A 32-year-old Rh-negative woman (G2P1) attends the antenatal clinic at 28 weeks of gestation. Her first child (Rh-positive) was delivered 3 years ago by normal vaginal delivery without complications. She did not receive anti-D prophylaxis postpartum. Current pregnancy: routine antibody screening at 28 weeks is positive for anti-D IgG (titre 1:16). Ultrasound shows normal fetal biometry, no ascites, and normal MCA Doppler (MCA-PSV 1.2 MoM). What is the most appropriate management at this stage?

Explanation

## Clinical Context: Secondary Rh Sensitisation This is a case of **Rh isoimmunisation in a previously sensitised mother** (anti-D IgG positive from prior delivery without prophylaxis) with **currently mild-to-moderate fetal anaemia** (MCA-PSV 1.2 MoM, which is <1.5 MoM threshold). ### Key Findings **High-Yield:** - **Anti-D IgG positive at titre 1:16** = prior sensitisation from first delivery (no anti-D given postpartum) - **MCA-PSV 1.2 MoM** = currently normal-to-mild anaemia (threshold for intervention is 1.5 MoM) - **No hydrops features** (no ascites, normal biometry) - **Gestational age 28 weeks** = still time for expectant management with surveillance ### Management Strategy ```mermaid flowchart TD A[Rh-negative, anti-D IgG positive]:::outcome --> B{MCA-PSV at 28 weeks?}:::decision B -->|< 1.5 MoM| C[Monthly MCA Doppler surveillance]:::action B -->|1.5–1.8 MoM| D[Cordocentesis + consider IUT]:::action B -->|> 1.8 MoM| E[Urgent cordocentesis + IUT]:::urgent C --> F{MCA-PSV rises to 1.5 MoM?}:::decision F -->|Yes| D F -->|No| G[Continue surveillance until delivery]:::action D --> H[Transfuse to Hb 15 g/dL]:::action G --> I[Deliver at 37–38 weeks]:::action ``` **Key Point:** - **MCA-PSV <1.5 MoM = no immediate intervention needed**; surveillance with **monthly Doppler** is the standard of care [cite:RCOG Green-top Guideline 18] - **Anti-D immunoglobulin is contraindicated** once the mother is sensitised (anti-D IgG present); it cannot reverse existing antibodies - **Cordocentesis is reserved for MCA-PSV ≥1.5 MoM** or signs of hydrops ### Why Monthly MCA Doppler Surveillance Is Correct 1. **Current MCA-PSV 1.2 MoM** is below the threshold for intervention (1.5 MoM) 2. **Sensitised mothers may show rapid progression** of fetal anaemia; monthly surveillance detects crossing of the 1.5 MoM threshold 3. **If MCA-PSV rises to ≥1.5 MoM**, cordocentesis and IUT are then performed 4. **If MCA-PSV remains <1.5 MoM**, expectant management continues until 37–38 weeks, when delivery is planned 5. **Avoids unnecessary invasive procedures** (cordocentesis carries 0.1–0.3% loss risk) while maintaining fetal safety **Clinical Pearl:** - In previously sensitised mothers, the **secondary immune response is faster and more severe** than the primary response; therefore, **close surveillance from 20 weeks onwards** is essential [cite:Williams Obstetrics 26e Ch 15]. ## Why Other Options Are Wrong **Anti-D immunoglobulin 500 IU/kg + monthly antibody titre:** - **Anti-D is preventive, not therapeutic**; it is indicated for unsensitised (anti-D IgG negative) mothers to prevent sensitisation - The mother is already sensitised (anti-D IgG positive); anti-D will not neutralise existing IgG or reduce antibody titre - Monitoring antibody titre is **not predictive of fetal anaemia severity**; **MCA-PSV Doppler is the gold standard** for assessing fetal anaemia - Anti-D administration in a sensitised mother is wasteful and ineffective **Cordocentesis immediately:** - **Cordocentesis is indicated only when MCA-PSV ≥1.5 MoM** or when hydrops is present - Current MCA-PSV 1.2 MoM indicates mild anaemia; cordocentesis at this stage is **unnecessary and exposes the fetus to 0.1–0.3% loss risk** - Performing cordocentesis without evidence of moderate-to-severe anaemia violates the principle of beneficence **Deliver at 36 weeks:** - **Delivery at 36 weeks is premature** and exposes the neonate to respiratory distress syndrome, intraventricular haemorrhage, and other complications of prematurity - Current fetal status (MCA-PSV 1.2 MoM, no hydrops) does not warrant preterm delivery - **Standard delivery timing in Rh disease is 37–38 weeks** if fetal anaemia is mild-to-moderate and stable - Hydrops fetalis is the indication for earlier delivery (≥34 weeks if feasible) ## Summary Table: Surveillance vs. Intervention in Rh Isoimmunisation | MCA-PSV (MoM) | Fetal Hb (approx.) | Action | Interval | |---|---|---|---| | <1.5 | >7 g/dL | Doppler surveillance | Monthly (or 2-weekly if rising) | | 1.5–1.8 | 5–7 g/dL | Cordocentesis + IUT | Repeat IUT every 2–3 weeks | | >1.8 | <5 g/dL | Urgent cordocentesis + IUT | Urgent, then every 2–3 weeks | | Any + hydrops | Variable | Urgent IUT ± delivery ≥34 weeks | Immediate | [cite:RCOG Green-top Guideline 18, Williams Obstetrics 26e Ch 15]