A 24-year-old unmarried woman attends the OPD 48 hours after unprotected intercourse. She is unsure of her cycle day. Which clinical feature best distinguishes the efficacy of copper intrauterine device (Cu-IUD) from levonorgestrel as an emergency contraceptive in this scenario?

Explanation

## Cycle-Dependent vs. Cycle-Independent Efficacy **Key Point:** The most clinically important distinction between Cu-IUD and levonorgestrel is that Cu-IUD efficacy is independent of the menstrual cycle, whereas levonorgestrel efficacy is cycle-dependent and significantly reduced in the periovulatory period. ### Levonorgestrel: Cycle-Dependent Mechanism **Mechanism:** Inhibits or delays ovulation by suppressing the LH surge **Cycle dependency:** - **Follicular phase (days 1–14):** Highly effective (~95% efficacy if given before LH surge) - **Periovulatory period (days 12–16):** Efficacy drops dramatically to ~25–50% because ovulation may already be imminent or occurring - **Luteal phase (days 17–28):** Moderately effective (~60–80%) but ovulation has already occurred **Clinical implication:** If the woman is in the periovulatory period (when she is most likely to conceive), levonorgestrel is least effective. ### Cu-IUD: Cycle-Independent Mechanism **Mechanism:** Primarily spermicidal and ovicidal; also causes endometrial inflammation and impairs sperm transport **Cycle independence:** - Effective regardless of cycle day - Works through direct toxic effects on gametes and embryo - Does not depend on hormonal suppression of ovulation - **Efficacy:** ~99% if inserted within 5 days (and up to 10 days) post-intercourse **Clinical Pearl:** Cu-IUD is the most effective form of emergency contraception (>99% efficacy) and is the WHO-recommended first-line EC, especially when cycle day is unknown or in the periovulatory period. ### Comparison Table | Feature | Levonorgestrel | Cu-IUD | | --- | --- | --- | | **Mechanism** | Ovulation inhibition | Spermicidal + endometrial inflammation | | **Cycle dependent** | Yes (poor in periovulatory period) | No | | **Efficacy in periovulatory period** | ~25–50% | ~99% | | **Window of insertion** | ≤72 hours | ≤5 days (efficacy up to 10 days) | | **Efficacy overall** | ~60% | >99% | | **Repeat dosing** | May be repeated | Single insertion | | **Contraindications** | Few | Pregnancy, STI, anatomical abnormality | **High-Yield:** In a woman with unknown cycle day, especially if she may be periovulatory, Cu-IUD is superior to levonorgestrel because its efficacy does not depend on the timing of ovulation. **Mnemonic:** **CYCLE** — Cu-IUD is independent of **CYCLE**, Levonorgestrel is **CYCLE**-dependent. **Warning:** Do not assume levonorgestrel is equally effective at all cycle days. Its efficacy is poorest when it matters most — in the periovulatory period when conception risk is highest.