Emergency contraception with ulipristal acetate (UPA, 30 mg) is effective up to how many hours after unprotected intercourse, and what is its primary mechanism?

• A 72 hours — prevents fertilisation by blocking tubal motility
• B 72 hours — prevents implantation by causing endometrial atrophy
• C 120 hours — selectively modulates progesterone receptors to delay or inhibit ovulation ✓
• D 120 hours — causes luteal phase disruption after fertilisation

Explanation

Ulipristal acetate (ellaOne) is a selective progesterone receptor modulator licensed for emergency contraception up to 120 hours (5 days) after unprotected intercourse — unlike levonorgestrel (LNG) EC, which is licensed for 72 hours. UPA's primary mechanism is inhibition or delay of ovulation, including when the LH surge has already commenced (a key advantage over LNG-EC). There is no confirmed post-fertilisation mechanism in human studies. UPA is more effective than LNG-EC, particularly 72–120 hours after intercourse.

Reference: Shaw's Textbook of Gynaecology, 17th ed.

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Written and medically reviewed by the StethoPrep medical team.