A woman using a copper intrauterine device (Cu-IUD) as emergency contraception presents 4 days after unprotected intercourse. Which mechanism primarily accounts for its >99% effectiveness in this setting?

• A Endometrial thinning prevents implantation of a fertilised ovum
• B Progesterone release suppresses ovulation in the current cycle
• C Cervical mucus thickening blocks sperm penetration
• D Copper ions create a hostile environment that is toxic to sperm and inhibits fertilisation ✓

Explanation

The primary mechanism of the Cu-IUD as emergency contraception is the spermicidal and fertilisation-inhibiting effect of released copper ions — copper is toxic to sperm, impairs motility, and induces acrosomal reaction prematurely, preventing fertilisation. This is the dominant mechanism when placed within 5 days of intercourse (before ovulation in many cases). The Cu-IUD does not release progesterone (that is the LNG-IUS), and endometrial effects are secondary at best.

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

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Written and medically reviewed by the StethoPrep medical team.