## Antenatal Anti-D Prophylaxis in Rh-Negative Pregnancy **Key Point:** Antenatal anti-D prophylaxis is administered to unsensitized Rh-negative pregnant women at 28 weeks and 34 weeks of gestation to prevent isoimmunization from fetomaternal hemorrhage during pregnancy. ### Dosing Schedule | Gestation | Dose | Route | Indication | |-----------|------|-------|------------| | 28 weeks | 500 IU (100 μg) | IM | Standard antenatal prophylaxis | | 34 weeks | 500 IU (100 μg) | IM | Second dose (optional in some guidelines) | | Postpartum | Based on FMH quantification | IM | After delivery, within 72 hours | **High-Yield:** The standard antenatal dose of 500 IU (100 μg) anti-D is given at 28 weeks in unsensitized Rh-negative women with negative ICT. This covers approximately 4 mL of fetal red cells or 15 mL of fetal whole blood. **Clinical Pearl:** At 34 weeks, a second dose of 500 IU may be given in some protocols (particularly in UK guidelines), but the primary prophylaxis occurs at 28 weeks. Since this patient is at 34 weeks with negative ICT (unsensitized), she should receive anti-D prophylaxis if she has not received it at 28 weeks. **Warning:** Do NOT defer anti-D prophylaxis until after delivery in an unsensitized Rh-negative woman — the risk of sensitization during pregnancy (especially in the third trimester) is significant. The Kleihauer–Betke test is used AFTER delivery to quantify fetomaternal hemorrhage and determine additional anti-D requirement, not as a prerequisite for antenatal prophylaxis. ### Mechanism of Action Anti-D immunoglobulin (passive antibody) coats fetal red blood cells that enter maternal circulation, preventing maternal B-cell recognition and active sensitization. This is passive immunization, not active vaccination. **Mnemonic:** **RhIG** = **Rh** Immunoglobulin given at **28 weeks** (and optionally 34 weeks) in unsensitized Rh-negative women.
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