Rhesus Incompatibility: Comprehensive Medical Lecture
For Medical Students and Healthcare Professionals
Learning Objectives
By the end of this lecture, learners will be able to:
Describe the molecular basis and genetics of the Rh blood group system
Explain the pathophysiology of Rh alloimmunization and hemolytic disease of the fetus and newborn (HDFN)
Identify high-risk clinical scenarios and appropriate screening protocols
Apply evidence-based prevention and management strategies
Analyze laboratory findings and imaging studies in Rh incompatibility
Evaluate treatment options and their mechanisms of action
Discuss current research and future directions in maternal-fetal medicine
I. Introduction and Historical Context
Definition
Rhesus (Rh) incompatibility refers to the discordant pairing of maternal and fetal Rh types. It is associated with the development of maternal Rh sensitization and hemolytic disease of the neonate (HDN).
Historical Significance
The discovery of the Rh system in 1940 by Landsteiner and Wiener revolutionized obstetric medicine. Prior to the introduction of Rho(D) immune globulin (RhIG) in 1968, Rh incompatibility was a leading cause of perinatal morbidity and mortality, affecting approximately 1 in 200 births.
Current Epidemiology
Global prevalence: 15-17% of Caucasians are Rh-negative
Ethnic variations:
African Americans: 5-8% Rh-negative
Asian populations: 1-3% Rh-negative
Hispanic populations: 10-15% Rh-negative
Clinical impact: This problem has become less common in places that provide good prenatal care. This is because special immune globulins called RhoGAM are routinely used
II. Molecular Biology and Genetics
The Rh Blood Group System
The Rh system is the most complex blood group system, comprising over 50 different antigens. The clinically significant components include:
Primary Antigens:
D antigen: Most immunogenic, primary clinical concern
C/c antigens: Secondary clinical importance
E/e antigens: Occasional clinical significance
Genetic Basis
Chromosomal Location: Chromosome 1p36.11 Gene Structure:
Two homologous genes: RHD and RHCE
RHD gene: Encodes D antigen (~97% of anti-D immune responses)
RHCE gene: Encodes C, c, E, e antigens
Inheritance Pattern: Autosomal codominant Phenotype Expression:
Rh-positive: Presence of D antigen (DD or Dd genotypes)
Rh-negative: Absence of D antigen (dd genotype)
Molecular Structure
The Rh proteins are transmembrane glycoproteins with 12 membrane-spanning domains. The D antigen contains multiple epitopes, with the most immunogenic located on the extracellular loops.
III. Pathophysiology of Rh Alloimmunization
Primary Sensitization Process
Step 1: Fetomaternal Hemorrhage (FMH) Even in normal pregnancies, a small number of fetal blood cells enter the maternal bloodstream (fetomaternal hemorrhage)
Volume Requirements for Sensitization:
Minimum sensitizing volume: 0.1-0.5 mL fetal blood
Average FMH during pregnancy: <0.1 mL
FMH during delivery: 15-25% of pregnancies >0.5 mL
High-Risk Events for FMH:
Delivery (especially traumatic)
Amniocentesis (2-3% risk)
Chorionic villus sampling
External cephalic version
Abdominal trauma
Placental abruption
Manual placental removal
Step 2: Antigen Presentation
Maternal antigen-presenting cells (APCs) process Rh-positive fetal RBCs
HLA class II presentation to CD4+ T cells
B cell activation and differentiation
Initial IgM response (primary immune response)
Step 3: Memory Formation
Class switching to IgG production
Formation of memory B cells
Establishment of immunological memory
Secondary Immune Response
Characteristics of Secondary Response:
Onset: More rapid (days vs. weeks)
Magnitude: 10-100x greater antibody production
Antibody type: Predominantly IgG (crosses placenta)
Duration: Persistent, often lifelong
