Emergency contraception with ulipristal acetate (UPA) is effective up to 120 hours post-coitus and is more effective than levonorgestrel. The mechanism of UPA is:

• A Copper-mediated spermicidal and inflammatory effect preventing fertilisation
• B Selective progesterone receptor modulator (SPRM) that delays or inhibits follicular rupture even after LH surge has begun, and may alter endometrial receptivity ✓
• C High-dose progestogen causing anovulation by suppressing FSH and LH peaks in the early follicular phase
• D GnRH antagonist that acutely suppresses LH surge preventing ovulation

Explanation

Ulipristal acetate is a selective progesterone receptor modulator (SPRM) with both agonist and antagonist properties. It inhibits or delays follicular rupture by blocking progesterone action on the follicle and LH-induced signalling, remaining effective even after the LH surge has begun (unlike levonorgestrel which is less effective post-LH surge). It may also alter endometrial receptivity. UPA must not be combined with progestogens (including the ongoing pill) as they competitively reduce its efficacy. The copper IUD is the most effective emergency contraception but works differently.

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

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Written and medically reviewed by the StethoPrep medical team.