## Failure of Levonorgestrel Emergency Contraception **Key Point:** The most common reason for failure of levonorgestrel emergency contraception is **administration after ovulation has already occurred**. ### Mechanism of Levonorgestrel Action Levonorgestrel works by **preventing or delaying ovulation**. It is effective only when given **before the LH surge** and before ovulation occurs. ```mermaid flowchart TD A[Unprotected intercourse] --> B{When is levonorgestrel given?} B -->|Before LH surge| C[Inhibits/delays LH surge]:::action B -->|After LH surge| D[Ovulation already triggered]:::urgent C --> E[Ovulation prevented]:::outcome D --> F[Levonorgestrel ineffective]:::outcome E --> G[Pregnancy prevented] F --> H[Pregnancy may occur] ``` ### Timing and Efficacy Relationship | Time Since Intercourse | Efficacy of Levonorgestrel | Likelihood of Ovulation | Success Rate | |------------------------|----------------------------|------------------------|--------------| | **0–24 hours** | Highest (95%) | Low | ~95% | | **24–48 hours** | Moderate (85%) | Increasing | ~85% | | **48–72 hours** | Lower (60%) | High | ~60% | | **72–120 hours** | Minimal (<20%) | Very high | <20% | **High-Yield:** Levonorgestrel **cannot reverse** an ovulation that has already occurred. If the woman is already in the luteal phase or ovulation has been triggered, levonorgestrel will not prevent pregnancy. ### Why This Is the Most Common Failure Reason 1. **Unpredictable ovulation timing:** Women often do not know exactly when they ovulate in their cycle 2. **Delay in seeking care:** Many women present 48–72 hours after intercourse, when ovulation may have already occurred 3. **Variable cycle length:** In women with irregular cycles, ovulation timing is harder to predict 4. **Biological variability:** The timing of the LH surge varies between individuals and cycles **Clinical Pearl:** A woman in the **late follicular phase** (days 12–14 of a 28-day cycle) is at highest risk of levonorgestrel failure because ovulation is imminent or may have already occurred. ### Why Other Options Are Less Common Causes of Failure #### Malabsorption due to concurrent diarrhoea - While malabsorption can reduce levonorgestrel bioavailability, this is a **less frequent** cause of failure - Diarrhoea would need to be severe and concurrent with levonorgestrel administration - Most women presenting for emergency contraception do not have acute GI symptoms #### Drug interaction with oral antibiotics - Broad-spectrum antibiotics (e.g., rifampicin) can induce hepatic metabolism of levonorgestrel - However, this is a **rare scenario** in emergency contraception practice - Most antibiotics do not significantly interact with levonorgestrel - This is a theoretical concern, not a common clinical failure reason #### Obesity with BMI >30 kg/m² - Recent evidence suggests levonorgestrel efficacy may be **reduced in obese women** (BMI >25 or >30) - However, this is a **newer finding** and not yet universally established as a major failure cause - The mechanism is unclear (altered pharmacokinetics vs. other factors) - This is **less common** as a failure reason compared to post-ovulation administration **Warning:** Do not confuse "failure of emergency contraception" (pregnancy still occurs) with "levonorgestrel is ineffective as a drug." Levonorgestrel is highly effective *if given before ovulation*; failure usually reflects **biological timing**, not drug failure. ### Clinical Counselling Points **Key Point:** When counselling a woman about emergency contraception: 1. Emphasize that **earlier is better** (within 24 hours is ideal) 2. Explain that it **cannot work if ovulation has already occurred** 3. Advise a **follow-up pregnancy test** if menses are delayed by >7 days 4. Consider **copper IUD** as an alternative if >72 hours have passed
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