Hormonal Emergency Contraception

Melissa Sanders Wanner, Pharm.D., Rachel L. Couchenour, Pharm.D.


Pharmacotherapy. 2002;22(1) 

In This Article

Mechanisms of Action of Estrogen and Progesterone in Emergency Contraception

Administration of a combination of ethinyl estradiol plus levonorgestrel (the Yuzpe regimen), high doses of estrogen alone, or levonorgestrel alone are effective postcoital contraceptive methods. Emergency contraception with these agents works before implantation. The mechanism of action of hormonal emergency contraception remains unclear, however, and more than one mechanism is thought to be involved.[11] The mechanisms may vary depending on when emergency contraception is used during the menstrual cycle.[12,13] Limited reports have been published that attempt to study the mechanism of emergency contraception in preventing pregnancy, and the majority of studies have focused on the Yuzpe regimen. Emergency contraceptives may prevent pregnancy by inhibiting ovulation, fertilization, gamete transport, or implantation.[1]

One of the first studies examined the administration of the Yuzpe regimen during or just before the midcycle peak of luteinizing hormone.[14] Ovarian effects suggested an inhibition or delay in ovulation. Similar results were found when the Yuzpe regimen was administered to 14 women who were midcycle at the time of unprotected intercourse.[15] In subsequent studies, the Yuzpe regimen was administered the day after, 36 hours after, or 48 hours after the luteinizing hormone surge.[12,13] The resultant endometrial environment found in these respective studies was thought to inhibit normal implantation. This is in contrast to the results of another study[16] in which 16 women received the Yuzpe regimen. Eight women were given the regimen on day 12 of the menstrual cycle before the luteinizing hormone surge, and the other eight women were treated 48 hours after the surge in luteinizing hormone. Relatively minor alterations in the endometrium were observed and were thought to be insufficient to prevent implantation.


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