A 32-year-old multigravida, Rh-negative, sensitised woman (indirect Coombs titre 1:16) presents at 28 weeks of gestation for routine antenatal care. Ultrasound shows no signs of hydrops fetalis. Her previous child had haemolytic disease of the newborn (HDN) requiring exchange transfusion. What is the most appropriate next step in management?

Explanation

## Clinical Context This is a sensitised Rh-negative woman (indirect Coombs titre 1:16) with a history of severe HDN in a previous child. At 28 weeks with no hydrops, the next critical step is to assess the severity of fetal haemolysis and guide further management. ## Amniocentesis and Amniotic Fluid Bilirubin: The Gold Standard **Key Point:** In a sensitised pregnancy, amniocentesis with amniotic fluid bilirubin (ΔOD450) measurement is the primary investigation to determine the severity of fetal haemolysis and guide decisions about intrauterine fetal transfusion (IUT), timing of delivery, and neonatal preparation. **High-Yield:** The Liley chart (or modified Queenan chart for >27 weeks) uses ΔOD450 values to classify risk into three zones: - **Zone 1 (Low risk):** Fetus unlikely to be severely affected; repeat amniocentesis in 2–3 weeks - **Zone 2 (Intermediate risk):** Fetus may be moderately affected; repeat in 1–2 weeks or proceed to IUT based on trend - **Zone 3 (High risk):** Fetus likely severely affected; IUT or delivery indicated ## Management Algorithm Based on Amniocentesis Results ```mermaid flowchart TD A[Sensitised Rh-negative at 28 weeks, no hydrops]:::outcome --> B[Amniocentesis + ΔOD450]:::action B --> C{Liley Zone?}:::decision C -->|Zone 1| D[Repeat amniocentesis in 2-3 weeks]:::action C -->|Zone 2| E[Repeat in 1-2 weeks or IUT if trend worsening]:::action C -->|Zone 3| F[IUT or delivery depending on gestational age]:::action D --> G[Monitor until delivery]:::action E --> H{Repeat ΔOD450 worsening?}:::decision H -->|Yes| F H -->|No| G ``` ## Why Amniocentesis Is Preferred Over Non-Invasive Tests **Clinical Pearl:** While Doppler assessment of peak systolic velocity (PSV) of the middle cerebral artery (MCA-PSV) is increasingly used as a non-invasive marker of fetal anaemia, amniocentesis with ΔOD450 remains the gold standard for quantifying bilirubin and determining the need for IUT. In a sensitised pregnancy with a history of severe HDN, amniocentesis is essential to guide invasive therapy decisions. **Mnemonic:** **Liley-Zone** — Amniocentesis → ΔOD450 → Liley chart → Zone classification → Management decision. ## Timing of Amniocentesis **High-Yield:** Amniocentesis should be performed: - At 28 weeks in a sensitised woman with a history of severe HDN (as in this case) - Earlier (18–20 weeks) if the previous child required IUT or had severe HDN - Repeated based on Liley zone and trend ## Why Other Options Are Incorrect **Warning:** IVIG has limited evidence in Rh isoimmunisation and is not first-line. IUT should only be performed if amniocentesis shows Zone 3 results. Early delivery at 34 weeks without assessment of fetal status risks unnecessary prematurity.