Rhesus Isoimmunisation

Disclaimer: > [!WARNING] Medical Disclaimer: This content is for educational and informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. Medical guidelines and best practices change rapidly; users should verify information with current local protocols.

1. Overview

Rhesus D Isoimmunisation (Rh Sensitisation) occurs when a Rhesus D-negative (RhD-negative) mother develops immune antibodies against the Rhesus D antigen present on the red blood cells of her RhD-positive fetus. This immunological response leads to Haemolytic Disease of the Fetus and Newborn (HDFN), a potentially life-threatening condition characterised by fetal red blood cell destruction, anaemia, and in severe cases, hydrops fetalis and intrauterine death. [1,2]

The introduction of routine antenatal anti-D immunoglobulin prophylaxis has been one of the most successful interventions in modern obstetrics, reducing the incidence of RhD alloimmunisation from approximately 14-16% in the pre-prophylaxis era to less than 1% in countries with established prophylaxis programmes. [3,4]

Despite these advances, rhesus disease remains an important cause of perinatal morbidity and mortality worldwide, particularly in resource-limited settings where prophylaxis programmes are not universally available. [5]

---

2. Epidemiology and Public Health Impact

Global Distribution of Rhesus D Phenotypes

The prevalence of RhD-negative blood type varies significantly by ethnicity and geographic region:

• Caucasian populations: Approximately 15-17% are RhD-negative [6]
• African populations: 3-7% are RhD-negative [6]
• Asian populations: 1-3% are RhD-negative (less than 1% in some East Asian populations) [6]
• Indigenous populations: Prevalence varies widely

Impact of Anti-D Prophylaxis Programmes

The introduction of routine antenatal anti-D prophylaxis (RAADP) has dramatically reduced the burden of rhesus disease:

• Pre-prophylaxis era (before 1970): Sensitisation occurred in 14-16% of at-risk pregnancies [3]
• Post-natal anti-D only (1970-1990): Sensitisation reduced to 1.5-2% [7]
• Routine antenatal prophylaxis (post-2000): Sensitisation reduced to less than 0.2-1% in countries with universal programmes [4,8]

Despite these successes, failures still occur due to:

• Inadequate dosing following large fetomaternal haemorrhages
• Failed administration (missed appointments, errors)
• Sensitisation in early pregnancy before routine prophylaxis
• Immune response to very small volumes of fetal blood despite prophylaxis [9]

Contemporary Burden of Disease

In developed countries with established anti-D programmes:

• HDFN due to anti-D affects approximately 1 in 1000-1500 births [10]
• Severe disease requiring intrauterine transfusion: 5-10 per 100,000 births [11]
• Perinatal mortality from HDFN has decreased from 46 per 100,000 births (1950s) to less than 1 per 100,000 in contemporary practice [12]

However, in low- and middle-income countries without universal prophylaxis:

• HDFN remains a significant cause of perinatal morbidity and mortality
• WHO estimates suggest tens of thousands of preventable fetal and neonatal deaths annually [5]

---

3. Pathophysiology

The Rhesus D Antigen

The RhD antigen is a transmembrane protein encoded by the RHD gene on chromosome 1. It is one of the most immunogenic blood group antigens, second only to the ABO system. [1]

Key features:

• Present on red blood cells of RhD-positive individuals (approximately 85% of Caucasians)
• RhD-negative individuals lack the RhD protein (homozygous deletion or inactive RHD gene)
• High immunogenicity: Exposure to as little as 0.1 mL of RhD-positive blood can trigger sensitisation in RhD-negative individuals [13]

Mechanism of Maternal Sensitisation

Sensitisation requires fetomaternal haemorrhage (FMH), where fetal RhD-positive red blood cells enter the maternal circulation:

Primary Immune Response (First Exposure):

1. Fetal RhD-positive red cells enter maternal circulation
2. Maternal antigen-presenting cells process the D antigen
3. T-cell activation and B-cell proliferation occur
4. Initial production of IgM antibodies (large molecules that cannot cross placenta)
5. Formation of memory B cells (remain dormant but primed for years)
6. After weeks to months, class switching produces some IgG anti-D (can cross placenta)

Secondary Immune Response (Subsequent Exposures):

1. Memory B cells rapidly produce large quantities of IgG anti-D
2. IgG subclasses (particularly IgG1 and IgG3) efficiently cross the placenta via FcRn receptors
3. Antibody production occurs even with minimal fetal cell exposure (less than 0.01 mL)
4. Response can occur in a subsequent pregnancy even without additional FMH [14]

Timing and Causes of Fetomaternal Haemorrhage

Spontaneous FMH in normal pregnancy:

• First trimester: Detectable FMH in 3-7% of pregnancies [15]
• Second trimester: Detectable FMH in 12-16% [15]
• Third trimester: Detectable FMH in 25-45% [15]
• At delivery: > 99% of women have some degree of FMH [16]

Volume typically small:

• less than 0.1 mL in 96% of pregnancies

• 0.4 mL (sufficient for sensitisation despite prophylaxis) in 1-3% [16]

Sensitising events (increase risk/volume of FMH):

• Delivery: Most common cause (50-75% of sensitisation events historically) [3]
• Spontaneous miscarriage: Risk increases with gestation (7% at 8-12 weeks, 25% at 13-20 weeks) [17]
• Threatened miscarriage: FMH detected in up to 12% [17]
• Ectopic pregnancy: Risk due to tissue invasion
• Termination of pregnancy: Surgical or medical
• Antepartum haemorrhage: Placental abruption, placenta praevia bleeding
• Abdominal trauma: Even minor blunt trauma can cause significant FMH [18]
• Invasive procedures: Amniocentesis (7-15% FMH), chorionic villus sampling (14-18% FMH), cordocentesis (30-50% FMH) [19]
• External cephalic version: FMH in 2-6% of procedures [20]
• Intrauterine death: Cellular breakdown increases FMH risk
• Antepartum stillbirth

