A 28-year-old woman uses the levonorgestrel-releasing IUS (Mirena, 52 mg) for contraception. Which mechanism accounts for its primary contraceptive efficacy?

• A Suppression of ovulation through systemic levonorgestrel absorption affecting the hypothalamic-pituitary axis
• B Local endometrial and cervical mucus effects — glandular atrophy, decidualization, and thick hostile mucus — with inconsistent ovulation suppression ✓
• C Copper ion-mediated spermicidal effect identical to the copper IUD mechanism
• D Systemic progestogenic effect causing consistent anovulation in >95% of cycles

Explanation

The LNG-IUS (52 mg) primarily works locally: it releases LNG directly into the endometrial cavity, causing profound endometrial decidualization/glandular atrophy that makes implantation impossible, and thickening of cervical mucus that blocks sperm penetration. Ovulation is suppressed in only 50–75% of cycles (variable by year of use and individual), meaning the primary mechanism is NOT ovulation suppression. Systemic absorption is low (150 pg/mL steady state) — insufficient for consistent hypothalamic suppression, unlike the 20 mcg pill. Copper ion toxicity is the copper IUD mechanism.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.