Emergency contraceptive pills (Levonorgestrel 1.5 mg) are most effective when taken within 72 hours of unprotected intercourse. The PRIMARY mechanism of action is:

• A Inhibition or delay of ovulation, with possible thickening of cervical mucus ✓
• B Inhibition of implantation of an already fertilized ovum
• C Abortifacient action via endometrial shedding
• D Direct spermicidal effect reducing fertilization

Explanation

High-dose levonorgestrel (LNG) emergency contraception acts primarily by inhibiting or delaying ovulation, preventing the LH surge necessary for follicle rupture. Secondary mechanisms include thickening of cervical mucus and possibly altering tubal motility. Evidence from studies shows LNG-EC does NOT prevent implantation of an already fertilized egg — when taken after ovulation has already occurred, it does not reduce pregnancy rates, confirming its pre-fertilization primary mechanism. It is not abortifacient. Efficacy is 95% within 24 hours, declining to ~58% at 72 hours, which is consistent with an ovulation-inhibiting mechanism.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.