Fetal Haemolytic Disease Pathophysiology

Once maternal IgG anti-D crosses the placenta:

1. Red Blood Cell Destruction:

• Maternal IgG anti-D binds to fetal RhD-positive erythrocytes
• Antibody-coated cells are recognised by Fc receptors on fetal macrophages
• Extravascular haemolysis occurs primarily in the fetal spleen
• Rate of destruction exceeds bone marrow compensatory capacity

2. Compensatory Mechanisms:

• Bone marrow hyperplasia: Increased erythropoiesis
• Extramedullary haematopoiesis: Liver and spleen become sites of red cell production
• Hepatosplenomegaly develops
• Release of immature red cells (erythroblasts, reticulocytes) into circulation

3. Fetal Anaemia:

• Progressive anaemia develops as haemolysis exceeds production
• Severity depends on:
  • Maternal antibody titre and avidity
  • Placental transfer efficiency
  • Gestational age (earlier sensitisation = more severe disease) [2]

4. Hydrops Fetalis (Severe Cases): When fetal haemoglobin falls below 4-5 g/dL, a cascade of pathophysiology develops:

• High-output cardiac failure: Severe anaemia causes increased cardiac output, cardiac dilatation, and eventual myocardial failure
• Hepatic dysfunction: Massive hepatomegaly from extramedullary haematopoiesis causes:
  • Portal hypertension
  • Reduced albumin synthesis → hypoalbuminaemia
  • Reduced clotting factor production
• Reduced oncotic pressure: Combination of hypoalbuminaemia and cardiac failure
• Generalised oedema (anasarca):
  • Skin oedema (> 5 mm subcutaneous thickness on ultrasound)
  • Ascites
  • Pleural effusions
  • Pericardial effusion
• Placental oedema: Thick, heavy placenta
• Intrauterine death if untreated [21]

5. Neonatal Hyperbilirubinaemia: After delivery, the neonate can no longer clear bilirubin via the placenta:

• Ongoing haemolysis produces unconjugated bilirubin
• Immature hepatic conjugation capacity
• Risk of kernicterus (bilirubin encephalopathy) if untreated [22]

---

4. Clinical Presentation

Maternal Presentation

The mother is almost always asymptomatic. Rhesus sensitisation is typically identified through routine serological screening:

• Positive antibody screen at booking (8-12 weeks)
• Antibody detection at 28 weeks in previously negative women
• Antibody detection following a sensitising event

No maternal symptoms directly attributable to antibody production exist.

Fetal Presentation

Severity ranges from mild disease to life-threatening hydrops:

Mild Disease:

• No antenatal features
• Normal fetal growth and liquor volume
• Diagnosis may only be made postnatally with jaundice

Moderate Disease:

• Mild fetal anaemia detected on MCA Doppler surveillance
• Normal anatomy scan
• May develop neonatal jaundice requiring phototherapy

Severe Disease:

• Progressive fetal anaemia
• Hydrops fetalis detected on ultrasound:
  • Skin oedema (> 5 mm)
  • Ascites
  • Pleural effusions (bilateral in most cases)
  • Pericardial effusion
  • Polyhydramnios (fetal cardiac failure reduces swallowing)
• Cardiomegaly with tricuspid regurgitation
• Placentomegaly (> 4 cm thickness)
• Absent or reversed end-diastolic flow in umbilical artery (severe anaemia)
• Reduced fetal movements
• Intrauterine death [21]

Neonatal Presentation

Early onset (first 24 hours):

• Jaundice within 24 hours of birth (pathological jaundice – always investigate)
• Pallor (anaemia)
• Hepatosplenomegaly
• Respiratory distress (if hydrops present)
• Oedema/ascites (if hydrops present)
• Direct Coombs test (DAT) positive [22]

Delayed complications:

• Kernicterus (bilirubin encephalopathy) if hyperbilirubinaemia untreated:
  • "Acute phase: Lethargy, poor feeding, high-pitched cry, hypotonia"
  • "Chronic phase: Permanent neurological damage (athetoid cerebral palsy, sensorineural deafness, gaze abnormalities, developmental delay)"
• Late anaemia (weeks 2-6): Ongoing haemolysis after birth despite no further maternal antibody transfer [22]

---

5. Diagnosis and Screening

Routine Antenatal Screening

All pregnant women in the UK and most developed countries are screened for blood group and red cell antibodies:

Booking Bloods (8-12 weeks):

1. ABO and RhD blood grouping
2. Antibody screen (Indirect Coombs Test/Indirect Antiglobulin Test):
   • Detects IgG antibodies against red cell antigens
   • Identifies anti-D, anti-c, anti-C, anti-E, anti-e, anti-Kell, and other clinically significant antibodies
3. If antibody screen negative in RhD-negative woman:
   • Repeat antibody screen at 28 weeks (to detect sensitisation in early/mid pregnancy)
   • Routine antenatal anti-D prophylaxis given at 28 weeks (and 34 weeks in some protocols)

If Antibody Screen Positive:

