The levonorgestrel intrauterine system (LNG-IUS, e.g., Mirena) primarily prevents pregnancy by which mechanism?
- A Consistent inhibition of ovulation in all cycles
- B Thickening of cervical mucus, endometrial decidualization/atrophy making implantation hostile, and impaired sperm function — with ovulation inhibition being inconstant ✓
- C Copper ion-mediated spermicidal effect
- D Systemic progestogen effect suppressing the hypothalamic-pituitary axis
Correct answer: B. Thickening of cervical mucus, endometrial decidualization/atrophy making implantation hostile, and impaired sperm function — with ovulation inhibition being inconstant
Explanation
The LNG-IUS releases levonorgestrel locally at 20 mcg/day, acting primarily via: (1) thickening of cervical mucus impeding sperm penetration, (2) endometrial decidualization and atrophy making the endometrium inhospitable to implantation, and (3) impaired sperm motility and capacitation. Ovulation inhibition occurs in approximately 50–75% of cycles but is inconsistent; the primary contraceptive mechanism is local endometrial and cervical mucus effects. Systemic levonorgestrel levels are much lower than oral pills.
Reference: Shaw's Textbook of Gynaecology, 17th ed.
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Written and medically reviewed by the StethoPrep medical team.