Does levonorgestrel emergency contraceptive have a post-fertilization effect? A review of its mechanism of action

Abstract

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.

Keywords:

• Levonorgestrel emergency contraception
• Mechanism of action
• Pre- and post-fertilization effects
• Inhibition of ovulation
• Luteal dysfunction

Acknowledgements

The authors thank Shaobo Jin, Ph.D. candidate, Department of Statistics, Uppsala University, Sweden, for his contribution to the statistical analysis and treatment of the data.

Notes

1 Due to the small number of studies, the controls from one study (Croxatto et al. 2002) were pooled with the other controls (Noé et al. 2011; Croxatto et al. 2004) since the methodologies were the same.

2 In Okewole's study, only four out of eight women had a “significant” delay over 5 days. Okewole reported that the women in Group A had a significant delay in LH peak by about 96–120 hours compared with their control cycles, but this finding was a reported mean value based on all eight subjects. So, for example, in subject 7, her delay in LH surge was as small as 1 day, and subject 4 had a 2-day delay. The mean reporting measure obscures these findings.

Additional information

Notes on contributors

Rebecca Peck

Biographical Note

Rebecca Peck, M.D., is a clinical assistant professor at Florida State University, College of Medicine, Daytona Beach, FL.

Walter Rella

Walter Rella, M.D., is a family practitioner, Institut für Medizinische Anthropologie und Bioethik (IMABE), Wien, Austria.

Julio Tudela

Julio Tudela, Pharm.D., Ph.D., is a member of the Observatory Bioethics of the Catholic University of Valencia and professor at the Catholic University of Valencia, Spain.

Justo Aznar

Justo Aznar, M.D., Ph.D., is a director of the Life Sciences Institute of the Catholic University of Valencia, Spain.

Bruno Mozzanega

Bruno Mozzanega, M.D., is a professor of gynecology in the Department of Woman's and Child's Health, University of Padua, Obstetrics and Gynecology Unit, University of Padova, Italy.