A 28-year-old Rh-negative primigravida presents to the antenatal clinic at 34 weeks of gestation. Her husband is Rh-positive. Routine antenatal blood work shows: Hb 10.5 g/dL, indirect bilirubin 2.8 mg/dL, and Kleihauer–Betke test reveals 2.5 mL of fetal red blood cells in maternal circulation. Antibody screening is positive for anti-D IgG. Doppler ultrasound shows peak systolic velocity in the middle cerebral artery (MCA) at 1.8 multiples of median (MoM). What is the most appropriate next step in management?

Explanation

## Clinical Assessment This is a case of **Rh isoimmunisation with moderate-to-severe fetal anaemia** requiring urgent fetal intervention. ### Key Findings Indicating Severity **High-Yield:** - **Anti-D IgG positive** = prior sensitisation (likely at delivery of previous pregnancy or unrecognised fetomaternal haemorrhage) - **MCA-PSV 1.8 MoM** = moderate fetal anaemia (threshold for intervention is 1.5 MoM; values >1.5 MoM indicate Hb <7 g/dL in the fetus) - **Kleihauer–Betke 2.5 mL fetal RBCs** = significant fetomaternal haemorrhage - **Indirect hyperbilirubinaemia** = evidence of ongoing haemolysis ### Management Algorithm ```mermaid flowchart TD A[Rh-negative mother with anti-D IgG]:::outcome --> B{MCA-PSV?}:::decision B -->|< 1.5 MoM| C[Repeat MCA Doppler weekly]:::action B -->|1.5–1.8 MoM| D[Cordocentesis + IUT if Hb < 7]:::action B -->|> 1.8 MoM| E[Urgent cordocentesis + IUT]:::urgent D --> F[Transfuse to Hb 15 g/dL]:::action E --> F F --> G[Repeat IUT every 2–3 weeks]:::action ``` **Key Point:** - **MCA-PSV >1.5 MoM is the non-invasive marker of fetal anaemia** that mandates cordocentesis and intrauterine transfusion in Rh disease [cite:RCOG Green-top Guideline 18] - IUT is the definitive intrauterine therapy; it corrects anaemia and reduces the need for postnatal exchange transfusion ### Why IUT (Cordocentesis) Is Correct Here 1. **MCA-PSV 1.8 MoM** exceeds the threshold (1.5 MoM) → fetal Hb is likely <7 g/dL 2. **Anti-D IgG already present** → anti-D immunoglobulin will not prevent further haemolysis 3. **Gestational age 34 weeks** → IUT is safe and effective; delivery at <35 weeks carries neonatal morbidity 4. **Cordocentesis allows direct fetal blood sampling** to confirm anaemia and transfuse packed RBCs to Hb 15 g/dL **Clinical Pearl:** - IUT success rate is >95% when performed by experienced operators; each transfusion corrects anaemia for ~2–3 weeks, after which repeat IUT is needed. ## Why Other Options Are Wrong **IVIG 2 g/kg + urgent delivery:** - IVIG is used in cases of **hydrops fetalis or critical anaemia** when IUT is not available or has failed - At 34 weeks, delivery exposes the neonate to respiratory distress, intraventricular haemorrhage, and other prematurity complications - MCA-PSV 1.8 MoM does not yet indicate hydrops; IUT is safer than preterm delivery **Anti-D immunoglobulin 500 IU/kg:** - Anti-D is **preventive**, not therapeutic - The mother is already sensitised (anti-D IgG positive); additional anti-D will not neutralise existing IgG or prevent further haemolysis - Anti-D is indicated for Rh-negative unsensitised mothers after delivery, abortion, or fetomaternal haemorrhage **Amniocentesis for ΔOD450 + repeat MCA Doppler:** - ΔOD450 (Liley chart) is an **older, invasive method** for assessing fetal bilirubin and is now largely replaced by **non-invasive MCA-PSV Doppler** - Amniocentesis carries a 0.1–0.3% risk of pregnancy loss and is **contraindicated when MCA-PSV already indicates anaemia** - Delaying IUT by 1 week risks worsening anaemia, hydrops, and fetal death ## Summary Table: Management of Rh Isoimmunisation by Severity | MCA-PSV (MoM) | Clinical Status | Management | |---|---|---| | <1.5 | Normal/mild anaemia | Repeat MCA Doppler weekly | | 1.5–1.8 | Moderate anaemia | Cordocentesis + IUT if Hb <7 g/dL | | >1.8 | Severe anaemia | Urgent cordocentesis + IUT | | Any + hydrops | Critical | Urgent IUT ± delivery if >34 weeks | [cite:RCOG Green-top Guideline 18, Williams Obstetrics 26e Ch 15]