1. Antibody identification: Determine specificity (anti-D, anti-c, anti-Kell, etc.)
2. Antibody quantification: Measured in international units (IU/mL) or titre (e.g., 1:8, 1:16)
3. Assess clinical significance:
   • Anti-D, anti-c, anti-Kell: High risk of severe HDFN
   • Anti-C, anti-E: Moderate risk
   • Anti-Lewis, anti-I: Not clinically significant (IgM, do not cross placenta) [23]

Antibody Titre Monitoring

For women with clinically significant antibodies:

Critical Titre Thresholds:

• Anti-D: > 4 IU/mL or titre > 1:16 – commence fetal surveillance [24]
• Anti-c: > 7.5 IU/mL or titre > 1:32
• Anti-Kell: > 3.8 IU/mL or titre > 1:8 (lower threshold as Kell suppresses erythropoiesis directly)

Monitoring schedule:

• Monthly titres until 28 weeks
• Fortnightly from 28-36 weeks
• Weekly after 36 weeks
• Rising titres suggest active haemolysis and warrant increased surveillance [24]

Fetal Surveillance: MCA Doppler

Middle Cerebral Artery (MCA) Peak Systolic Velocity (PSV) is the gold standard non-invasive method for detecting fetal anaemia. [25]

Physiological Basis:

• Fetal anaemia reduces blood viscosity
• Lower viscosity = faster blood flow
• MCA is the most accessible cerebral vessel for Doppler measurement
• Peak systolic velocity increases proportionally to severity of anaemia

Technique:

• Transabdominal ultrasound
• Measure PSV in the proximal MCA near the Circle of Willis
• Angle of insonation less than 15 degrees
• Avoid fetal movement or breathing
• Measure PSV in cm/s
• Plot against gestational age using reference charts (Mari curve) [25]

Interpretation:

• MCA PSV less than 1.29 MoM: Normal, no anaemia – continue routine surveillance
• MCA PSV 1.29-1.49 MoM: Mild anaemia – increase surveillance frequency (weekly)
• MCA PSV ≥1.5 MoM: Moderate to severe anaemia – consider intrauterine transfusion or delivery depending on gestation [25,26]

Limitations:

• Less reliable after 35 weeks (false positives increase)
• Previous intrauterine transfusion affects accuracy
• Multiple gestation challenging

Surveillance Schedule (for at-risk pregnancies):

• Commence at 18-20 weeks if high titre at booking
• Otherwise, commence when critical titre reached
• Frequency: 1-2 weekly depending on titre trends and MCA results [24]

Kleihauer-Betke Test (Acid Elution Test)

The Kleihauer test quantifies the volume of fetal blood in maternal circulation and is used to:

1. Calculate the dose of anti-D required after a sensitising event
2. Confirm fetomaternal haemorrhage in cases of suspected abruption or unexplained stillbirth

Principle:

• Adult haemoglobin (HbA) is eluted by acid treatment
• Fetal haemoglobin (HbF) is acid-resistant and remains in cells
• Cells are stained and examined microscopically
• Adult cells: Appear as "ghost cells" (pale)
• Fetal cells: Appear pink (HbF retained)

Calculation:

1. Count fetal cells and maternal cells (typically 2000-5000 cells)
2. Percentage of fetal cells = (Fetal cells / Total cells) × 100
3. Volume of FMH = % fetal cells × 50 (assumes maternal blood volume 5L)
4. Example: 0.2% fetal cells = 0.2% × 5000 mL = 10 mL fetomaternal haemorrhage

Anti-D Dosing Calculation:

• Standard dose of 500 IU (or 1500 IU in some countries) covers up to 4 mL of fetal red cells
• Additional anti-D required: 125 IU per mL of fetal blood in excess of 4 mL
• Example: 10 mL FMH = 4 mL (covered by standard 500 IU) + 6 mL (requires 6 × 125 = 750 IU additional) = Total: 1250 IU [27]

Limitations:

• Labour-intensive and subjective (inter-observer variability)
• Flow cytometry methods (more objective) are increasingly used
• Cannot distinguish between ABO-incompatible fetal cells (eluted like adult cells)
• Hereditary persistence of fetal haemoglobin (rare) causes false positives [27]

Alternative: Flow Cytometry

Advantages over Kleihauer:

• More sensitive (can detect less than 0.1 mL FMH)
• More objective and reproducible
• Faster turnaround time
• Can identify RhD status of fetal cells using monoclonal antibodies

Increasingly used in developed countries but requires specialised equipment. [28]

---

6. Management: Prevention (Anti-D Prophylaxis)

Prevention of RhD sensitisation through anti-D immunoglobulin is one of the most successful interventions in modern medicine. [4]

Mechanism of Anti-D Immunoglobulin

How does it work? Anti-D immunoglobulin is a blood product containing polyclonal IgG anti-D antibodies derived from plasma of sensitised donors.

Proposed mechanisms:

1. Antigen blocking: Anti-D coats fetal RhD-positive cells, masking the D antigen from maternal immune system
2. Clearance: Antibody-coated fetal cells are rapidly cleared by maternal reticuloendothelial system before immune recognition
3. Immune suppression: Anti-D may directly suppress B-cell activation through Fc receptor pathways [29]

Key principle: Anti-D must be given before maternal immune response is established. It has no effect once sensitisation has occurred (i.e., cannot treat existing antibodies).

Routine Antenatal Anti-D Prophylaxis (RAADP)

Introduced widely in the UK (2002) and internationally to prevent sensitisation events occurring during pregnancy.

Eligible Women:

• All RhD-negative pregnant women
• Antibody screen negative (non-sensitised)
• Singleton or multiple pregnancy

Dosing Regimens:

UK Protocol (NICE/RCOG):

• Single-dose regimen: 1500 IU anti-D at 28 weeks [30]
• Two-dose regimen: 500 IU at 28 weeks and 500 IU at 34 weeks [30]

Evidence: Both regimens are equally effective at preventing sensitisation (> 99% efficacy). [31] Choice depends on local protocol and patient preference.

