A 32-year-old woman with a history of two previous pregnancies (both delivered vaginally without complications) presents to the antenatal clinic at 28 weeks gestation. Her blood group is O Rh-negative. Routine antenatal serum screening shows an indirect Coombs test (IAT) positive with an anti-D titre of 1:16. Ultrasound shows no evidence of fetal hydrops. What is the most appropriate next step in management?

Explanation

## Clinical Context: Rh Alloimmunization in Pregnancy This patient has **Rh alloimmunization** (sensitization) — evidenced by a positive indirect Coombs test and detectable anti-D antibodies. The critical question is whether the fetus has developed hemolytic disease of the newborn (HDN) and, if so, its severity. ### Why Cordocentesis is the Next Step **Key Point:** In a sensitized Rh-negative mother with detectable anti-D antibodies, cordocentesis is indicated to: - Confirm fetal blood group (Rh status) - Measure fetal hemoglobin and reticulocyte count - Assess the degree of fetal anemia - Guide decisions on intrauterine transfusion (IUT) vs. expectant management vs. early delivery At 28 weeks with no ultrasonic signs of hydrops, the fetus may have mild-to-moderate anemia or no anemia at all. Cordocentesis provides the definitive hemodynamic status. ### Management Algorithm ```mermaid flowchart TD A[Rh-negative mother<br/>Anti-D positive<br/>No hydrops on USS]:::outcome A --> B{Cordocentesis<br/>Fetal Hb?}:::decision B -->|Hb > 10 g/dL| C[Expectant management<br/>Repeat cordocentesis<br/>in 2-4 weeks]:::action B -->|Hb 7-10 g/dL| D[Intrauterine transfusion<br/>Consider early delivery]:::action B -->|Hb < 7 g/dL| E[Urgent IUT or<br/>Delivery if viable]:::urgent ``` **High-Yield:** Cordocentesis allows **quantitative assessment** of fetal anemia, not just serological detection of antibodies. This is the gold standard for severity assessment in Rh disease. ### Why Other Options Are Premature - **IVIG** is used in severe cases with hydrops or very high anti-D titres; it is not first-line for asymptomatic alloimmunization. - **Anti-D immunoglobulin** is given to prevent sensitization in non-sensitized mothers; this patient is already sensitized and anti-D will not reverse existing antibodies. - **Emergency cesarean delivery** is premature without knowing fetal hemoglobin status; delivery at 28 weeks carries significant neonatal morbidity and is reserved for severe fetal anemia or hydrops. **Clinical Pearl:** The presence of anti-D antibodies alone does NOT indicate fetal hemolytic disease severity. Cord blood sampling (cordocentesis) is the only way to assess actual fetal compromise and guide intervention. [cite:Harrison 21e Ch 297]