Factors Affecting Response Intensity:
Volume of FMH
Maternal immune status
ABO compatibility (ABO incompatibility can be protective)
Previous sensitization events
IV. Hemolytic Disease of the Fetus and Newborn (HDFN)
Mechanism of Fetal Hemolysis
Antibody Transfer: In a second such pregnancy, pre-existing maternal RhD IgG antibodies can cross the placenta
Hemolysis Process:
Opsonization: Anti-D IgG binds to fetal RhD+ RBCs
Phagocytosis: Macrophages in fetal spleen and liver destroy opsonized cells
Compensation: Extramedullary hematopoiesis attempts to compensate
Decompensation: Severe anemia, cardiac failure, hydrops fetalis
Clinical Spectrum of HDFN
Mild Disease (50-60% of cases):
Mild anemia (Hb 10-14 g/dL)
Minimal jaundice
No treatment required
Moderate Disease (25-30% of cases):
Moderate anemia (Hb 7-10 g/dL)
Significant jaundice requiring phototherapy
Possible exchange transfusion
Severe Disease (10-20% of cases):
Severe anemia (Hb <7 g/dL)
Hydrops fetalis
Intrauterine fetal death
Kernicterus risk
Hydrops Fetalis Pathophysiology
High-Output Heart Failure:
Severe anemia → increased cardiac output
Venous congestion and increased capillary hydrostatic pressure
Fluid extravasation into body cavities
Portal Hypertension:
Extramedullary hematopoiesis in liver
Hepatomegaly and portal venous obstruction
Ascites formation
Hypoproteinemia:
Hepatic dysfunction from extramedullary hematopoiesis
Decreased albumin synthesis
Reduced oncotic pressure
V. Clinical Assessment and Risk Stratification
Maternal Assessment
Initial Evaluation:
ABO and Rh typing (both partners if possible)
Antibody screen (indirect Coombs test)
Antibody identification if screen positive
Antibody titration if anti-D detected
Antibody Titration Methodology:
Serial dilutions of maternal serum
Critical titer: 1:16-1:32 (laboratory-dependent)
Indicates need for enhanced fetal surveillance
Limitations of Titration:
Inter-laboratory variation
Technical inconsistencies
Poor correlation with disease severity in some cases
Fetal Assessment
Non-Invasive Monitoring:
1. Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV)
Gold standard for fetal anemia detection
Technique: Doppler ultrasound measurement
Normal values: <1.5 multiples of median (MoM)
Severe anemia: >1.5-1.7 MoM
Accuracy: Sensitivity 85-90%, specificity 95%
2. Ultrasonographic Signs:
Early signs: Polyhydramnios, placentomegaly
Progressive signs: Hepatosplenomegaly, cardiomegaly
Late signs: Ascites, pericardial effusion, skin edema
Invasive Monitoring:
Cordocentesis (PUBS)
Indications: MCA-PSV >1.5 MoM, severe hydrops
Parameters measured:
Hemoglobin concentration
Hematocrit
Bilirubin levels
Direct Coombs test
Risks: 1-2% fetal loss rate
Timing: Usually >18 weeks gestation
VI. Prevention Strategies
Rho(D) Immune Globulin (RhIG) - RhoGAM
Mechanism of Action: RhIG is known for its use in preventing hemolytic disease of the fetus and neonate, which is the result of transplacental passage of anti-D antibodies due to previous exposure to an incompatible Rh blood type
Proposed Mechanisms:
Antigen blocking: RhIG antibodies bind to D antigens, preventing recognition
Immune suppression: Feedback inhibition of B cell responses
Clearance enhancement: Rapid removal of Rh+ cells from circulation
Fc receptor blocking: Prevention of complement activation
Dosing Protocols:
Antenatal Prophylaxis: Pooled human anti-D immune globulin (Rh IgG or RhoGAM) treatment is indicated at 28 weeks
Standard dose: 300 μg (1500 IU) intramuscularly
Protection duration: ~12 weeks
Rationale: Prevents sensitization from late pregnancy FMH
Postnatal Prophylaxis: within 72 hours after delivery if the baby is Rh+
Standard dose: 300 μg (1500 IU) intramuscularly
Timing: Optimal within 72 hours, some benefit up to 28 days