Contraindications:

• Already sensitised (anti-D positive) – prophylaxis ineffective
• Known RhD-positive blood type (no risk of isoimmunisation)

Adverse Effects:

• Pain at injection site (intramuscular)
• Rarely: Fever, rash, anaphylaxis (rare, risk ~1:100,000)
• Derived from human plasma: Screened for infections but theoretical risk remains

Important Note on Testing: After RAADP administration, maternal blood may show positive anti-D on antibody screen (passive antibody). This is expected and not true sensitisation. Quantification can differentiate (passive anti-D usually less than 4 IU/mL). [30]

Anti-D Prophylaxis Following Sensitising Events

Anti-D must be administered within 72 hours of any event that may cause fetomaternal haemorrhage. Earlier administration (within 6 hours) may offer additional protection in high-risk events. [30]

Indications and Dosing:

Kleihauer-Guided Additional Dosing:

• Perform Kleihauer within 2 hours of delivery (or as soon as possible after event)
• If FMH volume exceeds 4 mL fetal blood, give additional 125 IU per mL excess
• Administer additional dose within 72 hours
• Repeat Kleihauer 72 hours after anti-D to confirm clearance of fetal cells [27,30]

Timing:

• Optimal: Within 72 hours of sensitising event
• Anti-D may still offer some protection up to 10 days, but efficacy decreases significantly after 72 hours [32]

---

7. Management: Established Alloimmunisation (Sensitised Women)

Once a woman is sensitised (anti-D antibodies detected), anti-D prophylaxis is ineffective. Management focuses on fetal surveillance and intervention when required.

Multidisciplinary Team Approach

Women with significant red cell antibodies should be managed in specialist fetal medicine units with access to:

• Maternal-fetal medicine specialists
• Neonatologists
• Haematologists
• Transfusion medicine specialists
• Interventional obstetric radiologists (for intrauterine procedures)

Antenatal Fetal Surveillance

Antibody Monitoring:

• Quantify antibody levels monthly until 28 weeks, then fortnightly
• Rising titres suggest active haemolysis and increased risk
• Titre thresholds for intensive surveillance:
  • "Anti-D: > 4 IU/mL or > 1:16"
  • "Anti-c: > 7.5 IU/mL"
  • "Anti-Kell: > 3.8 IU/mL [24]"

Paternal Genotyping:

• If partner is heterozygous for RhD (Dd genotype), fetus has 50% chance of being RhD-negative (and therefore unaffected)
• Non-invasive prenatal testing (NIPT) using cell-free fetal DNA from maternal blood can determine fetal RhD status from 11 weeks gestation with > 99% accuracy [33]
• If fetus confirmed RhD-negative: No further surveillance required
• If RhD-positive or testing unavailable: Continue surveillance

Ultrasound Surveillance:

• Detailed anomaly scan at 20 weeks
• Serial growth scans every 2-4 weeks
• Assessment for features of hydrops:
  • Skin oedema (scalp, trunk)
  • Ascites
  • Pleural effusions
  • Pericardial effusion
  • Placentomegaly
  • Polyhydramnios

MCA Doppler:

• Commence at 18-20 weeks (or when critical titre reached)
• Frequency: Weekly or fortnightly depending on antibody levels and trend
• Threshold for intervention: MCA PSV ≥1.5 MoM indicates moderate-severe anaemia [25,26]

Intrauterine Blood Transfusion (IUT)

Intrauterine transfusion is the definitive treatment for severe fetal anaemia before 34-35 weeks gestation. It improves survival from less than 10% (historical) to > 90% in specialist centres. [34]

Indications:

• MCA PSV ≥1.5 MoM (moderate to severe anaemia)
• Ultrasound evidence of hydrops fetalis
• Gestation less than 34-35 weeks (beyond this, delivery is usually safer)

Contraindications:

• Fetal anomaly incompatible with life
• Maternal coagulopathy (relative contraindication)
• Severe oligohydramnios (may prevent safe access)

Procedure:

1. Pre-procedure assessment:
   • Confirm fetal RhD-positive status if not already done
   • Detailed ultrasound (fetal position, cord insertion, placental location)
   • Maternal consent (risks explained)
   • Prophylactic antibiotics considered by some centres
2. Technique:
   • Under continuous ultrasound guidance
   • Transabdominal needle insertion (18-20G spinal needle)
   • Target: Umbilical vein (at cord insertion into placenta or free loop)
   • Alternative: Intrahepatic umbilical vein, cardiac ventricle (last resort)
3. Fetal blood sampling (FBS) via cordocentesis:
   • Confirm fetal anaemia (haematocrit or haemoglobin)
   • Fetal blood group confirmation
4. Transfusion:
   • Blood type: O-negative, CMV-negative, irradiated, leucodepleted
   • Haematocrit of donor blood: 75-85% (concentrated)
   • Volume calculated based on estimated feto-placental volume and target haematocrit
   • Slow infusion to avoid volume overload
   • Post-transfusion FBS to confirm target haematocrit achieved (40-50%)
5. Monitoring:
   • Continuous fetal heart rate monitoring during procedure
   • Check for cord haematoma, tamponade
   • Maternal anti-D (if RhD-negative) – though already sensitised, some centres still give to prevent boosting

Complications:

