An Rh-negative, unsensitised woman at 32 weeks of gestation undergoes external cephalic version for breech presentation. What is the drug of choice to prevent Rh sensitisation following this procedure?

Explanation

## Anti-D Prophylaxis for Sensitising Events in Pregnancy ### Definition of Sensitising Events **Key Point:** Sensitising events are obstetric procedures or complications that carry a risk of **fetomaternal haemorrhage (FMH)** and can trigger Rh isoimmunisation in unsensitised Rh-negative women [cite:RCOG Green-top Guideline 41]. ### Sensitising Events in Pregnancy | Event | Gestation | Anti-D Dose | Timing | |-------|-----------|-------------|--------| | External cephalic version | Any trimester | 500 IU | Within 72 hrs | | Amniocentesis | Any trimester | 500 IU | Within 72 hrs | | Chorionic villus sampling | First trimester | 500 IU | Within 72 hrs | | Antepartum haemorrhage | Any trimester | 500 IU + FMH screen | Within 72 hrs | | Miscarriage/termination | First trimester | 250–500 IU | Within 72 hrs | | Ectopic pregnancy | N/A | 500 IU | Within 72 hrs | ### Dosing Rationale 1. **Standard dose for sensitising events:** 500 IU (100 µg) is the baseline dose 2. **FMH assessment:** Kleihauer–Betke test or flow cytometry quantifies fetal RBCs 3. **Additional dosing:** If FMH > 4 mL fetal RBCs (8 mL fetal whole blood), give extra 100 IU per mL fetal RBCs 4. **Time window:** Must be administered **within 72 hours** of the event for maximum efficacy **High-Yield:** The 500 IU dose is standard for all first-line sensitising events. Additional Anti-D is calculated only if FMH exceeds the threshold covered by the standard dose. ### External Cephalic Version (ECV) as a Sensitising Event **Clinical Pearl:** ECV carries a **5–15% risk of FMH** and is classified as a sensitising event requiring Anti-D prophylaxis in unsensitised Rh-negative women, regardless of success or failure of the procedure. **Key Point:** Anti-D should be given **within 72 hours** of ECV. The standard dose of 500 IU is sufficient for most cases; FMH screening is performed to determine if additional Anti-D is needed. ### Mechanism of Prophylaxis 1. Anti-D antibodies bind to fetal D antigen on RBCs 2. Prevents maternal immune recognition 3. Fetal RBCs are cleared without primary immune response 4. Suppresses B-cell sensitisation and memory formation **Warning:** Prophylaxis is **ineffective if Coombs test is already positive** (prior sensitisation). Always check indirect Coombs before administering Anti-D.