A 28-year-old primigravida with Rh-negative blood group presents at 34 weeks gestation for a routine antenatal visit. Her indirect Coombs test (ICT) is negative. She has no history of transfusion or previous pregnancy. On examination, the fetus is in vertex presentation with normal biometry. Her husband is Rh-positive. What is the most appropriate management regarding Rh isoimmunisation prevention at this stage?

Explanation

## Rh Isoimmunisation Prevention in Unsensitised Rh-Negative Primigravida ### Antenatal Anti-D Prophylaxis Protocol **Key Point:** Unsensitised Rh-negative women require routine antenatal anti-D prophylaxis to prevent Rh sensitisation during pregnancy. The patient is currently presenting at **34 weeks**, and the question asks about management **at this stage**. ### Standard Dosing Schedule (RCOG / Indian Guidelines) | Timing | Dose (IU) | Route | Indication | |--------|-----------|-------|------------| | 28 weeks | 500 IU | IM | Routine antenatal prophylaxis (1st dose) | | 34 weeks | 500 IU | IM | Routine antenatal prophylaxis (2nd dose) | | Within 72 hrs postpartum | 500 IU (or more based on Kleihauer–Betke) | IM | Prevent sensitisation from fetal–maternal haemorrhage | **High-Yield:** Per RCOG guidelines (2011) and Indian obstetric practice, the two-dose antenatal prophylaxis regimen consists of **500 IU anti-D IM at 28 weeks and 500 IU anti-D IM at 34 weeks**. The patient is now at 34 weeks, so the appropriate action is to administer **500 IU anti-D intramuscularly** — the second scheduled dose of the two-dose regimen. ### Why Option A (1500 IU + repeat at 34 weeks) is Incorrect - Option A describes administering 1500 IU and then repeating at 34 weeks — this is internally contradictory since the patient **is already at 34 weeks**. - The ACOG single-dose regimen uses 300 mcg (~1500 IU) at **28 weeks only** (not repeated at 34 weeks). - Neither RCOG nor ACOG recommends 1500 IU with a repeat dose at 34 weeks as a standard protocol. ### Why Negative ICT Does Not Change Management - A negative indirect Coombs test confirms she is **unsensitised** (no anti-D antibodies present). - This makes her eligible for and in need of **prophylactic** anti-D to **prevent** sensitisation. - Prophylaxis is given regardless of ICT status as long as she remains unsensitised. ### Why Other Options Are Incorrect - **Option C (Amniocentesis):** Invasive and not indicated in an unsensitised woman; amniocentesis is used to assess fetal anaemia in *sensitised* women with rising antibody titres. - **Option D (IV route):** Anti-D immunoglobulin is **always administered intramuscularly** for routine prophylaxis; intravenous administration is not standard practice for this indication. **Clinical Pearl (RCOG 2011 / Dutta's Obstetrics):** The goal of antenatal anti-D is to neutralise any fetal Rh-positive red cells that may enter maternal circulation during pregnancy (e.g., from placental microtrauma), preventing the maternal immune response that would cause haemolytic disease of the fetus/newborn in future pregnancies. At 34 weeks, 500 IU IM is the correct dose per the two-dose schedule. **Warning:** Do not defer anti-D prophylaxis based on negative ICT — this is a **common exam trap**. Prophylaxis is standard of care for all unsensitised Rh-negative women regardless of perceived risk.