A 32-year-old G2P1 Rh-negative woman presents at 36 weeks of gestation with a history of delivery of an Rh-positive baby 3 years ago. She did not receive anti-D prophylaxis after her first delivery. Current antenatal investigations show: Indirect Coombs test (ICT) positive at 1:16 dilution, hemoglobin 9.2 g/dL, and ultrasound shows mild polyhydramnios with normal fetal biometry. What is the most appropriate next step in management?

Explanation

## Management of Sensitized Rh-Negative Pregnancy (Rh Isoimmunisation) ### Clinical Context: This is a Sensitized Pregnancy **Key Point:** This patient is **already sensitized** (positive ICT) due to lack of anti-D prophylaxis after her first Rh-positive delivery. Management now focuses on **assessing and preventing fetal hemolytic disease**, not preventing sensitization. ### Distinguishing Sensitized vs. Unsensitized Pregnancies | Feature | Unsensitized | Sensitized | |---|---|---| | ICT | Negative | Positive | | Anti-D indication | Prophylaxis (prevent sensitization) | Assessment & fetal monitoring | | Management | Routine anti-D at 28 & 34 weeks | Non-invasive + invasive fetal assessment | | Monitoring | Clinical + repeat ICT if needed | MCA-PSV, cordocentesis, IUT | **High-Yield:** Once sensitization is confirmed (positive ICT), anti-D prophylaxis is **contraindicated** — it will not help and may worsen hemolysis. The focus shifts to **fetal assessment and intrauterine intervention if needed**. ### Assessment of Fetal Hemolytic Disease in Sensitized Pregnancy ```mermaid flowchart TD A[Sensitized Rh-negative pregnancy<br/>ICT positive]:::outcome --> B{Assess fetal severity}:::decision B -->|Non-invasive: MCA-PSV| C[MCA-PSV < 1.5 MoM]:::outcome B -->|Non-invasive: MCA-PSV| D[MCA-PSV 1.5-1.8 MoM]:::outcome B -->|Non-invasive: MCA-PSV| E[MCA-PSV > 1.8 MoM]:::outcome C --> F[Low risk of anemia<br/>Expectant management]:::action D --> G[Intermediate risk<br/>Repeat MCA-PSV in 1-2 weeks]:::action E --> H[High risk of anemia<br/>Cordocentesis ± IUT]:::urgent H --> I{Fetal Hb low?}:::decision I -->|Yes| J[Intrauterine transfusion]:::action I -->|No| K[Repeat assessment<br/>Plan delivery at 37-38 weeks]:::action ``` ### Why MCA-PSV is the Gold Standard 1. **Non-invasive** — avoids cordocentesis unless MCA-PSV suggests severe anemia 2. **Sensitivity & Specificity** — MCA-PSV > 1.5 MoM predicts moderate-to-severe fetal anemia with ~90% sensitivity 3. **Timing of Intervention** — guides decision for cordocentesis and IUT 4. **Reduces unnecessary invasive procedures** — cordocentesis carries 1-2% miscarriage risk **Clinical Pearl:** In this case, the presence of mild polyhydramnios and maternal anemia (Hb 9.2) suggests **possible fetal hemolytic disease**. MCA-PSV assessment is the next logical step to quantify fetal risk before considering cordocentesis or transfusion. ### Why NOT the Other Options? **Anti-D prophylaxis in sensitized pregnancy:** Contraindicated. The patient is already sensitized; giving anti-D will not prevent further sensitization and may increase hemolysis. **Immediate cordocentesis:** Invasive and carries fetal loss risk (1-2%). Reserved for when MCA-PSV suggests severe anemia (MCA-PSV > 1.8 MoM) or to confirm fetal anemia before IUT. **Induction at 36 weeks:** Premature. At 36 weeks with mild disease signs, expectant management with serial MCA-PSV is safer. Delivery is planned at 37-38 weeks if fetal condition permits. **Mnemonic:** **MCA-PSV FIRST** — In sensitized pregnancies, use Middle Cerebral Artery Peak Systolic Velocity as the first-line non-invasive assessment tool to guide further invasive intervention. [cite:Williams Obstetrics 25e Ch 5, RCOG Guideline on Rh D Alloimmunisation]