Mechanisms of action of oral emergency contraception

Abstract

This review gives an overview of the mechanisms of action of oral emergency contraception pills (ECPs), focusing on the levonorgestrel (LNG) and ulipristal acetate (UPA) containing ECPs. In vivo and in vitro studies have addressed the effect of EC on various possible targets. Based on these studies as well as on clinical trials it is clear that the efficacy of ECPs to prevent an unintended pregnancy depends on their mechanism of action as well as on their use in relation to the fertile window. While the main effect of both available ECPs is to prevent or delay ovulation the window of action for UPA is wider than that of LNG. This provides the biological explanation for the difference observed in clinical trials and the higher efficacy of UPA. Neither LNG nor UPA impairs endometrial receptivity or embryo implantation. Correct knowledge on the mechanism of action of ECPs is important to avoid overestimating their effectiveness and to advise women on correct use.

Chinese abstract

本文将综述口服紧急避孕药（ECPs）的作用机制，尤其针对含有左炔诺孕酮（LNG）与醋酸乌利司他（UPA）的ECPs。体内与体外研究均证实EC可作用于不同的靶点。基于这些研究及临床试验，已明确ECPs防止意外妊娠的效力取决于它们的作用机制，且与服药的时间窗有关。两种ECPs的主要作用均为阻滞或延迟排卵时间，而UPA的作用时间窗要宽于LNG。这从生物学角度解释了为何临床试验中UPA的有效性更高。LNG与UPA均不会干扰子宫容受性及胚胎植入。正确认知ECPs的作用机制对于避免高估其有效性及指导女性合理用药有着重要意义。

• Emergency contraception pills
• endometrium
• follicular development
• implantation
• levonorgestrel
• ovulation
• ulipristal acetate