# Hemolytic Disease of the Fetus and Newborn

## Introduction

Hemolytic disease of the fetus and newborn (HDFN) results from maternal alloantibodies crossing the placenta and destroying fetal red blood cells. While anti-D prophylaxis has dramatically reduced RhD-related HDFN, other alloantibodies continue to cause significant perinatal morbidity and mortality.

## Pathophysiology

### Maternal Alloimmunization

Alloimmunization occurs when the mother is exposed to fetal red cell antigens she lacks, through **fetomaternal hemorrhage** (FMH) or prior transfusion. The mother produces **IgG alloantibodies** that cross the placenta (IgM does not cross). The first sensitizing pregnancy rarely causes severe disease; subsequent pregnancies with antigen-positive fetuses are at greatest risk due to the **anamnestic response**. The volume of FMH needed for sensitization may be as small as **0.1 mL**.

### Mechanisms of Fetal Hemolysis

Maternal IgG coats fetal RBCs bearing the target antigen, leading to **extravascular hemolysis** in the fetal spleen and liver via Fc receptor-mediated phagocytosis. Fetal anemia triggers compensatory **extramedullary hematopoiesis** with resulting hepatosplenomegaly. Severe cases progress to **hydrops fetalis**, characterized by generalized edema, ascites, pleural and pericardial effusions, and placental edema. Anti-K (Kell) antibodies are unique in that they also suppress fetal erythropoiesis, causing anemia disproportionate to the degree of hemolysis.

## Implicated Antibodies

### Anti-D

Anti-D was historically the most common and severe cause of HDFN. Its incidence has been dramatically reduced by **Rh immune globulin (RhIG)** prophylaxis, though cases still occur due to failure of prophylaxis, inadequate dosing, or sensitization from transfusion.

### Other Clinically Significant Antibodies

**Anti-K (Kell)** is the second most common cause of severe HDFN and suppresses erythropoiesis, resulting in poor correlation between titer and disease severity. **Anti-c and anti-E** are Rh system antibodies that can cause moderate to severe HDFN. **Anti-Fya (Duffy)** can cause moderate HDFN, while **anti-Jka (Kidd)** is a rare cause that is usually mild. **ABO HDFN** is the most common form overall but is usually mild, primarily affecting group A or B infants born to group O mothers whose anti-A,B IgG crosses the placenta.

| Antibody | Severity | Mechanism | Special Considerations |
|---|---|---|---|
| Anti-D | Severe | Extravascular hemolysis | Preventable with RhIG prophylaxis |
| Anti-K (Kell) | Severe | Hemolysis + erythropoiesis suppression | Titer poorly predicts severity |
| Anti-c | Moderate–severe | Extravascular hemolysis | Second most common Rh cause |
| Anti-E | Mild–moderate | Extravascular hemolysis | Usually low titers |
| Anti-Fya (Duffy) | Moderate | Extravascular hemolysis | Less common |
| Anti-Jka (Kidd) | Mild | Extravascular hemolysis | Rare cause |
| ABO (anti-A,B) | Mild | IgG crosses placenta | Most common overall; rarely requires exchange |

![Peripheral blood smear from a neonate with HDFN showing spherocytes and polychromasia](images/hdfn-smear.jpg)

## Prenatal Assessment

### Maternal Antibody Screening

ABO/Rh type and antibody screen are performed at the **first prenatal visit** and again at 28 weeks. If an antibody is identified, its specificity is determined and a **titer** is performed for non-ABO antibodies. The **critical titer** is typically 1:16 or 1:32 (varying by institution), and at or above this level further fetal assessment is indicated. For anti-K, there is no reliable critical titer, and any detectable anti-K warrants close monitoring.

### Fetal Antigen Determination

**Cell-free fetal DNA (cffDNA)** in maternal plasma enables noninvasive determination of fetal RHD, RHCE, and KEL genotype. Accuracy exceeds 99% for RHD genotyping after 10-12 weeks gestation, eliminating unnecessary monitoring of antigen-negative fetuses. Amniocentesis for fetal genotyping is rarely needed with the availability of cffDNA testing.