Coverage: A 300-μg dose, which can neutralize 15 mL of fetal blood, is administered for all other indications
Additional Indications: It is also indicated following spontaneous or induced termination and any event that could lead to transplacental hemorrhage. A single dose (50 micrograms [μg]), which is administered following first-trimester pregnancy termination, is enough to neutralize 2.5 mL of fetal blood
Specific Clinical Scenarios:
Ectopic pregnancy: 50 μg if <12 weeks
CVS: 300 μg
Amniocentesis: 300 μg
External cephalic version: 300 μg
Antepartum hemorrhage: 300 μg
Abdominal trauma: 300 μg
Kleihauer-Betke Testing
Purpose: Quantify FMH volume to determine adequate RhIG dosing
Methodology:
Acid elution technique
Counts fetal cells in maternal blood
Calculates estimated fetal blood volume
Clinical Application:
Standard FMH: <30 mL - standard 300 μg dose adequate
Massive FMH: >30 mL - additional RhIG needed (10 μg per mL fetal blood)
Limitations:
Technical complexity
Observer variability
False positives (hereditary persistence of fetal hemoglobin)
Effectiveness of Prevention
Primary prevention success rate: >99% when properly administered
Failure rate: <0.1-0.2% with appropriate protocols
Population impact: 85% reduction in HDFN since introduction
VII. Management of Established Alloimmunization
Antenatal Management
Mild to Moderate Alloimmunization:
Serial antibody titrations every 4 weeks
MCA-PSV monitoring every 1-2 weeks starting at 18-20 weeks
Fetal growth assessment
Standard obstetric care
Severe Alloimmunization (Critical titer or previous HDFN):
Weekly MCA-PSV starting at 16-18 weeks
Cordocentesis if MCA-PSV >1.5 MoM
Intrauterine transfusion if fetal Hb <10 g/dL
Intrauterine Transfusion (IUT)
Types:
Intravascular transfusion (IVT): Direct into umbilical vein
Intraperitoneal transfusion (IPT): Into fetal peritoneal cavity (historical)
Blood Product Specifications:
Type: O-negative, irradiated, CMV-negative
Hematocrit: 75-85%
Crossmatch: Compatible with maternal serum
Leukoreduction: Mandatory
IVT Procedure:
Ultrasound guidance: Continuous visualization
Access site: Placental cord insertion or free loop
Volume calculation: Based on estimated fetal blood volume
Target Hct: 40-50% post-transfusion
Monitoring: Fetal heart rate, umbilical venous pressure
Complications:
Immediate: Cord hematoma, bradycardia, fetal death (1-2%)
Delayed: Membrane rupture, infection, preterm labor
Long-term: Developmental outcomes generally favorable
Success Rates:
Survival to delivery: 85-95% for non-hydropic fetuses
Neurologic outcomes: 95% normal development
Gestational age at delivery: Usually 35-37 weeks
Timing of Delivery
Non-hydropic fetuses:
Target gestation: 37-38 weeks
Lung maturity: Amniocentesis if delivery <38 weeks
Mode of delivery: Vaginal delivery unless obstetric indications
Hydropic fetuses:
Early delivery: Often indicated at 32-35 weeks
Steroid administration: For lung maturity
NICU coordination: Essential for immediate postnatal care
VIII. Neonatal Management
Immediate Assessment
Laboratory Evaluation:
Hemoglobin/Hematocrit: Assess degree of anemia
Bilirubin levels: Total and direct bilirubin
Direct Coombs test: Confirms antibody coating of RBCs
Blood type and Rh: Confirm fetal genotype
Reticulocyte count: Indicates bone marrow response
Clinical Assessment:
Signs of anemia: Pallor, tachycardia, poor feeding
Jaundice: Onset, progression, distribution
Hepatosplenomegaly: Indicates extramedullary hematopoiesis
Heart failure: Tachypnea, poor perfusion, edema
Treatment Modalities
Phototherapy:
Mechanism: Converts bilirubin to water-soluble photoisomers
Initiation criteria: Based on bilirubin levels and risk factors
Intensive phototherapy: Multiple light sources, maximum surface area
Exchange Transfusion:
Indications:
Severe anemia (Hb <7 g/dL)
