What Is EC, and How Does It Work?
Most forms of emergency contraception work by delaying or preventing ovulation, and perhaps by preventing fertilization. These medications therefore cannot be considered abortifacients, because they will not disrupt an existing pregnancy.
Currently, 2 pill-based methods and 1 intrauterine device (IUD) are approved for use by the US Food and Drug Administration (FDA) for EC. Two other methods can also be used, but these are not FDA-approved.
Levonorgestrel (LNG) is a progesterone analogue that works to prevent or delay ovulation during a narrow window of time -- starting from the selection of the dominant follicle and ending when luteinizing hormone (LH) begins to rise. If administered at least 2 days before the LH surge, LNG may delay or prevent the LH surge, thus preventing ovulation. However, if the LH levels are already beginning to rise, LNG will not prevent ovulation, making a pregnancy possible. Furthermore, LNG does not have any effects on hormonal levels of progesterone, nor does it modulate the endometrial development.
If LNG is used within the first 24 hours of exposure, pregnancy rates are approximately 1.5%. As time elapses between 48 and 72 hours, the pregnancy rate increases to 2.6%.
Of note, LNG is available by prescription and over the counter, in both branded and generic formulations, to women of any age, although the package insert of the generics may still indicate that it is not recommended for those younger than 17 years.
Ulipristal acetate (UPA) is an active selective progesterone receptor modulator, which plays a key role during ovulation. UPA given during the mid-follicular phase will prevent lead follicle growth. When given during the LH rise, UPA inhibits 100% of follicular ruptures. Furthermore, when given during the LH peak, ovulation is effectively delayed to approximately 24-48 hours after administration of the drug.
This suggests that UPA has a unique inhibitory effect on follicular rupture, even during times when LNG would be ineffective. Furthermore, UPA maintains its efficacy for up to 5 days after intercourse, which parallels sperm survival. Because UPA delays ovulation, it does significantly increase the user's vulnerability to pregnancy for the remainder of that menstrual cycle. If UPA is administered, providers should counsel women on barrier methods or back-up contraception until their next menses. UPA may be obtained by prescription only.
The copper IUD is the most effective form of postcoital EC and may be used up to 5 days after intercourse. Unlike pill-based EC methods, the copper IUD does not modulate hormones or affect ovulation. Rather, its copper composition is toxic to both the ovum and sperm. Likewise, it creates a local inflammatory state that is perceived as a hostile environment by ovum and sperm, thus inhibiting embryo implantation even if fertilization occurred. An additional and distinctive benefit of the copper IUD is that it can be left in place for 10-12 years and provide highly effective, continual contraception. However, it requires insertion by a skilled professional.
High-dose combined estrogen/progesterone oral contraceptives were the first method of EC available. This method, known as the Yuzpe method, used 2 doses of combination ethinylestradiol/LNG within 72 hours of intercourse, 12 hours apart. Although an agent using this combination was approved by the FDA in 1998, it was withdrawn from the market in 2004 with the emergence of other products with lower side-effect profiles and higher efficacy.
Mifepristone is an antiprogesterone synthetic steroid that, when used at low doses together with LNG, may be effective in preventing pregnancy. It is used in this way in Eastern Europe and East Asia. In the United States, however, mifepristone is only used at higher doses as an abortifacient in early pregnancy.
Medscape Ob/Gyn © 2014
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