A 32-year-old woman attends the family planning clinic asking about emergency contraception options. She weighs 85 kg (BMI 32). Which is the most common reason for failure of levonorgestrel-based emergency contraception?

Explanation

## Most Common Reason for Levonorgestrel EC Failure **Key Point:** The most common reason for levonorgestrel failure is administration **after ovulation has already occurred**, as the drug works by inhibiting or delaying ovulation. ### Why Timing Matters: Mechanism of Action Levonorgestrel's efficacy depends on the **cycle phase** at administration: 1. **Follicular Phase (Pre-ovulation)**: Highly effective - Inhibits or delays LH surge - Prevents ovulation - Efficacy: 60–90% 2. **Periovulatory Phase (LH surge already initiated)**: Reduced efficacy - Cannot stop an LH surge already in progress - Efficacy drops significantly 3. **Luteal Phase (Post-ovulation)**: Ineffective - Ovulation has occurred; corpus luteum is established - Levonorgestrel cannot reverse this - Efficacy: <10% **High-Yield:** Levonorgestrel is **ineffective after ovulation** because it lacks the ability to prevent fertilization or implantation once the oocyte has been released. ### Timing Window and Efficacy Decay | Time After Intercourse | Efficacy | Notes | |------------------------|----------|-------| | 0–12 hours | 95% | Optimal window | | 12–24 hours | 85% | Still effective | | 24–48 hours | 60–70% | Declining efficacy | | 48–72 hours | 40–50% | Significantly reduced | | >72 hours | <10% | Minimal benefit | **Clinical Pearl:** Patients should be counseled to take levonorgestrel **as soon as possible** after unprotected intercourse; delays reduce effectiveness. The "72-hour window" is a guideline, not a guarantee. ### Why Other Options Are Less Common Causes of Failure - **GI disease (malabsorption)**: Rare; would require significant pathology (e.g., severe diarrhea, malabsorption syndrome) - **Drug interactions with antibiotics**: Levonorgestrel interactions are minimal; rifampicin is the main culprit (not commonly co-prescribed) - **Obesity**: While some studies suggest reduced efficacy in BMI >25–30, this is **not the most common reason** for failure; timing is far more critical **Warning:** Do NOT confuse obesity as a primary failure mechanism — the **timing of administration relative to ovulation** is the dominant factor. [cite:Park 26e Ch 6] [cite:Harrison 21e Ch 297]