Overview
Hemolytic disease of the newborn (HDNB) occurs when maternal IgG antibodies cross the placenta into fetal circulation and destroy fetal red cells. These antibodies are most commonly directed against Rh (particularly D antigen) or ABO blood group antigens.
While ABO incompatibility is more common, Rh incompatibility is clinically more severe. Timely antenatal screening, monitoring of maternal antibody titres, and Anti-D prophylaxis have dramatically reduced the incidence of severe HDNB worldwide.
Learning Objectives
After this lesson you will be able to…- Explain the pathology of hemolytic disease of the newborn.
- Discuss the antenatal assessment of hemolytic disease of the newborn.
- Describe the tests performed on maternal and cord blood at delivery.
- Explain the prophylaxis for hemolytic disease of the newborn.
- Differentiate the severity and mechanism of Rh versus ABO incompatibility.
Clinical Story
Why This MattersAn Rh-negative woman in her second pregnancy is found to have a rising anti-D titre on routine antenatal screening. The lab's serial monitoring and amniotic fluid optical density analysis (Liley curve) allow the obstetric team to plan early delivery and prepare for possible intrauterine transfusion — averting the severe fetal anemia and hydrops that claimed her first unmonitored pregnancy.
Core Concepts
Maternal IgG antibodies cross the placenta and react with fetal red cell antigens, causing their destruction. Risk depends on the volume of fetomaternal hemorrhage and the immunogenicity of the antigen plus maternal immune response. In Rh incompatibility, an Rh-negative mother carrying an Rh-positive fetus becomes sensitized when fetal cells cross into her circulation — the first pregnancy is usually unaffected, but subsequent Rh-positive pregnancies trigger a severe secondary IgG response.
More common than Rh incompatibility but usually milder. Antibodies are typically IgM (cannot cross placenta); IgG anti-A/anti-B occurs more often when the mother is blood group O carrying a group A or B fetus. It can occur even in a first pregnancy but is milder because fetal A/B antigens are weakly expressed and are also neutralized by widespread carbohydrate antigens in fetal tissues/fluids.
About 50% of affected infants are asymptomatic or mildly affected, 30% have moderate anemia and hyperbilirubinemia, and approximately 20% are severely affected and may develop hydrops fetalis.
All pregnant women should have ABO/Rh grouping and antibody screening (indirect Coomb's test) at first antenatal visit. Cord blood tests: ABO & Rh group, Direct Coomb's test, hemoglobin, bilirubin. Maternal blood tests: ABO & Rh group, anti-D titre (indirect Coomb's), Kleihauer-Betke test (quantifies fetomaternal hemorrhage to guide Anti-D dosing), amniotic fluid spectrophotometry (Liley curve based on optical density at 450nm), and Doppler-based non-invasive fetal monitoring.
Anti-D immunoglobulin is administered to Rh-negative women during pregnancy and after delivery or abortion to neutralize any fetal Rh-positive cells before the mother's immune system can mount a sensitizing response, minimizing the risk of HDNB in future pregnancies.
Laboratory Principle
The Kleihauer-Betke acid dilution test exploits the fact that fetal hemoglobin (HbF) resists acid elution while adult hemoglobin (HbA) is eluted from the red cell, leaving "ghost" cells. Counting resistant fetal cells against total cells quantifies the volume of fetomaternal hemorrhage. Amniotic fluid spectrophotometry uses the principle that optical density at 450 nm rises proportionally with bilirubin concentration, reflecting the severity of fetal hemolysis via the Liley curve.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Anti-A, Anti-B, Anti-D reagents | Monoclonal typing sera | ABO & Rh blood grouping | 2–8°C |
| AHG reagent | Polyspecific/monospecific | Direct & Indirect Coomb's testing | 2–8°C |
| Acid elution buffer (Kleihauer-Betke) | Citric acid-phosphate buffer | Elutes adult Hb, leaving fetal cells stained | Room temperature, freshly prepared |
| Anti-D immunoglobulin | Standard prophylactic dose (per protocol) | Prevention of maternal Rh sensitization | 2–8°C |
Step-by-Step Procedure
Determine maternal ABO and Rh group; perform indirect Coomb's test to screen for red cell antibodies.
In Rh-negative, sensitized women, serially monitor anti-D titre across pregnancy to track rising risk.
Perform spectrophotometric analysis of amniotic fluid optical density at 450nm and plot against the Liley curve to assess fetal hemolysis severity.
Test cord blood for ABO/Rh group, Direct Coomb's test, hemoglobin and bilirubin.