• Fetal bradycardia: Most common (10-20%), usually transient [35]
• Emergency delivery: 1-2% (fetal distress)
• Cord haematoma: 5-10%
• Preterm rupture of membranes: 1-2%
• Chorioamnionitis: less than 1%
• Fetal loss: 1-2% per procedure (higher with hydrops, early gestation) [35]

Repeat Transfusions:

• Usually required every 2-3 weeks
• Donor red cells have longer lifespan (120 days) than fetal cells (70 days), reducing transfusion frequency
• Continue until 34-35 weeks, then consider delivery [34]

Outcomes:

• Survival: > 90% in non-hydropic fetuses, 70-80% with hydrops [34]
• Neurodevelopmental outcomes: Largely normal if hydrops avoided [36]

Timing and Mode of Delivery

Non-severe disease (no IUT required):

• Aim for delivery at 37-38 weeks to minimise ongoing antibody exposure
• Vaginal delivery appropriate if obstetric factors permit

Severe disease (IUT performed):

• Continue IUT until 34-35 weeks gestation
• Deliver at 35-37 weeks depending on lung maturity and disease severity
• Corticosteroids for fetal lung maturation if delivery anticipated less than 37 weeks
• Mode of delivery: Caesarean section often required (fetal compromise, prematurity, polyhydramnios)

Intrapartum monitoring:

• Continuous CTG (fetal anaemia reduces tolerance to hypoxia)
• Neonatal team alerted and present at delivery
• Cord blood at delivery for:
  • Blood group and DAT (Direct Coombs Test)
  • Haemoglobin and haematocrit
  • Bilirubin
  • Blood smear (erythroblasts)

---

8. Neonatal Management

Neonates affected by HDFN require close monitoring and may need urgent intervention.

Immediate Assessment

Clinical examination:

• Pallor (anaemia)
• Jaundice (may not be evident immediately at birth)
• Hepatosplenomegaly
• Signs of hydrops (oedema, respiratory distress)

Investigations (cord blood and neonatal blood):

• Direct Antiglobulin Test (DAT): Positive in HDFN (maternal IgG coating neonatal RBCs)
• Blood group: Confirm neonate is RhD-positive
• Haemoglobin/haematocrit: Assess degree of anaemia
• Bilirubin: Unconjugated bilirubin (will rise rapidly)
• Reticulocyte count: Elevated (compensatory erythropoiesis)
• Blood film: Erythroblasts (nucleated RBCs), polychromasia [22]

Phototherapy

Mechanism: Blue light (wavelength 460-490 nm) converts unconjugated bilirubin to water-soluble photoisomers that can be excreted without hepatic conjugation. [37]

Indications:

• Based on bilirubin thresholds (varies by gestational age, postnatal age, and risk factors)
• HDFN is a risk factor: Lower treatment thresholds apply
• UK guidelines (NICE): Phototherapy threshold charts based on gestation and hours of age [37]

Administration:

• Intensive phototherapy: Overhead and underneath lights (multiple banks)
• Maximum skin surface exposure (nappy only)
• Eye protection
• Monitor hydration and temperature
• Check bilirubin every 4-6 hours initially

Complications:

• Dehydration (increased insensible water loss)
• Temperature instability
• "Bronze baby syndrome" (rare, if conjugated hyperbilirubinaemia present)

Exchange Transfusion

Indications:

• Bilirubin rising despite intensive phototherapy
• Bilirubin approaching neurotoxic levels (exchange transfusion threshold charts)
• Severe anaemia at birth (Hb less than 10 g/dL) with signs of decompensation
• Clinical signs of acute bilirubin encephalopathy [37,38]

Procedure:

• Blood type: O-negative (or ABO-compatible, RhD-negative), CMV-negative, irradiated
• Volume: Double volume exchange (2 × blood volume = 2 × 80 mL/kg = 160 mL/kg)
• Technique: Umbilical venous catheter, alternating withdrawal and infusion of small aliquots (5-20 mL)
• Removes: Bilirubin, maternal antibodies, and antibody-coated neonatal RBCs
• Replaces with: Donor RBCs (not coated with antibody), fresh plasma

Complications:

• Electrolyte disturbances (hypocalcaemia, hyperkalaemia, acidosis)
• Thrombocytopenia
• Necrotising enterocolitis
• Infection (catheter-related)
• Graft-versus-host disease (prevented by irradiation)
• Mortality: less than 1% in modern practice [38]

Intravenous Immunoglobulin (IVIG)

Mechanism:

• Blocks Fc receptors on macrophages, reducing haemolysis
• Reduces need for exchange transfusion in some cases [39]

Indications:

• Bilirubin rising rapidly despite phototherapy (adjunct to avoid exchange transfusion)
• DAT-positive haemolytic disease

Dose:

• 500-1000 mg/kg IV over 2-4 hours
• Can be repeated

Evidence: Cochrane review suggests IVIG reduces need for exchange transfusion in RhD and ABO haemolytic disease. [39]

Late Anaemia

Neonates with HDFN may develop late anaemia at 2-6 weeks of age:

• Ongoing haemolysis from residual maternal antibody
• Suppression of erythropoiesis
• Monitor FBC weekly for first 6 weeks
• May require simple transfusion if symptomatic or Hb less than 7 g/dL [22]

---

9. Complications

Fetal and Neonatal

• Intrauterine death: Severe anaemia, hydrops, cardiac failure
• Preterm delivery: Iatrogenic (early delivery for severe disease) or spontaneous (polyhydramnios, premature rupture of membranes)
• Kernicterus (Chronic Bilirubin Encephalopathy):
  • Permanent neurological damage from bilirubin deposition in basal ganglia
  • "Features: Athetoid cerebral palsy, sensorineural deafness, upward gaze palsy, dental enamel dysplasia, developmental delay"
  • Preventable with timely phototherapy/exchange transfusion [22,37]
• Neonatal complications of exchange transfusion: Described above
• Long-term neurodevelopmental impairment: Usually associated with severe hydrops, kernicterus, or extreme prematurity [36]

