How are Rh‑negative maternal alloimmunization, ABO incompatibility, and indirect and direct Coombs testing managed during pregnancy and in the newborn?

Understanding Rh Negativity, ABO Incompatibility, and Coombs Testing in Pregnancy and Newborns

Rh Negative Pregnancy Management

All Rh-negative pregnant women should receive RhD immune globulin (RhIg) at 28 weeks gestation and within 72 hours after delivery of an Rh-positive infant, which reduces alloimmunization rates from 1.8% to 0.1-0.2%. 1

Why This Matters

• Fetal red blood cells display RhD antigens from as early as 6 weeks of gestation, making maternal sensitization possible even in very early pregnancy 1, 2
• Without prophylaxis, postpartum alloimmunization occurs in 13-17% of Rh-negative mothers delivering Rh-positive infants 1
• Once sensitized, a mother remains immunized for life and faces risk of hemolytic disease of the fetus and newborn (HDFN) in subsequent pregnancies, which can cause fetal hydrops, stillbirth, and need for fetal transfusion 1, 3

The Critical 72-Hour Window

• RhIg must be administered within 72 hours of any potentially sensitizing event for optimal effectiveness 2
• If delayed beyond 72 hours, administration up to 28 days postpartum still provides some benefit and is preferable to no administration 1
• As little as 0.03-0.1 mL of fetal Rh-positive red blood cells can trigger primary alloimmunization 2

When to Give RhIg During Pregnancy

Standard prophylaxis:

• Routine dose at 28 weeks gestation 1
• Postpartum dose within 72 hours if infant is Rh-positive 1

Additional doses required for:

• Any vaginal bleeding at any gestational age 1
• Spontaneous or induced abortion (50 μg dose adequate before 12 weeks; use 300 μg if 50 μg unavailable) 1
• Threatened abortion with heavy bleeding or abdominal pain 1
• Minor trauma (28% of pregnant patients with minor trauma have fetomaternal hemorrhage) 1
• Invasive procedures like amniocentesis, chorionic villus sampling, or external cephalic version 2

Common Pitfalls to Avoid

• Do not assume early gestational age eliminates risk - fetal RBCs with D-antigen are present from 6 weeks onward, yet many clinicians mistakenly withhold RhIg for first-trimester events 1, 2
• Do not withhold RhIg for "minimal" bleeding - fetomaternal hemorrhage occurs in 48% of threatened abortions, 36% of complete abortions, and 22% of incomplete abortions 1
• Most sensitization (90%) occurs at delivery itself, but antepartum exposure increases with gestational age: 7% in first trimester, 16% in second trimester, and 29% in third trimester 2

ABO Incompatibility

ABO blood group mismatch between mother and infant is the most common cause of positive direct Coombs test in newborns, accounting for 73.6% of cases, though it typically causes milder disease than Rh incompatibility. 4

Key Differences from Rh Disease

• ABO incompatibility most commonly occurs when the mother is blood type O and the infant is type A or B 4
• Unlike Rh disease, ABO incompatibility can affect first pregnancies because mothers have naturally occurring anti-A and anti-B antibodies 4
• The disease is generally less severe than Rh-mediated hemolysis 4

Clinical Consequences

• Approximately 47.6% of newborns with positive direct Coombs test require treatment for neonatal jaundice 4
• Most cases (93.3%) respond to phototherapy alone 4
• Exchange transfusion is rarely needed for ABO incompatibility (only 3 of 8 exchange transfusions in one series were for ABO mismatch, compared to 5 for Rh antibodies) 4

No Prevention Available

• Unlike Rh disease, there is no prophylactic treatment to prevent ABO incompatibility 4
• Management is entirely postnatal, focusing on monitoring and treating neonatal jaundice 4

Coombs Testing Explained

Indirect Coombs Test (Antibody Screen)

The indirect Coombs test detects antibodies circulating in maternal serum and is used to screen pregnant women for red cell alloimmunization. 3

• Performed on maternal blood during pregnancy 3
• Detects IgG antibodies against red blood cell antigens before they cause fetal harm 3
• Should be performed at first prenatal visit and repeated at 28 weeks in Rh-negative women 1
• If positive, indicates maternal sensitization has already occurred and requires specialist management 1

Direct Coombs Test (Direct Antiglobulin Test/DAT)

The direct Coombs test detects antibodies already bound to fetal/newborn red blood cells and is performed on cord blood or newborn blood to diagnose hemolytic disease. 3, 4

• Performed on infant's blood (cord blood or direct sample) 4
• Positive result indicates antibodies are coating the baby's red cells and causing or threatening hemolysis 4
• In Iceland, DAT is routinely performed on cord blood if the mother is Rh-negative or has non-A/B red cell alloantibodies 4

When Each Test is Used

Maternal screening (indirect Coombs):

• First prenatal visit for all pregnant women 3
• 28 weeks gestation for Rh-negative women 1
• After any potentially sensitizing event in Rh-negative women 1
• At least 4 weeks after exposure to allow time for antibody development 2

Newborn testing (direct Coombs):

• Cord blood of all infants born to Rh-negative mothers 4
• Cord blood of infants whose mothers have known red cell alloantibodies 4
• Any newborn with unexplained jaundice or anemia 4

Advanced Testing: Cell-Free Fetal DNA

Cell-free fetal DNA testing from maternal plasma can determine fetal RhD status with 97.2% sensitivity and 96.8% specificity, and exceeds 99% accuracy when performed after 11 weeks gestation. 1, 5

• This non-invasive test allows targeted RhIg administration only to mothers carrying Rh-positive fetuses 1, 5
• Avoids unnecessary RhIg administration and indirect Coombs testing in mothers carrying Rh-negative fetuses 5
• Cost-efficient approach that is becoming standard of care in many countries 5

Why Failures Still Occur

Despite comprehensive prophylaxis programs, anti-D remains the most common cause of severe HDFN 6. Failures occur due to:

• Silent sensitization without identified precipitating events 2
• Missed doses or inadequate dosing for large fetomaternal hemorrhage 1
• Individual immune variability (only 60% of exposed individuals develop antibodies, but those who do may have been missed) 2
• True immunization rates are likely higher than observed 17% because low-level antibodies may remain undetectable until an anamnestic response in a subsequent pregnancy 2