An Rh-negative, unsensitised woman at 20 weeks of gestation undergoes therapeutic termination of pregnancy for fetal anomalies. Which is the most common route of administration of anti-D immunoglobulin to prevent Rh sensitisation?

Explanation

## Route of Anti-D Immunoglobulin Administration **Key Point:** **Intramuscular (IM) injection** is the standard and most common route for anti-D immunoglobulin administration in clinical practice across all indications (prophylaxis, treatment of sensitisation). ### Routes of Anti-D Administration | Route | Frequency of Use | Indication | Onset of Action | Advantages | Disadvantages | |-------|------------------|-----------|-----------------|------------|----------------| | **Intramuscular** | Most common (>95%) | All indications | 24–48 hours | Cost-effective, standard formulation, reliable absorption | Local pain, depot effect | | Intravenous | Less common (<5%) | Urgent sensitisation, large FMH, ITP | Immediate | Rapid action, no local reaction | Higher cost, IV formulation required | | Subcutaneous | Rare | Contraindication to IM (e.g., severe thrombocytopenia) | Slower | Avoids IM injection | Unpredictable absorption, not standard | | Intrauterine | Not used | — | — | — | Invasive, no clinical benefit, risk of infection | **High-Yield:** Standard anti-D dose is **500 IU (100 µg)** for prophylaxis in unsensitised Rh-negative women. This covers up to 2 mL fetal RBCs (4 mL fetal whole blood). For larger FMH, additional anti-D is calculated based on Kleihauer–Betke test or flow cytometry. ### Indications for Anti-D Prophylaxis in Pregnancy ```mermaid flowchart TD A[Rh-negative, unsensitised woman]:::outcome --> B{Potentially sensitising event?}:::decision B -->|First trimester| C[Spontaneous/induced abortion]:::outcome C --> D[Anti-D 500 IU IM<br/>if FMH > 4 mL RBCs]:::action B -->|Second trimester| E[Amniocentesis, CVS,<br/>external cephalic version]:::outcome E --> F[Anti-D 500 IU IM<br/>within 72 hours]:::action B -->|Third trimester| G[Antepartum haemorrhage,<br/>trauma, abruption]:::outcome G --> H[Anti-D 500 IU IM<br/>+ Kleihauer–Betke]:::action B -->|Labour/Delivery| I[After delivery of baby]:::outcome I --> J[Anti-D IM based on<br/>cord blood Rh status]:::action ``` **Clinical Pearl:** Anti-D must be administered **within 72 hours** of the sensitising event for maximum efficacy. Earlier administration (within 24–48 hours) is preferred. The drug works by **immune suppression** — anti-D antibodies bind to fetal RBCs in maternal circulation, preventing maternal B cell recognition and sensitisation. **Mnemonic:** **IM-ASAP** — Intramuscular route, Administer as soon as possible (within 72 hours). **Warning:** ~~Intravenous anti-D is the standard route~~ — IV formulation is reserved for specific situations (large FMH, urgent need, severe thrombocytopenia) and is more expensive. ~~Intrauterine injection~~ has no role in modern obstetrics and carries unnecessary risk.