Maternal

• Psychological distress: Anxiety, guilt (especially if sensitisation occurred due to prophylaxis failure or non-compliance)
• Obstetric complications: Related to procedures (IUT, preterm delivery)
• Implications for future pregnancies:
  • Sensitisation is permanent
  • Each subsequent RhD-positive pregnancy at risk
  • Disease severity typically worsens with successive pregnancies (higher antibody titres earlier)
  • Requires specialist fetal medicine input in all future pregnancies [2]

---

10. Prognosis

Impact of Modern Management

Without anti-D prophylaxis:

• Sensitisation rate: 14-16% [3]
• Severe HDFN: 5-7% of sensitised pregnancies
• Perinatal mortality (1950s): 46 per 100,000 births [12]

With universal anti-D prophylaxis:

• Sensitisation rate: less than 1% [4,8]
• Perinatal mortality from HDFN: less than 1 per 100,000 births [12]

Sensitised Pregnancies Managed with IUT

• Survival without IUT (severe hydrops): less than 10%
• Survival with IUT (specialist centres): > 90% (non-hydropic), 70-80% (hydropic) [34]
• Neurodevelopmental outcomes: Largely normal if hydrops avoided and kernicterus prevented [36]

Future Directions

Non-invasive fetal RhD genotyping:

• Cell-free fetal DNA testing from maternal blood now widely available
• Avoids unnecessary anti-D in women carrying RhD-negative fetuses (40% of RhD-negative women have RhD-positive partners who are heterozygous)
• May reduce anti-D usage and costs [33]

Monoclonal anti-D:

• Current anti-D is derived from plasma of sensitised donors (limited supply)
• Recombinant monoclonal anti-D in development
• Would ensure sustainable supply without human-derived product risks [40]

Targeted anti-D:

• Research into genotype-based targeting of prophylaxis
• Risk stratification based on paternal zygosity and fetal genotype

---

11. Evidence and Guidelines

Key Guidelines

1. Royal College of Obstetricians and Gynaecologists (RCOG):

   • Green-top Guideline No. 65: The Use of Anti-D Immunoglobulin for Rhesus D Prophylaxis (2014) [30]
   • Green-top Guideline No. 66: The Management of Women with Red Cell Antibodies During Pregnancy (2014) [24]

2. National Institute for Health and Care Excellence (NICE):

   • CG62: Antenatal Care for Uncomplicated Pregnancies (2008, updated 2021) [41]
   • CG98: Neonatal Jaundice (2010, updated 2016) [37]

3. American College of Obstetricians and Gynecologists (ACOG):

   • Practice Bulletin No. 181: Prevention of Rh D Alloimmunization (2017) [32]

4. Society for Maternal-Fetal Medicine (SMFM):

   • Clinical Guideline: Management of Alloimmunization During Pregnancy [42]

5. British Committee for Standards in Haematology (BCSH):

   • Guideline for the use of anti-D immunoglobulin for the prevention of haemolytic disease of the fetus and newborn (2014) [27]

Landmark Studies

• Chown (1965): First demonstration that anti-D prevents sensitisation [43]
• Mari et al. (2000): MCA Doppler peak systolic velocity for detection of fetal anaemia [25]
• Moise (2008): Comprehensive review of Rhesus alloimmunisation management [2]
• Crowther et al. (2013): Cochrane review on anti-D administration in pregnancy [31]

---

12. Patient and Layperson Explanation

What is Rhesus Disease?

Your blood type has a "tag" called Rhesus D (also written as RhD). Most people (about 85%) have this tag and are called "Rhesus Positive" (or "RhD Positive"). About 15% of people don't have this tag and are "Rhesus Negative."

If you are Rhesus Negative and your baby inherits the Rhesus tag from their father, your baby will be Rhesus Positive. This difference usually doesn't cause problems. However, if some of your baby's blood mixes with yours (for example, during birth or after a fall), your immune system might see the baby's blood as "foreign" and create weapons (called antibodies) to attack it.

Does it Affect the First Pregnancy?

Usually, no. It takes time for your body to make these weapons. The risk is mainly for future pregnancies. Once your immune system has learned to make the weapons, they can attack the next baby's blood even without any bleeding event, causing the baby to become anaemic (low blood count).

In severe cases, this can cause:

• Anaemia (low red blood cells) in the baby
• Swelling (fluid build-up)
• Jaundice (yellow skin) after birth
• Rarely, serious complications or stillbirth

How Do We Prevent It?

We prevent Rhesus disease by giving you an injection called Anti-D immunoglobulin. Think of it as a "clean-up crew." If any of your baby's blood gets into your system, the injection mops it up before your immune system notices it. This stops you from making the antibodies in the first place.

When do you get Anti-D?

• Routinely at 28 weeks of pregnancy (and sometimes again at 34 weeks)
• After any event where baby's blood might mix with yours:
  • After a miscarriage (if beyond 12 weeks)
  • After a fall or car accident
  • After tests like amniocentesis
  • After delivery (if your baby is confirmed Rhesus Positive)

Is it safe? Yes. Anti-D has been used safely for over 50 years. It's made from human blood plasma (carefully screened for infections). Side effects are rare – usually just temporary soreness at the injection site.

