A 28-year-old woman presents to the emergency department 18 hours after unprotected intercourse during her fertile window. She is hemodynamically stable, with a regular menstrual cycle of 28 days and last menstrual period 12 days ago. She has no contraindications to any form of emergency contraception. Which of the following is the most effective option for her at this time?

Explanation

## Emergency Contraception Efficacy at 18 Hours Post-Intercourse **Key Point:** The copper IUD is the most effective emergency contraceptive method available, with failure rates of 0.1–0.2%, and remains effective up to 5 days (120 hours) after unprotected intercourse. ### Mechanism of Action The copper IUD works primarily through: 1. Spermicidal effect (toxic to sperm) 2. Prevention of gamete transport 3. Alteration of endometrial environment 4. Does NOT work by preventing implantation (hence acceptable in many ethical frameworks) ### Comparative Efficacy Table | Method | Failure Rate | Window | Optimal Timing | Mechanism | |--------|--------------|--------|----------------|----------| | **Copper IUD** | 0.1–0.2% | Up to 120 hrs | Within 72 hrs (but effective up to 5 days) | Spermicidal + transport inhibition | | Levonorgestrel | 0.6–2.6% | Up to 72 hrs | Within 24 hrs (efficacy ↓ after 72 hrs) | Ovulation inhibition | | Mifepristone | 0.4–1.5% | Up to 120 hrs | Within 72 hrs | Progesterone antagonism | | Yuzpe regimen | 2–3% | Up to 72 hrs | Within 24 hrs | Ovulation inhibition + altered transport | **Clinical Pearl:** At 18 hours post-intercourse (well within the 72-hour window), the copper IUD provides the highest efficacy and is the gold standard, especially in women with no contraindications to IUD insertion. **High-Yield:** Copper IUD efficacy does NOT decline significantly with time up to 120 hours, whereas levonorgestrel efficacy drops sharply after 72 hours and is least effective beyond 72 hours. ### Why Copper IUD Is Superior Here - Patient is at 18 hours (optimal window) - No mention of contraindications (nulliparity, STI risk, anatomical abnormality) - Provides dual benefit: emergency contraception + long-acting reversible contraception (LARC) for 10 years - Unaffected by body weight (unlike levonorgestrel, which has reduced efficacy in obese women) [cite:Contraception Guidelines FIGO 2023; Park 26e Ch 33]