviews the case under the product label and local policy.^{4,27,284 American College of Obstetricians and… Practice Bulletin No. 2017.27 AABB. Association Bulletin #24-02. 2024.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.}

Before 12 weeks, ACOG and the Society for Maternal-Fetal Medicine differ on routine D testing and prophylaxis after pregnancy loss. The obstetric service follows its adopted policy. The laboratory performs D testing and fetomaternal hemorrhage studies when the event and policy require them.^{29,3029 American College of Obstetricians and… Rh D immune globulin administration after abortion or pregnancy… 2024.30 Prabhu M, et al. Society for Maternal-Fetal Medicine Statement. 2024.}

Laboratory eligibility and dose support

The postpartum pathway is conditional:

1. Establish the mother's D status. A clearly D-positive patient does not need anti-D prophylaxis. A weak, partial, discrepant, or indeterminate D result requires molecular testing or management under the laboratory's D-variant policy. The 2015 multisociety statement supports D-positive management for weak D types 1, 2, and 3. AABB Association Bulletin 24-02 also lists type 4.1 as not at risk for D alloimmunization. Type 4.1 and the other variants are handled according to the genotype report and the transfusion service's adopted policy. An unresolved variant is managed as being at risk.^{27,3127 AABB. Association Bulletin #24-02. 2024.31 AABB, et al. Joint Statement on Phasing-In RHD Genotyping for Pregnant Women… 2015.}
2. Determine whether anti-D is immune. Confirmed immune anti-D makes prophylaxis ineffective. Documented passive anti-D after prophylaxis, or anti-D whose status remains uncertain, does not by itself make the patient ineligible.^{14,2814 Regan F, et al. Guideline for the investigation and management of red cell… 2025.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.}
3. Test the infant for D, including a serologic weak-D test when the initial result is D-negative. Prophylaxis is unnecessary only when both results are negative. An unavailable, indeterminate, discrepant, or blocked-D result is managed as D-positive or unknown until resolved.^{2727 AABB. Association Bulletin #24-02. 2024.}
4. For an eligible mother with a D-positive or D-unknown infant, obtain the maternal hemorrhage specimen after delivery and before Rh immune globulin at the interval specified by the laboratory. Report promptly when quantitation is pending so the clinical service can meet the administration target.
5. Use a rosette screen only when the maternal and infant D results make that test valid. A maternal D variant, weakly expressed or blocked infant D, unknown infant D, or another source of unreliable reactivity requires a quantitative method instead. A valid negative screen supports one standard dose. A positive or unsuitable screen is followed by Kleihauer-Betke testing or flow cytometry and dose calculation.^{2727 AABB. Association Bulletin #24-02. 2024.}

Maternal anti-D strength or titer does not measure fetomaternal hemorrhage volume and is not used to calculate the postpartum dose.^{14,27,2814 Regan F, et al. Guideline for the investigation and management of red cell… 2025.27 AABB. Association Bulletin #24-02. 2024.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.}

Quantifying large hemorrhages

The laboratory specifies when to collect the postpartum maternal specimen so fetal cells have mixed into the circulation while prophylaxis can still be given promptly. A rosette screen detects D-positive fetal cells in a clearly D-negative mother and therefore depends on valid maternal and infant D results. It can miss a weakly expressed fetal D antigen or produce misleading reactivity when the mother has a D variant.^{2727 AABB. Association Bulletin #24-02. 2024.}

The Kleihauer-Betke test acid-treats and counterstains a maternal blood film. Fetal cells retain stain because they contain acid-resistant fetal hemoglobin, while most maternal cells appear as pale ghosts. The laboratory's validated counting and calculation procedure converts the fetal-cell percentage to an estimated hemorrhage volume. Maternal F cells can produce a falsely high estimate, and differences in staining and counting add imprecision. Flow cytometry can provide a more precise measurement when available, although an anti-hemoglobin F assay must still account for maternal F cells.^{2727 AABB. Association Bulletin #24-02. 2024.}

One 300 microgram (1,500 IU) dose covers 30 mL of D-positive whole blood or 15 mL of D-positive red cells. The current RhoGAM label directs the calculated result to be rounded up to the next whole syringe.^{27,2827 AABB. Association Bulletin #24-02. 2024.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.} The fetal-cell count is imprecise, so the AABB calculation adds a margin: round to the nearest whole syringe (a fraction of .5 or more rounds up), then add one syringe. The result is never below the label's rounded-up count.^{11 Bloch EM, et al. Technical Manual. 2026.}