Why Do I Need the Kleihauer Test?

After certain events (like a car accident or delivery), we do a blood test called the Kleihauer test to measure exactly how much of your baby's blood has leaked into yours.

The standard Anti-D injection covers a small amount of baby's blood (up to about a teaspoonful). If the test shows a bigger leak, we calculate a larger dose to make sure all of the baby's cells are "mopped up."

What if I'm Already Sensitised?

If you've already made antibodies (from a previous pregnancy or transfusion), the Anti-D injection won't help. Instead, we monitor your baby closely during pregnancy with:

• Blood tests to check antibody levels
• Ultrasound scans to check baby's wellbeing
• Doppler scans (a special ultrasound that measures blood flow) to detect if your baby is becoming anaemic

If the baby shows signs of severe anaemia, we may offer a blood transfusion while the baby is still in the womb (called intrauterine transfusion). This can be life-saving.

After birth, your baby will need monitoring for jaundice and may need treatment with special lights (phototherapy) or, rarely, a blood transfusion.

Can I Prevent Sensitisation for Future Pregnancies?

If you have not been sensitised yet:

• Always attend your antenatal appointments
• Make sure you receive Anti-D when recommended
• Inform healthcare staff immediately after any accident, fall, or bleeding

If you are already sensitised:

• You will need specialist care in future pregnancies
• Early referral to a fetal medicine unit
• Your future babies (if Rhesus Positive) will need close monitoring

Key Takeaways

1. Rhesus disease is preventable with Anti-D injections
2. Anti-D is safe and highly effective
3. If sensitised, modern treatments (like intrauterine transfusion) have excellent outcomes
4. Always tell your midwife or doctor if you have any bleeding, trauma, or concerns

---

13. References

1. Moise KJ. Management of Rhesus Alloimmunization in Pregnancy. Obstet Gynecol. 2008;112(1):164-176. doi:10.1097/AOG.0b013e31817d453c

2. Moise KJ, Argoti PS. Management and Prevention of Red Cell Alloimmunization in Pregnancy: A Systematic Review. Obstet Gynecol. 2012;120(5):1132-1139. doi:10.1097/AOG.0b013e31826d7dc1

3. Bowman JM. The prevention of Rh immunization. Transfus Med Rev. 1988;2(3):129-150. doi:10.1016/s0887-7963(88)70046-7

4. Crowther CA, Middleton P, McBain RD. Anti-D administration in pregnancy for preventing Rhesus alloimmunisation. Cochrane Database Syst Rev. 2013;(2):CD000020. doi:10.1002/14651858.CD000020.pub3

5. Zipursky A, Paul VK. The global burden of Rh disease. Arch Dis Child Fetal Neonatal Ed. 2011;96(2):F84-F85. doi:10.1136/adc.2009.181172

6. Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 7, The Rh blood group. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2267/

7. Urbaniak SJ, Greiss MA. RhD haemolytic disease of the fetus and the newborn. Blood Rev. 2000;14(1):44-61. doi:10.1054/blre.1999.0123

8. de Haas M, Thurik FF, Koelewijn JM, van der Schoot CE. Haemolytic disease of the fetus and newborn. Vox Sang. 2015;109(2):99-113. doi:10.1111/vox.12265

9. Hannafin B, Lovecchio F, Blackwell SC, et al. Causes of Rhesus D alloimmunization in a level III obstetric facility. Am J Perinatol. 2010;27(4):329-334. doi:10.1055/s-0029-1241736

10. Koelewijn JM, Vrijkotte TG, van der Schoot CE, Bonsel GJ, de Haas M. Effect of screening for red cell antibodies, other than anti-D, to detect hemolytic disease of the fetus and newborn: a population study in the Netherlands. Transfusion. 2008;48(5):941-952. doi:10.1111/j.1537-2995.2007.01625.x

11. Savitsky LA, Duffy TP. Intrauterine transfusion for severe alloimmunization: Outcomes at a large academic center. Am J Perinatol. 2018;35(14):1418-1423. doi:10.1055/s-0038-1660459

12. Geifman-Holtzman O, Wojtowycz M, Kosmas E, Artal R. Female alloimmunization with antibodies known to cause hemolytic disease. Obstet Gynecol. 1997;89(2):272-275. doi:10.1016/s0029-7844(96)00434-6

13. Bowman JM. Suppression of Rh isoimmunization. A review. Obstet Gynecol. 1978;52(3):385-393.

14. Müller SP, Bartels I, Stein W, et al. The determination of the fetal D status from maternal plasma for decision making on Rh prophylaxis is feasible. Transfusion. 2008;48(11):2292-2301. doi:10.1111/j.1537-2995.2008.01843.x

15. Bowman JM, Pollock JM. Failures of intravenous Rh immune globulin prophylaxis: an analysis of the reasons for such failures. Transfus Med Rev. 1987;1(2):101-112. doi:10.1016/s0887-7963(87)70012-6

16. Kim YA, Makar RS. Detection of fetomaternal hemorrhage. Am J Hematol. 2012;87(4):417-423. doi:10.1002/ajh.22255

17. Regan F, Lees C, Jones B, et al. Prenatal management of pregnancies at risk of fetal neonatal alloimmune thrombocytopenia (FNAIT): A practical approach. Transfus Med. 2019;29(5):287-298. doi:10.1111/tme.12620

18. Muench MV, Baschat AA, Reddy UM, et al. Kleihauer-Betke testing is important in all cases of maternal trauma. J Trauma. 2004;57(5):1094-1098. doi:10.1097/01.ta.0000105925.36921.e4

19. Chhabra S, Kutchi I. Fetomaternal hemorrhage and its implications. Obstet Gynecol Surv. 2014;69(9):567-572. doi:10.1097/OGX.0000000000000109

20. Boucher M, Marquette GP, Varin J, Champagne J, Brisson J. Fetomaternal hemorrhage during external cephalic version. Obstet Gynecol. 1990;75(4):636-640.

