A 28-year-old primigravida with Rh-negative blood group is evaluated at 28 weeks gestation. Indirect Coombs test (ICT) is positive at 1:16 dilution. Which single feature best distinguishes Rh isoimmunisation from ABO incompatibility in the newborn?

## Distinguishing Feature: Severity and Hydrops Fetalis **Key Point:** Rh isoimmunisation causes severe haemolytic disease of the newborn (HDN) with risk of hydrops fetalis and intrauterine death, whereas ABO incompatibility is typically mild and self-limited. ### Comparison Table: Rh vs ABO Incompatibility | Feature | Rh Isoimmunisation | ABO Incompatibility | | --- | --- | --- | | **Severity** | Severe (can cause hydrops, IUFD) | Mild to moderate | | **Hydrops Fetalis** | Common in sensitised pregnancies | Rare | | **Anaemia** | Severe (Hb often <7 g/dL) | Mild to moderate | | **Jaundice onset** | 24–48 hours (can be earlier) | First 24 hours (often within 12 hrs) | | **DAT (Cord blood)** | Strongly positive, IgG predominant | Weakly positive, IgM + IgG | | **Exchange transfusion need** | Frequent (>50% of cases) | Rare (<5% of cases) | | **Recurrence in next pregnancy** | Yes, often more severe | No (ABO antibodies naturally present) | **High-Yield:** Rh isoimmunisation is a **progressive disease** — each sensitised pregnancy worsens due to anamnestic IgG response. Severe anaemia, hydrops, and intrauterine death are hallmarks. ABO incompatibility, by contrast, is mild because: - ABO antibodies are IgM (do not cross placenta efficiently) - Fetal RBCs have fewer A/B antigens than adult RBCs - Natural ABO antibodies limit severe sensitisation **Clinical Pearl:** A Rh-negative primigravida with a positive ICT at 28 weeks signals active sensitisation; the risk of severe HDN and fetal hydrops is substantial. Monitoring with serial ultrasound (peak systolic velocity of MCA) and possible intrauterine transfusion are indicated. In contrast, ABO incompatibility rarely necessitates intrauterine intervention. ### Why the Correct Answer Stands Option 2 (severe anaemia and hydrops fetalis) is the **single most discriminating feature**. Rh isoimmunisation uniquely carries the risk of: - Profound fetal anaemia (Hb <5 g/dL in severe cases) - Hydrops fetalis (ascites, pericardial effusion, pulmonary oedema) - Intrauterine fetal death if untreated ABO incompatibility does not produce these severe manifestations because of the IgM nature of ABO antibodies and reduced antigen density on fetal RBCs.