Rapidly rising bilirubin despite phototherapy
Signs of bilirubin encephalopathy
Blood product: O-negative, crossmatched with mother
Volume: Double blood volume exchange (160-180 mL/kg)
Technique: Arterial and venous access, gradual replacement
Simple Transfusion:
Indications: Severe anemia without significant hyperbilirubinemia
Volume: 10-15 mL/kg packed RBCs
Rate: Slow infusion over 3-4 hours
IVIG Therapy:
Mechanism: Blocks Fc receptors, reduces hemolysis
Dosage: 0.5-1 g/kg IV over 2 hours
Efficacy: May reduce need for exchange transfusion
Timing: Early administration most effective
Long-term Follow-up
Hematologic Monitoring:
Late anemia: Common 2-6 weeks post-birth
Mechanism: Shortened RBC lifespan, delayed erythropoiesis
Management: Iron supplementation, possible transfusion
Neurodevelopmental Assessment:
Risk factors: Severe hyperbilirubinemia, exchange transfusion
Monitoring: Hearing, visual, cognitive development
Early intervention: If developmental delays identified
IX. Special Clinical Situations
ABO-Rh Combined Incompatibility
Clinical Significance:
ABO incompatibility may offer protection against Rh sensitization
Rapid clearance of ABO-incompatible cells reduces Rh exposure
May still require RhIG prophylaxis
Management Considerations:
Standard RhIG protocols apply
Monitor for both ABO and Rh HDFN
ABO HDFN typically milder presentation
Multiple Gestation
Risk Factors:
Higher rates of FMH
Potential for twin-twin transfusion syndrome
Increased obstetric interventions
Management Modifications:
Standard RhIG dosing usually adequate
Enhanced fetal surveillance
Consider early consultation with maternal-fetal medicine
Massive Fetomaternal Hemorrhage
Definition: FMH >30 mL (>1% maternal blood volume)
Clinical Recognition:
Severe fetal compromise/demise
Maternal symptoms rare
Kleihauer-Betke testing essential
Management:
Calculate appropriate RhIG dose: 10 μg per mL fetal blood
Maximum single dose: 1500 μg (may require multiple injections)
Close maternal monitoring for anemia
Previous IUT Recipients
Immunologic Considerations:
Exposure to multiple donor antigens
Risk of developing additional alloantibodies
May require specialized blood banking
Reproductive Counseling:
Increased risk with subsequent pregnancies
Early referral to maternal-fetal medicine
Preconceptional counseling important
X. Laboratory Considerations
Blood Bank Protocols
Maternal Testing:
ABO/Rh typing with weak D testing
Antibody screening at first visit and 28 weeks
Antibody identification and titration if positive
Paternal Testing:
ABO/Rh typing if maternal Rh-negative
Weak D testing in Rh-positive fathers
Genetic counseling if both parents Rh-negative
Neonatal Testing:
Cord blood: ABO/Rh, direct Coombs test
Peripheral blood if cord blood unavailable
Additional antibody studies if indicated
Quality Assurance
Critical Values:
Antibody titers ≥1:16 (critical titer)
MCA-PSV >1.5 MoM
Neonatal Hb <10 g/dL
Bilirubin levels approaching exchange thresholds
Communication Protocols:
Immediate notification of critical results
Clear documentation in medical records
Interdisciplinary team communication
XI. Pharmacoeconomics and Healthcare Impact
Cost-Effectiveness Analysis
Prevention vs. Treatment Costs:
RhIG prophylaxis: $200-300 per pregnancy
HDFN management: $50,000-200,000 per case
Cost-effectiveness ratio: Highly favorable for prevention
Population Health Impact:
Prevented cases: >10,000 annually in developed countries
Reduced maternal morbidity and anxiety
Decreased healthcare resource utilization
Global Health Considerations
Resource-Limited Settings:
Limited access to RhIG
Higher rates of Rh sensitization
Need for affordable prevention strategies
Emerging Technologies:
Recombinant anti-D production
Non-invasive fetal genotyping
Point-of-care testing solutions
