A 28-year-old primigravida at 34 weeks of gestation presents for routine antenatal care. She is Rh-negative and unsensitised. On investigation, her partner is found to be Rh-positive. What is the most common cause of Rh isoimmunisation in this pregnancy?

## Most Common Cause of Rh Isoimmunisation **Key Point:** Fetomaternal haemorrhage (FMH) during labour and delivery is the most common cause of Rh isoimmunisation, accounting for approximately 70–80% of all sensitisation events in Rh-negative women. ### Timing and Mechanism 1. **Labour and delivery** — The placenta separates, causing the largest volume of FMH 2. **Fetal red blood cells** cross into maternal circulation 3. **Maternal immune response** — If the mother is unsensitised, she develops anti-D antibodies (IgG) over 6–12 weeks 4. **Subsequent pregnancies** — These IgG antibodies cross the placenta and cause haemolytic disease of the fetus and newborn (HDFN) ### Why This Is Most Common | Cause | Frequency | Volume of FMH | Timing | |-------|-----------|---------------|--------| | Labour & delivery | **70–80%** | **4–15 mL fetal RBCs** | Intrapartum/postpartum | | Antepartum FMH | 10–15% | 0.1–1 mL fetal RBCs | Third trimester | | Invasive procedures | 5–10% | Variable | Depends on procedure | | Placental abruption | 5% | Variable | Antepartum | **High-Yield:** The critical window for sensitisation is **within 72 hours of delivery**. This is why anti-D immunoglobulin (500 IU/mL or 100 μg/mL) must be given **within 72 hours postpartum** in all Rh-negative unsensitised women who deliver Rh-positive infants. ### Clinical Pearl Sensitisation can occur with as little as 0.1 mL of fetal red blood cells (or 4 mL of fetal whole blood). A standard dose of anti-D (500 IU) covers up to 4 mL of fetal RBCs; larger FMH requires additional doses based on Kleihauer–Betke test or flow cytometry. ### Prevention Strategy **Mnemonic: ANTI-D TIMING** - **A**ntepartum prophylaxis: 500 IU at 28 weeks (in some guidelines, also at 34 weeks) - **N**eed to repeat postpartum if FMH detected - **T**est for FMH volume (Kleihauer–Betke or flow cytometry) - **I**mmunoglobulin within 72 hours of delivery - **D**ose adjusted based on FMH volume [cite:Williams Obstetrics 26e Ch 5]