### Fetal Monitoring

**Middle cerebral artery peak systolic velocity (MCA-PSV)** by Doppler ultrasound is the noninvasive standard for assessing fetal anemia. An MCA-PSV greater than 1.5 multiples of the median (MoM) for gestational age suggests moderate-to-severe anemia. This technique has largely replaced **amniotic fluid spectrophotometry** (delta OD450/Liley curve). Serial monitoring is performed every 1-2 weeks when maternal titers are at or above the critical threshold.

## Fetal and Neonatal Treatment

### Intrauterine Transfusion (IUT)

**Intravascular IUT** via cordocentesis (percutaneous umbilical blood sampling) delivers a direct transfusion into the umbilical vein. The donor units are group O, RhD-negative, antigen-negative, irradiated, CMV-safe, leukoreduced packed RBCs with a hematocrit of approximately 75-85%. IUT is performed when MCA-PSV indicates moderate-to-severe anemia, typically before 34 weeks. Repeat IUTs are given every **2-3 weeks** as needed, and survival rates exceed **90%** in experienced centers.

### Neonatal Management

**Phototherapy** is the first-line treatment for hyperbilirubinemia, converting bilirubin to water-soluble isomers. **Exchange transfusion** removes antibody-coated RBCs and circulating bilirubin and is indicated for severe or rapidly rising bilirubin unresponsive to phototherapy. **IVIG** may reduce the need for exchange transfusion by blocking Fc receptors on fetal and neonatal macrophages. Monitoring for **late-onset anemia** (during weeks 1-3 of life) is important because ongoing hemolysis and suppressed erythropoiesis may require simple transfusion.

![MCA Doppler ultrasound demonstrating elevated peak systolic velocity in a fetus with anemia](images/mca-doppler-hdfn.jpg)

## RhD Prophylaxis

### Rh Immune Globulin (RhIG)

RhIG works by providing anti-D IgG that clears fetal D-positive RBCs from maternal circulation before sensitization occurs. The **standard dose** of 300 micrograms (1500 IU) protects against up to **30 mL of fetal whole blood** (15 mL of fetal RBCs). It is administered at **28 weeks gestation** (antepartum) and within **72 hours of delivery** (postpartum) if the infant is RhD-positive. Additional indications include amniocentesis, chorionic villus sampling, abdominal trauma, ectopic pregnancy, miscarriage, and manual removal of placenta.

### Quantifying Fetomaternal Hemorrhage

The **rosette test** is a qualitative screen for FMH sensitive to approximately 10 mL of fetal blood. The **Kleihauer-Betke (KB) test** uses acid elution, with fetal hemoglobin (HbF) being resistant to acid, allowing counting of the percentage of fetal cells among maternal cells. **Flow cytometry** provides quantitative measurement and is more accurate and precise than the KB test, making it the preferred method. Additional RhIG doses are calculated by dividing the volume of fetal bleed in mL by 30.

![Kleihauer-Betke test showing dark-staining fetal red cells among ghost maternal cells](images/kleihauer-betke.jpg)

## ABO Hemolytic Disease

ABO HDFN is the most common form but is generally **mild**. It occurs primarily in group O mothers with group A or B infants, as naturally occurring anti-A,B IgG in group O individuals crosses the placenta. The DAT may be weakly positive or negative, and spherocytes are seen on the peripheral smear. Exchange transfusion is rarely required, and phototherapy is usually sufficient.

## Clinical Pearls

Cell-free fetal DNA testing from maternal plasma allows noninvasive determination of fetal RhD and other blood group antigen status, avoiding unnecessary invasive procedures. Anti-Kell HDFN causes anemia primarily through suppression of erythropoiesis rather than hemolysis alone, making antibody titers a poor predictor of disease severity. MCA-PSV Doppler has replaced amniocentesis-based methods as the standard noninvasive assessment of fetal anemia. RhIG must be given within 72 hours of delivery or other sensitizing events, and quantification of fetomaternal hemorrhage is necessary to ensure adequate dosing.

## References

1. Moise KJ. Management of rhesus alloimmunization in pregnancy. *Obstet Gynecol*. 2008;112(1):164-176.
2. Mari G, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. *N Engl J Med*. 2000;342(1):9-14.
3. de Haas M, et al. Anti-red blood cell antibodies causing hemolytic disease of the fetus and newborn. *Curr Opin Hematol*. 2015;22(6):519-527.
4. Fung MK, et al., eds. *AABB Technical Manual*. 20th ed. AABB Press; 2020.