XII. Current Research and Future Directions
Non-Invasive Fetal Genotyping
Cell-Free Fetal DNA (cffDNA):
Detection of fetal RHD gene in maternal plasma
Accuracy >99% after 10 weeks gestation
Potential to avoid unnecessary RhIG in Rh-negative pregnancies
Clinical Implementation:
Reduces RhIG usage by 35-40%
Cost-effective in many healthcare systems
Requires robust laboratory infrastructure
Novel Therapeutic Approaches
Monoclonal Anti-D Antibodies:
Synthetic alternative to human-derived RhIG
Consistent potency and safety profile
Unlimited supply potential
Immune Modulation Strategies:
Targeted B-cell depletion
T-regulatory cell enhancement
Complement inhibition
Precision Medicine Applications
Pharmacogenomics:
FcγR polymorphisms affecting antibody clearance
HLA typing for risk stratification
Personalized prophylaxis protocols
Biomarker Development:
Early sensitization detection
Severity prediction models
Treatment response monitoring
XIII. Case Studies
Case 1: Standard Prevention Scenario
Clinical Presentation: 24-year-old G1P0 presents for routine prenatal care at 8 weeks gestation.
Laboratory Results:
Blood type: A-negative
Antibody screen: Negative
Partner: O-positive
Management:
RhIG prophylaxis at 28 weeks
Postnatal RhIG if infant Rh-positive
Routine prenatal care
Outcome:
Healthy term delivery
Infant: A-positive, direct Coombs negative
Mother received postnatal RhIG
No sensitization at 6-month follow-up
Case 2: Established Alloimmunization
Clinical Presentation: 28-year-old G2P1 with history of neonatal jaundice requiring phototherapy.
Laboratory Results:
Blood type: O-negative
Anti-D antibody titer: 1:64
Previous infant: Exchange transfusion required
Management:
Referral to maternal-fetal medicine at 16 weeks
Weekly MCA-PSV monitoring
Cordocentesis at 26 weeks: Fetal Hb 7.2 g/dL
Serial intrauterine transfusions (3 procedures)
Outcome:
Delivery at 35 weeks
Neonatal Hb: 12.4 g/dL
Phototherapy only
Normal neurodevelopmental outcome
Case 3: Massive Fetomaternal Hemorrhage
Clinical Presentation: 32-year-old G3P2 presents with decreased fetal movement at 36 weeks.
Assessment:
Fetal demise confirmed
Kleihauer-Betke test: 3.2% fetal cells
Estimated FMH: 150 mL
Management:
Calculated RhIG dose: 1500 μg (maximum single dose)
Additional 300 μg dose at 48 hours
Maternal CBC monitoring
Delivery management
Follow-up:
No evidence of sensitization at subsequent pregnancy
Successful prevention despite massive hemorrhage
XIV. Clinical Practice Guidelines
Professional Society Recommendations
American College of Obstetricians and Gynecologists (ACOG):
Routine antenatal RhIG at 28 weeks
Postnatal RhIG within 72 hours if infant Rh-positive
Additional RhIG for sensitizing events
Society for Maternal-Fetal Medicine (SMFM):
MCA-PSV monitoring for alloimmunized pregnancies
Intrauterine transfusion protocols
Multidisciplinary team approach
International Federation of Gynecology and Obstetrics (FIGO):
Global prevention strategies
Resource-appropriate care protocols
Quality improvement initiatives
Implementation Strategies
Healthcare System Requirements:
Standardized protocols and order sets
Staff education and competency assessment
Quality assurance programs
Patient education materials
Performance Metrics:
RhIG administration rates
Sensitization prevention rates
Neonatal outcomes
Cost-effectiveness measures
XV. Ethical and Legal Considerations
Informed Consent
Patient Education Requirements:
Risk stratification based on maternal/paternal blood types
Benefits and risks of RhIG prophylaxis
Consequences of untreated Rh incompatibility
Alternative management strategies
Decision-Making Support:
Shared decision-making approach
Cultural and religious considerations
Language-appropriate materials
Written documentation
Medicolegal Issues
Standard of Care:
