The levonorgestrel-releasing intrauterine system (LNG-IUS, Mirena) releases 20 μg/day of levonorgestrel. Its primary contraceptive mechanism is:
- A Systemic suppression of ovulation through hypothalamic-pituitary inhibition
- B Thickening of cervical mucus and endometrial atrophy suppressing sperm penetration and implantation; ovulation is suppressed in only ~25% of cycles ✓
- C Copper ion-mediated spermicidal effect similar to copper IUD
- D Progesterone-mediated prevention of implantation by inducing decidualisation
Explanation
The primary contraceptive mechanisms of LNG-IUS are local: thickening of cervical mucus (preventing sperm penetration) and endometrial atrophy (suppressing implantation potential). Unlike combined oral contraceptives, the LNG-IUS does not reliably suppress ovulation — ovulation occurs in approximately 75% of cycles, and the systemic progestogen levels are very low. This is why it is safe in women with contraindications to oestrogen (VTE history, migraine with aura) and is associated with fewer systemic side effects. Its non-contraceptive benefits include treatment of menorrhagia and endometriosis.
Reference: Shaw's Textbook of Gynaecology, 17th ed.
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Written and medically reviewed by the StethoPrep medical team.