Product-specific reporting

The laboratory report states the estimated fetomaternal hemorrhage, the product-specific coverage, and the calculated number of whole syringes or vials. The clinical service confirms the order and administration. Route, dose form, contraindications, and warnings are product-specific. RhoGAM and MICRhoGAM, for example, are labeled for intramuscular use, while some other anti-D products permit intravenous or intramuscular administration. The transfusion service should identify the product actually supplied so the clinical service can follow its label.^{27,2827 AABB. Association Bulletin #24-02. 2024.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.}

Pitfalls

Rh immune globulin cannot prevent anti-D formation once immune anti-D is established. Passive and immune anti-D are separated with the pre-dose result, timing and dose of prophylaxis, test method, history, and serial behavior. No single reaction strength or titer proves the distinction. While the status remains uncertain, an otherwise eligible patient continues to receive prophylaxis. Maternal D variants and a newborn's blocked-D result require resolution under the laboratory's policy. Rh immune globulin is administered to the mother, never to the newborn. Before each postpartum dose decision, check the pre-dose screen and the prophylaxis record, because together they help separate passive from immune anti-D.^{14,27,2814 Regan F, et al. Guideline for the investigation and management of red cell… 2025.27 AABB. Association Bulletin #24-02. 2024.28 Kedrion Biopharma Inc. RhoGAM Ultra-Filtered PLUS and MICRhoGAM Ultra-Filtered PLUS… 2025.}

ABO hemolytic disease

Maternal IgG anti-A, anti-B, or anti-A,B can cross the placenta and bind ABO-incompatible fetal red cells. Severe fetal anemia is rare. The usual concern is hemolysis with early neonatal hyperbilirubinemia. Jaundice during the first 24 hours requires prompt bilirubin measurement and clinical evaluation.^{9,329 Kemper AR, et al. Clinical practice guideline revision. 2022.32 Jajosky RP, et al. ABO blood group antigens and differential glycan expression. 2023.}

ABO disease is usually mild because ABO antigens are not fully developed on fetal and neonatal red cells. Antigen expression increases after birth and approaches adult levels at about 2 to 4 years. Neonatal red cells therefore offer fewer A or B sites for antibody binding. Maternal IgG concentration, subclass, antigen density, and neonatal bilirubin handling all contribute, so a high maternal titer does not determine an individual infant's course.^{32,3332 Jajosky RP, et al. ABO blood group antigens and differential glycan expression. 2023.33 Wiener AS, et al. Pathogenesis of ABO hemolytic disease. 1960.} Neonatal ABO typing uses reagents and methods validated for weak antigen expression.^{11 Bloch EM, et al. Technical Manual. 2026.}

High-titer IgG anti-A,B occurs more often in group O individuals than in group A or B individuals, so most ABO HDFN affects a group A or B infant of a group O mother. Rare cases occur with a non-O mother. Naturally occurring ABO antibodies can be present before pregnancy, allowing a first pregnancy to be affected, and recurrence and severity vary between pregnancies.^{1,32,331 Bloch EM, et al. Technical Manual. 2026.32 Jajosky RP, et al. ABO blood group antigens and differential glycan expression. 2023.33 Wiener AS, et al. Pathogenesis of ABO hemolytic disease. 1960.}

Microspherocytes are common on the neonatal blood film in ABO HDFN, while severe Rh-system disease more often produces marked anemia and circulating erythroblasts. DAT strength and eluate reactivity do not measure clinical severity. Follow the bilirubin rate of rise, hemoglobin or hematocrit, reticulocyte count, blood film, feeding, gestational age, and other neurotoxicity risk factors. Phototherapy is usually sufficient. For a DAT-positive infant at least 35 weeks' gestation, intravenous immune globulin is considered only under the restricted AAP circumstances.^{9,32,339 Kemper AR, et al. Clinical practice guideline revision. 2022.32 Jajosky RP, et al. ABO blood group antigens and differential glycan expression. 2023.33 Wiener AS, et al. Pathogenesis of ABO hemolytic disease. 1960.}

Maternal ABO antibody class and titer have limited value for prenatal prediction, so ABO disease is recognized after birth, and no single test establishes it. Obtain the infant's blood type and DAT when the maternal antibody screen is positive or unknown, or when the clinical course warrants testing. For an infant of a group O D-positive mother with a negative antibody screen, testing may be omitted when reliable bilirubin surveillance and follow-up are available. Interpret the DAT with the bilirubin trajectory and evidence of hemolysis. Cord blood collection by venipuncture can be used when testing is indicated, with care to avoid contamination by maternal blood or Wharton's jelly.^{9,249 Kemper AR, et al. Clinical practice guideline revision. 2022.24 Kaufman DA, et al. Postnatal cord blood sampling. 2025.}

Practice