Rh Negative Pregnancy - Postgraduate Level

1. The Rh Blood Group System - Molecular Basis

• RHD gene - codes for the D antigen (the most immunogenic antigen in the Rh system)
• RHCE gene - codes for C/c and E/e antigens via alternative splicing

• Occurs in ~15% of white donors; ~7-8% in Black populations
• In most white individuals, Rh-negative reflects complete deletion of the RHD gene
• In Black individuals, ~60% of D-negative individuals inherit a mutant RHD allele - the RHD pseudogene (RHDpsi) - containing a 37-bp internal duplication, frameshift, and premature stop codon
• No true "d" antigen exists; antibody against "d" cannot be raised

2. Pathophysiology of Rh Alloimmunization

Mechanism of Sensitization

• As little as 0.1 µL of Rh-positive fetal blood entering maternal circulation can sensitize the mother
• Initial exposure generates predominantly IgM antibodies (do not cross the placenta)
• On re-exposure, a rapid anamnestic response produces IgG anti-D (crosses the placental freely)
• IgG anti-D crosses the placenta via Fc receptors, enters fetal circulation, coats fetal Rh-positive RBCs, and triggers extravascular hemolysis

When Does FMH Occur?

• Spontaneously throughout pregnancy (undetectable small amounts)
• Delivery (largest FMH - the primary sensitizing event in classical cases)
• Invasive procedures: amniocentesis, chorionic villus sampling (CVS), cordocentesis
• First-trimester events: abortion (spontaneous or induced), ectopic pregnancy, threatened miscarriage
• External cephalic version (ECV)
• Abdominal trauma

Risk Factors for Sensitization

• First Rh-positive baby rarely causes clinical sensitization (only ~1% after delivery without prophylaxis)
• ~3% of second Rh-positive babies show signs of hemolytic disease
• ~10% of third babies are affected
• Risk rises progressively with each subsequent Rh-positive pregnancy

3. Hemolytic Disease of the Fetus and Newborn (HDFN)

Pathophysiology

Consequences of Fetal Anemia

4. Antenatal Management of the Unsensitized (Rh-negative, Antibody-negative) Mother

First Visit Assessment

• ABO and Rh typing
• Indirect Coombs test (ICT) / indirect antiglobulin test (IAT)
• If ICT is negative = unsensitized

Paternal Testing

• Father homozygous RhD-negative → no risk; no further monitoring needed
• Father homozygous RhD-positive → fetus certainly Rh-positive; full prophylaxis required
• Father heterozygous → 50% chance fetus is Rh-positive → fetal RhD genotyping recommended

Fetal RhD Genotyping

• Cell-free fetal DNA (cffDNA) from maternal plasma is now the preferred non-invasive method (available from ~10 weeks gestation)
• If fetus is RhD-negative genotype, no prophylaxis needed; if RhD-positive, full prophylaxis protocol applies
• 2025 JAMA Network Open Practice Guideline (Moise et al.) recommends use of cffDNA to identify the at-risk fetus early in pregnancy as a key recommendation

Anti-D Immunoglobulin (RhIG) Prophylaxis

5. Management of the Sensitized (Rh-negative, Anti-D positive) Mother

Initial Assessment

1. Confirm maternal anti-D (and rule out passive RhIG-induced anti-D from prior prophylaxis)
2. Antibody titration by indirect Coombs test (serial dilutions)
3. Paternal Rh phenotype and zygosity
4. Fetal RhD genotyping (by cffDNA or amniocentesis if needed)

Critical Anti-D Titer

• 1:8 to 1:32 is the critical titer range associated with risk for fetal hydrops (exact threshold varies by lab; most centers use 1:16 or 1:32)
• Below critical titer → serial titrations monthly until 24 weeks, then every 2 weeks
• At or above critical titer → intensive fetal surveillance begins

Monitoring Fetal Anemia

Middle Cerebral Artery (MCA) Doppler - Current Gold Standard

• MCA Peak Systolic Velocity (PSV) > 1.5 MoM (multiples of the median) for gestational age = severe fetal anemia
• Non-invasive, accurate, has replaced amniocentesis as primary surveillance tool
• False-positive rate ~12% (acceptable)
• After 35 weeks, higher false-positive rates occur; amniocentesis may be considered for confirmation
• Creasy & Resnik's Maternal-Fetal Medicine cites the landmark study by Mari et al. (N Engl J Med 2000) establishing this threshold
• 2025 JAMA Practice Guideline recommends implementation of MCA-PSV Doppler measurements to detect fetal anemia earlier in pregnancy

Amniocentesis + ΔOD450 (Historical/Supplemental)

• Spectrophotometric measurement of amniotic fluid bilirubin at 450 nm wavelength
• Plotted on the Liley graph (third trimester) or Queenan curve (second and third trimester with four zones)
• Liley zones:
  • Zone 1 (lower) = mild hemolysis; no immediate intervention
  • Zone 2 (middle) = moderate hemolysis; repeat in 10-14 days
  • Zone 3 (upper) = severe hemolysis; fetal blood sampling ± intrauterine transfusion
• ΔOD450 value at 80th percentile of Zone 3 or entering the intrauterine transfusion zone on Queenan curve → fetal blood sampling (cordocentesis)

