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Contraception - Background information
How does the levonorgestrel-releasing intrauterine system (IUS) prevent pregnancy?
- The Mirena® intrauterine system is a T-shaped plastic frame with a reservoir on the vertical stem containing 52 milligrams of levonorgestrel. A rate-limiting membrane releases levonorgestrel into the uterine cavity at a constant rate of 20 micrograms per day for at least 5 years.
- The contraceptive effects of the levonorgestrel-releasing intrauterine system (IUS) are mainly due to its progestogenic effect on the endometrium, which prevents implantation of the fertilized ovum. In addition, changes in cervical mucus inhibit penetration into the uterus by sperm [National Collaborating Centre for Women's and Children's Health, 2005].
- Most women (> 75%) will continue to ovulate while using this method [FFPRHC, 2004d; National Collaborating Centre for Women's and Children's Health, 2005].
- The levonorgestrel-releasing IUS has a minimal effect on the hypothalamo-pituitary-ovarian axis.
- Peak serum estradiol levels are greater than 100 picograms/mL in most women, which indicates that follicles are developing.
- A judicial review ruled that pregnancy begins, not at fertilization, but at implantation; thus the levonorgestrel-releasing IUS is not legally considered an abortifacient [DH, 2002].
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