Failure to offer appropriate RhIG prophylaxis
Inadequate monitoring of alloimmunized pregnancies
Delayed recognition of fetal anemia
Suboptimal neonatal management
Risk Management:
Clear documentation of all interventions
Timely consultation with specialists
Patient communication and education
Follow-up care coordination
XVI. Summary and Key Learning Points
Essential Clinical Concepts
Rh incompatibility is largely preventable with appropriate use of RhIG prophylaxis
MCA-PSV is the gold standard for non-invasive fetal anemia detection
Intrauterine transfusion can successfully treat severe fetal anemia
Multidisciplinary team approach is essential for optimal outcomes
Long-term follow-up is important for both maternal and neonatal care
Critical Decision Points
Recognition of risk factors for FMH and sensitization
Appropriate timing and dosing of RhIG prophylaxis
Escalation to maternal-fetal medicine for alloimmunized patients
Timing of invasive procedures and delivery planning
Coordination of neonatal intensive care for severely affected infants
Future Considerations
Implementation of non-invasive fetal genotyping to optimize RhIG use
Development of synthetic alternatives to human-derived RhIG
Advancement of precision medicine approaches to risk stratification
Global health initiatives to improve access to prevention strategies
XVII. Assessment and Evaluation
Self-Assessment Questions
A 22-year-old G1P0 woman with O-negative blood type presents at 30 weeks gestation after a motor vehicle accident with minor abdominal trauma. She received RhIG at 28 weeks. What is the most appropriate management?
In a pregnancy complicated by anti-D alloimmunization, at what MCA-PSV threshold should cordocentesis be considered?
Calculate the appropriate RhIG dose for a patient with a Kleihauer-Betke test showing 1.8% fetal cells.
Clinical Scenarios for Discussion
Complex sensitization: Patient with multiple alloantibodies including anti-D, anti-C, and anti-K
Religious considerations: Jehovah's Witness patient declining blood products
Resource limitations: Management in low-resource settings without access to intrauterine transfusion
Competency Assessment
Knowledge Domain:
Understanding of Rh system genetics and immunology
Recognition of clinical presentations and risk factors
Knowledge of prevention and treatment protocols
Skills Domain:
Interpretation of laboratory results and imaging studies
Communication with patients and families
Coordination of multidisciplinary care
Attitudes Domain:
Commitment to evidence-based practice
Sensitivity to cultural and ethical considerations
Dedication to quality improvement and patient safety
XVIII. References and Further Reading
Primary Literature
Moise KJ Jr. Fetal anemia due to non-Rhesus-D red-cell alloimmunization. Semin Fetal Neonatal Med. 2008;13(4):207-214.
Mari G, Deter RL, Carpenter RL, 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.
Zwiers C, Lindenburg IT, Klumper FJ, et al. Complications in intrauterine intravascular blood transfusion: lessons learned after 42 years. Ultrasound Obstet Gynecol. 2017;50(2):180-186.
Guidelines and Position Statements
ACOG Practice Bulletin No. 181: Prevention of Rh D Alloimmunization. Obstet Gynecol. 2017;130(2):e57-e70.
Society for Maternal-Fetal Medicine (SMFM). Electronic address: pubs@smfm.org. Management of alloimmunization during pregnancy. Am J Obstet Gynecol. 2018;219(4):B2-B20.
Contemporary Reviews
Tiblad E, Taune Wikman A, Ajne G, et al. Targeted routine antenatal anti-D prophylaxis in the prevention of RhD immunisation-outcome of a new antenatal screening and prevention program. PLoS One. 2013;8(8):e70984.
Vivanti AJ, Benachi A, Huchet FX, et al. Management of fetal and neonatal alloimmune anemia: A systematic review. Transfus Med Rev. 2020;34(2):124-130.
