Concerns regarding the mechanisms of action of emergency contraception (EC) create major barriers to widespread use and could also lead to incorrect use of EC and overestimation of its effectiveness. While the copper intrauterine device (Cu-IUD) is the most effective method available for EC, the hormonal methods are frequently considered to be more convenient and acceptable. Today, the most commonly used method for hormonal EC is levonorgestrel (LNG). More recently, the progesterone receptor modulator ulipristal acetate (UPA) has been shown to be more effective than LNG to prevent an unwanted pregnancy. The main mechanism of action of both LNG and UPA for EC is delaying or inhibiting ovulation. However, UPA appears to have a direct inhibitory effect on follicular rupture which allows it to be effective even when administered shortly before ovulation, a time period when use of LNG is no longer effective. The main mechanism of action of the Cu-IUD is to prevent fertilization through the effect of Cu ions on sperm function. In addition, if fertilization has already occurred, Cu ions influence the female reproductive tract and prevent endometrial receptivity. Based on this review of the published literature, it can be concluded that existing methods used today for EC act mainly through inhibition of ovulation or prevention of fertilization. An additional effect on the endometrium as occurs for the Cu-IUD, but not for the hormonal alternatives, seems to increase the efficacy of the method.
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