A 28-year-old primigravida at 32 weeks presents with a positive indirect Coombs test. She is Rh-negative and her partner is Rh-positive. Which single feature best distinguishes Rh isoimmunisation from ABO incompatibility in the newborn?

Explanation

## Discriminating Feature: Parity Pattern **Key Point:** The cardinal distinguishing feature of Rh isoimmunisation is that severity **increases with successive pregnancies**, whereas ABO incompatibility remains relatively constant or even decreases. ### Pathophysiology of Rh Isoimmunisation **High-Yield:** Rh isoimmunisation is a **secondary immune response** (IgG antibodies) that develops after sensitisation in a prior pregnancy or transfusion. Each subsequent pregnancy triggers a stronger anamnestic response, leading to progressive worsening of hemolytic disease. ### Comparison Table: Rh Isoimmunisation vs ABO Incompatibility | Feature | Rh Isoimmunisation | ABO Incompatibility | | --- | --- | --- | | **Parity pattern** | Worsens with each pregnancy | Constant or improves | | **First pregnancy** | Rarely affected | Often affected (natural IgG) | | **Antibody type** | IgG (secondary response) | IgG (natural, primary) | | **Jaundice onset** | Day 2–7 of life | Day 1–2 (can be within 24 hrs) | | **Hydrops fetalis** | Common in severe cases | Rare | | **Direct Coombs test** | Strongly positive | Weakly positive or negative | | **Severity trajectory** | Progressive worsening | Stable or improves | ### Why Other Options Are Misleading **Clinical Pearl:** While jaundice within 24 hours *can* occur in severe Rh disease, it is **not pathognomonic** — ABO incompatibility also presents with early jaundice (often day 1–2). The **parity pattern** is the true discriminator. **Tip:** On NEET PG, when comparing Rh isoimmunisation to ABO incompatibility, always think: *"Rh gets worse; ABO stays the same."* ### Mechanism 1. **First Rh-negative pregnancy with Rh-positive fetus:** Minimal sensitisation → usually no hemolytic disease of newborn (HDN). 2. **Subsequent pregnancies:** Anamnestic IgG response → increasing antibody titres → worsening fetal hemolysis, anemia, and hydrops. 3. **ABO incompatibility:** Natural IgG antibodies present from birth; no worsening with parity. [cite:Park 26e Ch 8]