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Rationale and Objectives: Emergency contraceptives pills (ECPs) are described as drugs that work by either inhibiting or delaying ovulation without affecting implantation. In our opinion, as we aim at demonstrating, both EMA documents and the experimental papers indicate that they prevalently inhibit embryo-implantation. LNG-ECPs: literature: LNG-ECPs never prevent ovulation when are taken in the most fertile days (EMA-EPAR on ellaOne ® p. 9, first table). Conversely, they prevent the formation of an adequate corpus luteum. When they are taken pre-ovulatory ovulations occur regularly, but pregnancies do not appear. Taken after ovulation, they seem ineffective in preventing pregnancies. UPA-ECPs: literature: EllaOne ® prevents ovulation only when is taken in the first fertile day. Thereafter, its anti-ovulatory effect drops sharply and becomes insignificant (8%) 36 h before ovulation, in the most fertile days (Brache); its decreasing anti-ovulatory effect cannot explain a consistently high effectiveness in preventing pregnancies (≥80%) that does not decrease depending on which of the 5 d it is taken after unprotected intercourse. Besides, ovulation occurs regularly in 91.7% of women taking ellaOne ® weekly, for eight consecutive weeks (EMA-CHMP-Assessment Report 'EMA/73099/2015': study HRA2914-554, p. 7). Lastly, Lira-Albarrán administered ellaOne ® to women in the most fertile pre-ovulatory days: they had normal ovulation, but their endometrium, evaluated through samples obtained in the implantation window, became inhospitable: the expression of 1183 genes was exactly the opposite of that observed in the receptive pro-gestational endometrium. This agrees with information by EMA-CHMP-Assessment Report 'EMEA/261787/2009' (p. 8): after UPA administration 'the proteins necessary to begin and maintain pregnancy are not synthesized'., Conclusions: Emergency Contraceptives work prevalently by preventing embryo-implantation. People shall receive correct information.

Background: Recent studies have identified that levonorgestrel administered orally in emergency contraception (LNG-EC) is only efficacious when taken before ovulation. However, the drug does not consistently prevent follicular rupture or impair sperm function., Objective: The present systematic review is performed to analyze and more precisely define the extent to which pre-fertilization mechanisms of action may explain the drug's efficacy in pregnancy avoidance. We also examine the available evidence to determine if pre-ovulatory drug administration may be associated with post-fertilization effects., Conclusion: The mechanism of action of LNG-EC is reviewed. The drug has no ability to alter sperm function at doses used in vivo and has limited ability to suppress ovulation. Our analysis estimates that the drug's ovulatory inhibition potential could prevent less than 15 percent of potential conceptions, thus making a pre-fertilization mechanism of action significantly less likely than previously thought. Luteal effects (such as decreased progesterone, altered glycodelin levels, and shortened luteal phase) present in the literature may suggest a pre-ovulatory induced post-fertilization drug effect., Lay Summary: Plan B is the most widely used emergency contraceptive available. It is important for patients and physicians to clearly understand the drug's mechanism of action (MOA). The drug was originally thought to work by preventing fertilization. Recent research has cast doubt on this. Our review of the research suggests that it could act in a pre-fertilization capacity, and we estimate that it could prevent ovulation in only 15 percent or less of cases. The drug has no ability to alter sperm function and limited ability to suppress ovulation. Further, data suggest that when administered pre-ovulation, it may have a post-fertilization MOA.