Emergency contraception with levonorgestrel (Plan B) is most effective when taken within 72 hours of unprotected intercourse. Its primary mechanism of action when used for emergency contraception is:

• A Inhibiting or delaying ovulation by suppressing the LH surge ✓
• B Preventing implantation of a fertilised ovum by thinning the endometrium
• C Causing luteolysis and reducing progesterone to below threshold levels
• D Thickening cervical mucus to prevent sperm transport — primary mechanism

Explanation

Current pharmacological evidence shows that levonorgestrel's primary emergency contraceptive mechanism is inhibition or delay of ovulation by suppressing the mid-cycle LH surge, thereby preventing follicle rupture. If taken before or around the LH surge, ovulation does not occur and fertilisation cannot take place. Studies show it does not prevent implantation of an already fertilised egg with statistical significance. Thickened cervical mucus is a secondary effect and less relevant as a primary mechanism for emergency contraception given the short window of intercourse-to-pill timing.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.