Mechanism of action of levonorgestrel contraceptive implants

Abstract

Levonorgestrel implants release constant low doses of the hormone over a period of several years. The levonorgestrel exerts its contraceptive action by causing changes in the cervical mucus ,by inhibiting ovulation and by promoting ovulatory dysfunction. Cervical mucus becomes viscous and scanty. Postcoital studies and sperm penetration tests in vitro have shown that few sperm penetrate the mucus and that this effect persists ,even in cycles with high endogenous estradiol production. Ovulation is inhibited in over 85% of the cycles in the first year of use ,when the release rate of levonorgestrel is highest. The percentage of ovulation inhibition decreases to near 65% of the cycles in years 2 and 3 ,while luteal activity occurs in around 50% of the cycles in the last 2 years of use. However ,in these apparently ovulatory cycles ,a dissociation of the normal ovulatory process has been observed. The peak of follicle stimulating hormone present in non-users does not occur ,and the luteinizing hormone peak is blunted and of short duration. It is known that the mid-cycle gonadotropin surge plays a major role in the maturation of the oocyte; it is therefore possible that ,due to this inadequate gonadotropin surge ,the oocyte may not be capable of fertilization in the event of follicular rupture. Luteal phase defect has also been reported. Ultrasound evaluation of follicular activity during levonorgestrel implant use has shown that luteinization of unruptured follicles occurs in nearly 30% of the cycles with luteal activity. Persistent follicles are also a common observation among implant users. Since low-dose progestins do not completely inhibit the gonadotropin stimulus ,follicular growth occurs. However ,the positive feedback of estradiol on the mid-cycle gonadotropin surge is frequently blocked ,thus preventing follicular rupture. This dominant follicle remains functional for about 21 days ,but the anatomic structure remains echographically visible for around 1-2 months before spontaneously disappearing. Hypoestrogenism is not a concern in women with levonorgestrel implants since mean estradiol levels in samples taken twice weekly for 4-5 weeks were not significantly different in women using implants from those acting as controls.

• Levonorgestrel Implant
• Contraception
• Cervical Mucus
• Norplant
• Two-ROD
• System
• Ovulation Inhibition
• Ovulatory Dysfunction