A 28-year-old woman presents to the emergency department 36 hours after unprotected intercourse. She is considering emergency contraception. Which feature best distinguishes levonorgestrel from mifepristone as an emergency contraceptive?

Explanation

## Mechanism of Action: The Critical Discriminator **Key Point:** The fundamental difference between levonorgestrel and mifepristone lies in their mechanism and stage of pregnancy at which they act. ### Levonorgestrel (Progestin-Only EC) - **Mechanism:** Primarily inhibits or delays ovulation by suppressing the LH surge - **Timing:** Most effective within 72 hours of unprotected intercourse (some efficacy up to 120 hours) - **Stage:** Acts *before* fertilization or implantation - **Pregnancy status:** Does NOT disrupt an established pregnancy - **Efficacy:** ~60% reduction in pregnancy risk if given within 72 hours ### Mifepristone (Selective Progesterone Receptor Modulator) - **Mechanism:** Acts as a competitive antagonist at progesterone receptors - **Timing:** Effective up to 120 hours (5 days) post-intercourse - **Stage:** Can act at multiple stages — before ovulation, after fertilization, and even after implantation - **Pregnancy status:** Can disrupt an established pregnancy (hence classified as an abortifacient in many countries) - **Efficacy:** ~98% reduction in pregnancy risk if given within 72 hours **High-Yield:** Mifepristone's ability to antagonize progesterone means it can prevent implantation or disrupt early pregnancy — making it fundamentally different from the ovulation-inhibiting action of levonorgestrel. **Clinical Pearl:** In India, mifepristone is restricted under the Medical Termination of Pregnancy (MTP) Act and is used primarily for medical abortion, not routine emergency contraception. Levonorgestrel (Plan B, I-Pill) is the standard EC available over-the-counter. ### Comparison Table | Feature | Levonorgestrel | Mifepristone | | --- | --- | --- | | **Mechanism** | Ovulation inhibition | Progesterone antagonism | | **Window** | ≤72 hrs (efficacy up to 120 hrs) | ≤120 hrs | | **Can disrupt pregnancy** | No | Yes | | **Availability (India)** | OTC | Restricted (MTP Act) | | **Efficacy** | ~60% | ~98% | | **Endometrial effect** | Minimal | Decidualization reversal | **Warning:** Do not confuse levonorgestrel with ulipristal acetate (selective progesterone receptor modulator, similar to mifepristone but available as EC in some countries). Ulipristal is more effective than levonorgestrel but less commonly used in India.