Mifepristone acts as a medical abortifacient primarily through which mechanism?

• A Inhibition of prostaglandin synthesis, causing uterine contractions
• B Competitive antagonism of progesterone receptors, withdrawing progesterone support and sensitising the uterus to prostaglandins ✓
• C Direct agonism of oxytocin receptors, stimulating myometrial contractions
• D Inhibition of hCG binding to luteal cell receptors, causing luteolysis

Explanation

Mifepristone is a 19-norsteroid that binds progesterone receptors with high affinity (3–5× that of progesterone) and acts as a competitive antagonist. This withdraws progesterone support from the decidua, causing decidual necrosis and cervical ripening (by increasing prostaglandin sensitivity and collagenase activity). It also directly sensitises the myometrium to prostaglandins. When followed by misoprostol (a prostaglandin E1 analogue) after 24–48 hours, complete abortion rates exceed 95% in the first trimester. The combination is more effective than either agent alone.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.