## Prevention of Rh Isoimmunisation in Pregnancy **Key Point:** Rho(D) immune globulin (Anti-D immunoglobulin) prophylaxis is the drug of choice for preventing Rh sensitization in Rh-negative, unsensitized pregnant women. ### Standard Dosing for Routine Antenatal Prophylaxis The internationally accepted standard dose for routine antenatal prophylaxis at **28 weeks gestation** is **300 mcg (1500 IU)** of Rho(D) immune globulin as a **fixed dose** — this is the regimen endorsed by ACOG, RCOG, and WHO guidelines. | Gestation | Fixed Dose | Indication | |-----------|------------|------------| | 28 weeks | **300 mcg (1500 IU)** | Routine antenatal prophylaxis | | Postpartum (within 72 hrs) | 300 mcg (1500 IU) | After delivery of Rh-positive infant | | <12 weeks (sensitizing event) | 50–75 mcg (250–375 IU) | Miscarriage, ectopic, CVS | | >12 weeks (sensitizing event) | 300 mcg (1500 IU) | APH, amniocentesis, ECV | **High-Yield:** The standard fixed dose of **1500 IU (300 mcg)** Rho(D) immune globulin at 28 weeks is the globally recognized regimen for routine antenatal prophylaxis. Weight-based dosing (e.g., 500 IU/kg) is NOT the standard for routine prophylaxis; it may be used in specific contexts such as large fetomaternal hemorrhage quantification, but not for routine antenatal care. ### Why Other Options Are Incorrect - **Option A (100 IU/kg):** Weight-based dosing not used for routine antenatal prophylaxis; this dose is used for sensitizing events before 20 weeks in some UK protocols. - **Option B (500 IU/kg):** Not a standard regimen for routine prophylaxis; weight-based dosing is not recommended for routine antenatal care by ACOG or RCOG. - **Option C (IVIG 2 g/kg):** IVIG is used for *treatment* of already-sensitized pregnancies (e.g., severe Rh alloimmunization), not for *prevention* of isoimmunisation. ### Mechanism of Action 1. Anti-D antibodies coat fetal RBCs expressing D antigen that enter maternal circulation 2. Prevents maternal B-cell sensitization by blocking antigen recognition 3. Coated RBCs are rapidly cleared by the reticuloendothelial system 4. Suppresses the primary immune response to the D antigen **Clinical Pearl:** The negative indirect Coombs test confirms the mother is unsensitized and eligible for prophylaxis. Once sensitization occurs (positive indirect Coombs), anti-D prophylaxis is ineffective and management shifts to monitoring and treatment. **Reference:** ACOG Practice Bulletin No. 181; RCOG Green-top Guideline No. 65; Williams Obstetrics, 26th edition. **Warning:** Anti-D must be given within 72 hours of a sensitizing event for maximum efficacy, though some benefit may extend to 10 days.
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