Quantify fetomaternal hemorrhage using the Kleihauer-Betke acid elution test to calculate the correct prophylactic Anti-D dose for the mother.
Flow Diagram
Quality Control
Run positive and negative controls with every Direct Coomb's test on cord blood. Validate Kleihauer-Betke acid elution staining with known fetal and adult red cell controls each batch.
Blood bank and antenatal screening laboratories should participate in EQAS programs for blood grouping and antibody titration to maintain inter-laboratory consistency.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Positive Direct Coomb's test on cord blood | Maternal antibody has coated fetal RBCs (Rh or ABO HDNB) | Monitor bilirubin/hemoglobin; consider phototherapy/exchange transfusion |
| Rising maternal anti-D titre across pregnancy | Increasing risk of fetal hemolysis | Increase monitoring frequency; consider amniotic fluid analysis |
| High optical density at 450nm (Liley zone 3) | Severe fetal hemolysis | Consider early delivery or intrauterine transfusion |
Common Errors & How to Avoid Them
Cause: Failure to give Anti-D promptly after delivery, abortion, or sensitizing events (e.g. amniocentesis).
Prevention: Follow institutional protocols for Anti-D timing strictly within the recommended window post-delivery/event.
Cause: Inaccurate counting in the Kleihauer-Betke test due to poor smear quality or counting too few cells.
Prevention: Count a sufficient number of cells (typically 2000+) across multiple fields for accurate quantification.
Cause: Maternal blood contamination of the cord sample gives false grouping/Coomb's results.
Prevention: Collect cord blood carefully from the umbilical vein away from the maternal surface of the placenta.
Laboratory Tips from the Bench
Always determine maternal ABO and Rh group as early as possible in pregnancy — this single test sets the entire monitoring pathway for the rest of the pregnancy.
Remember that ABO incompatibility can occur even in a first pregnancy, unlike Rh incompatibility which typically spares the first pregnancy.
"Rh needs a repeat, ABO strikes first" — Rh HDNB usually needs sensitization from a prior pregnancy; ABO HDNB can occur even the first time.
Important Notes
Although ABO incompatibility occurs more frequently than Rh incompatibility, Rh disease is clinically far more severe and historically responsible for the most serious cases of HDNB.
Sensitizing events include delivery (vaginal or caesarean), abortion, invasive procedures like amniocentesis, and other clinical maneuvers — Anti-D should be considered after any of these in Rh-negative women.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeSecond pregnancy; did not receive Anti-D after her first delivery. Routine antenatal antibody screening at 28 weeks shows a positive indirect Coomb's test.
Rising anti-D titre with elevated amniotic fluid bilirubin and increased fetal MCA Doppler velocity indicate significant fetal hemolysis and anemia, requiring close monitoring and possible early intervention.
- →Missed Anti-D prophylaxis after a prior pregnancy increases future sensitization risk.
- →Serial antibody titres and Doppler studies guide the timing of delivery or intervention.
- →Early antenatal screening is key to identifying at-risk pregnancies.
Frequently Asked Questions
The mother's initial exposure to Rh-positive fetal cells (often during delivery) triggers a slow primary IgM response that does not significantly affect the current pregnancy. Only in subsequent pregnancies does the faster, larger IgG secondary response cause severe disease.
Yes, HDNB is not limited to Rh(D) — it can also occur due to ABO incompatibility or other minor blood group antigens, regardless of the mother's Rh status.
Anti-D is typically given routinely during pregnancy per protocol (e.g. around 28 weeks), and additionally after any potentially sensitizing event such as delivery, abortion, amniocentesis or abdominal trauma in Rh-negative women.
Quick Revision
10-Minute ReviewKey Takeaways
- HDNB results from maternal IgG antibody-mediated destruction of fetal red cells.
- Rh incompatibility is clinically severe; ABO incompatibility is common but milder.
- Antenatal screening with ABO/Rh grouping and indirect Coomb's test identifies at-risk pregnancies.
- Cord and maternal blood testing at delivery confirm diagnosis and guide management.
- The Kleihauer-Betke test quantifies fetomaternal hemorrhage to guide Anti-D dosing.
- Anti-D prophylaxis has dramatically reduced the incidence of severe Rh HDNB.
Competency Checklist
Track Your MasteryReferences
- NIOS Hematology and Blood Bank Technique — Lesson 26: Hemolytic Disease of the New Born (HDNB).
- Liley AW. Liquor amnii analysis in the management of pregnancy complicated by rhesus sensitization.