Fetal Blood Sampling (Cordocentesis)

• Indicated when MCA-PSV > 1.5 MoM
• Directly measures fetal hematocrit
• Fetal hematocrit < 30% → intrauterine transfusion (IUT)
• Has operator-dependent risks: fetal loss ~0.5-2%, worsened FMH (anamnestic antibody response)

6. Treatment of Fetal Anemia

Intrauterine Transfusion (IUT)

• Intravascular transfusion (IVT): transfusion directly into the umbilical vein under ultrasound guidance - now the preferred method
• Intraperitoneal transfusion (IPT): older technique; still used when umbilical access is difficult (hydrops with massive ascites)
• Blood used: O negative, CMV-negative, leukoreduced, irradiated, cross-matched against maternal serum, packed RBCs
• Goal post-transfusion hematocrit: 40-50%
• IUT can be repeated every 2-4 weeks as needed
• 2025 JAMA Practice Guideline: Continue IUT therapy until end of 35th week; prolong delivery to 37+0 to 38+6 weeks if possible

IVIG (Intravenous Immunoglobulin)

• Used in select cases as immunomodulatory adjunct
• 2025 JAMA Practice Guideline recommends: IVIG for patients with a documented antigen-positive fetus with either prior fetal anemia or fetal loss due to HDFN before 24 weeks' gestational age in a previous pregnancy
• Mechanism: reduces maternal anti-D production and decreases placental transfer of IgG antibodies

Timing of Delivery

• 2025 JAMA Guideline: Delivery between 37+0 to 38+6 weeks (late preterm/early term)
• If fetal hydrops develops late (week 35+) or multiple IUTs given: balance risks of continued IU development vs. prematurity
• After delivery: prepare for exchange transfusion of neonate

7. Neonatal Management

Exchange Transfusion

• Gold standard for severe neonatal HDFN
• Replace neonate's Rh-positive blood with Rh-negative blood
• Removes sensitized RBCs, anti-D antibodies, and bilirubin simultaneously
• Performed over 1.5+ hours; may be repeated
• Goal: keep bilirubin below kernicterus threshold
• Rh-negative donor cells are gradually replaced by infant's own RBCs (6+ weeks); by then maternal anti-D is cleared

Phototherapy

• For milder cases with indirect hyperbilirubinemia
• Converts bilirubin to water-soluble photoisomers

IVIG in Neonate

• Can reduce need for exchange transfusion in moderate HDFN
• Blocks Fc receptors on macrophages, slowing RBC destruction

8. Other Clinically Important Points (Exam High-Yield)

Weak D and Partial D

• Weak D (previously "Du"): reduced D antigen expression; Weak D types 1, 2, 3 are NOT at risk for alloimmunization and do NOT require RhIG
• Partial D: qualitatively different D antigen; these women CAN make anti-D and SHOULD receive RhIG
• Proposed algorithm: RHD genotyping for women with discrepant or weak (1+/2+) D serology reactions

The "G" Antigen

• High-frequency antigen present on virtually all D-positive AND C-positive RBCs
• Anti-G behaves as anti-C + anti-D; RhIG should still be given to Rh-negative women with anti-G (they can still become sensitized to true D epitopes)

Other Antigens Causing HDFN (after anti-D)

• Anti-c (RH4) - second most common cause of severe HDFN
• Anti-E, anti-C, anti-Kell (Kell system - particularly dangerous; causes HDFN by suppressing fetal erythropoiesis in addition to hemolysis)
• Kell-sensitized pregnancies: same monitoring with MCA-PSV Doppler

ABO Incompatibility vs Rh

• ABO incompatibility causes usually mild HDFN (IgM antibodies predominate; don't cross placenta)
• Does NOT worsen with subsequent pregnancies (unlike Rh)
• Actually protective - A or B fetal cells entering maternal circulation are rapidly cleared by pre-existing maternal ABO isohemagglutinins before D sensitization can occur

9. Recent Evidence Updates (2025-2026)

2025 JAMA Network Open Practice Guideline (Moise et al., PMID 41284292)

1. Use cffDNA to identify the at-risk fetus early in pregnancy
2. Implement MCA-PSV Doppler earlier (not just after critical titer is reached)
3. IVIG for pregnancies with prior HDFN before 24 weeks
4. Continue IUT until end of 35th week
5. Deliver at 37+0 to 38+6 weeks (not earlier)
6. Emerging concern: use of low-titer, O, RhD-positive whole blood (LTOWB) in trauma/hemorrhage is creating a new cohort of RhD-sensitized females of reproductive age

2026 Systematic Review - First-Trimester Anti-D Prophylaxis (Gemzell-Danielsson et al., PMID 40728267, IJGO)

• WHO 2022 guidelines and subsequent evidence suggest anti-D may NOT be required for first-trimester medical/surgical abortion before 12 weeks
• Risk of FMH at <12 weeks is extremely low
• Older guidelines (pre-2022) uniformly recommended prophylaxis even before 12 weeks
• Current practice: discrepancy exists between national guidelines; shared decision-making recommended
• This represents a significant change from classical teaching - acknowledge both positions in exams

Summary Table for Quick Revision