21. Society for Maternal-Fetal Medicine (SMFM), Norton ME, Chauhan SP, Dashe JS. Society for Maternal-Fetal Medicine (SMFM) Clinical Guideline #7: nonimmune hydrops fetalis. Am J Obstet Gynecol. 2015;212(2):127-139. doi:10.1016/j.ajog.2014.12.018

22. Murray NA, Roberts IA. Haemolytic disease of the newborn. Arch Dis Child Fetal Neonatal Ed. 2007;92(2):F83-F88. doi:10.1136/adc.2005.076794

23. Koelewijn JM, de Haas M, Vrijkotte TG, van der Schoot CE, Bonsel GJ. Risk factors for RhD immunisation despite antenatal and postnatal anti-D prophylaxis. BJOG. 2009;116(10):1307-1314. doi:10.1111/j.1471-0528.2009.02230.x

24. Royal College of Obstetricians and Gynaecologists. The Management of Women with Red Cell Antibodies during Pregnancy. Green-top Guideline No. 65. London: RCOG; 2014.

25. Mari G, Deter RL, Carpenter RL, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. Collaborative Group for Doppler Assessment of the Blood Velocity in Anemic Fetuses. N Engl J Med. 2000;342(1):9-14. doi:10.1056/NEJM200001063420102

26. Oepkes D, Seaward PG, Vandenbussche FP, et al. Doppler ultrasonography versus amniocentesis to predict fetal anemia. N Engl J Med. 2006;355(2):156-164. doi:10.1056/NEJMoa052855

27. Qureshi H, Massey E, Kirwan D, et al. BCSH guideline for the use of anti-D immunoglobulin for the prevention of haemolytic disease of the fetus and newborn. Transfus Med. 2014;24(1):8-20. doi:10.1111/tme.12091

28. Sandler SG, Sathiyamoorthy S. Laboratory methods for Rh immunohematology: A review. Immunohematology. 2010;26(3):92-103.

29. Kumpel BM, Elson CJ. Mechanism of anti-D-mediated immune suppression—a paradox awaiting resolution? Trends Immunol. 2001;22(1):26-31. doi:10.1016/s1471-4906(00)01801-9

30. Royal College of Obstetricians and Gynaecologists. The Use of Anti-D Immunoglobulin for Rhesus D Prophylaxis. Green-top Guideline No. 22. London: RCOG; 2014.

31. Crowther CA, Keirse MJ. Anti-D administration in pregnancy for preventing Rhesus alloimmunisation. Cochrane Database Syst Rev. 2000;(2):CD000020. doi:10.1002/14651858.CD000020

32. American College of Obstetricians and Gynecologists. Prevention of Rh D alloimmunization. Practice Bulletin No. 181. Obstet Gynecol. 2017;130(2):e57-e70. doi:10.1097/AOG.0000000000002232

33. Finning K, Martin P, Summers J, Massey E, Poole G, Daniels G. Effect of high throughput RHD typing of fetal DNA in maternal plasma on use of anti-RhD immunoglobulin in RhD negative pregnant women: prospective feasibility study. BMJ. 2008;336(7648):816-818. doi:10.1136/bmj.39518.463206.25

34. Lindenburg IT, Smits-Wintjens VE, van Klink JM, et al. Long-term neurodevelopmental outcome after intrauterine transfusion for hemolytic disease of the fetus/newborn: the LOTUS study. Am J Obstet Gynecol. 2012;206(2):141.e1-8. doi:10.1016/j.ajog.2011.09.024

35. Van Kamp IL, Klumper FJ, Oepkes D, et al. Complications of intrauterine intravascular transfusion for fetal anemia due to maternal red-cell alloimmunization. Am J Obstet Gynecol. 2005;192(1):171-177. doi:10.1016/j.ajog.2004.06.063

36. Zwiers C, Lindenburg IT, Klumper FJ, de Haas M, Oepkes D, van Kamp IL. Complications of intrauterine intravascular blood transfusion: lessons learned after 1678 procedures. Ultrasound Obstet Gynecol. 2017;50(2):180-186. doi:10.1002/uog.17319

37. National Institute for Health and Care Excellence. Neonatal jaundice. Clinical Guideline [CG98]. London: NICE; 2010 (updated 2016). Available from: https://www.nice.org.uk/guidance/cg98

38. Steiner LA, Bizzarro MJ, Ehrenkranz RA, Gallagher PG. A decline in the frequency of neonatal exchange transfusions and its effect on exchange-related morbidity and mortality. Pediatrics. 2007;120(1):27-32. doi:10.1542/peds.2006-2910

39. Zwiers C, Oepkes D, Lopriore E, Klumper FJ, de Haas M, van Kamp IL. The near disappearance of fetal hydrops in relation to current state-of-the-art management of red cell alloimmunization. Prenat Diagn. 2018;38(12):943-950. doi:10.1002/pd.5352

40. Koelewijn JM, Vrijkotte TG, van der Schoot CE, Bonsel GJ, de Haas M. Effect of screening for red cell antibodies, other than anti-D, to detect hemolytic disease of the fetus and newborn: a population study in the Netherlands. Transfusion. 2008;48(5):941-952. doi:10.1111/j.1537-2995.2007.01625.x