Emergency Contraception: A Primer for Pediatric Providers

Alana L. Clements; Alison Moriarty Daley


Pediatr Nurs. 2006;32(2):147-153. 

In This Article

Physiological Mechanism of Action for EC

The start of pregnancy is scientifically defined as implantation of the blastocyst in the endometrium of the uterus, an event that generally takes place 5-7 days after fertilization of the egg (Glaiser, 1997). There are three ways that hormonal EC may function to prevent pregnancy: delaying ovulation, preventing fertilization, or preventing implantation of a fertilized egg; the method of action depends on the part of the menstrual cycle the woman is in at the time of unprotected intercourse (Croxatto et al., 2001).

EC is not an abortifacient, and will not harm or cause teratogenic effects to an existing pregnancy (Glaiser, 1997; Croxatto et al., 2001). EC does not function in the same way as Mifepristone (also known as Mifeprex or RU-486), used for early first trimester abortions, which works by blocking the action of progesterone thus interfering with the establishment and maintenance of the placenta. Mifepristone also binds to glucocorticoid and androgen receptors and stimulates prostaglandin synthesis by early deciduas cells (Stewart, Ellertson, & Cates, 2004).

For maximum efficacy, the first dose of either method of EC should be taken as soon as possible after unprotected sex. It is most effective if administered within 12-24 hours, but retains some efficacy for up to 5 days after intercourse (Ellertson et al., 2003; Rodrigues, Grou, & Joly, 2001). EC has traditionally been given in 2 equal doses, 12 hours apart. This method of administration was intended to reduce the gastrointestinal side effects commonly seen with administration of the combined estrogen-progestin EC regimen. Plan B has the added benefit of having a significantly lower side effect profile secondary to the absence of estrogen (von Hertzen et al., 2002). Recent literature shows that Plan B is equally effective when the two doses are taken together (von Hertzen et al., 2002). Thus, for Plan B, the two equal dosing plan is no longer indicated and both doses can be given at once. Because of the gastrointestinal side effects experienced by many women taking combined estrogen-progestin EC, it is important to continue to administer that form of EC in equally divided doses, 12 hours apart.

The most common side effects of all forms of EC are nausea and vomiting, and some women may experience headache, dizziness, fatigue, breast tenderness, and changes in the next menstrual period (Glasier, 1997; Harper, Rocca, Darney, von Hertzen & Raine, 2004; Task Force on Postovulatory Methods of Fertility Regulation, 1998). A large-scale randomized controlled trial conducted by the World Health Organization (WHO) indicated substantially greater tolerability of progestin-only EC, such as Plan B, versus the estrogen-progestin regimen (Task Force on Postovulatory Methods of Fertility Regulation, 1998) (see Table 2 , Comparison of Side Effects Between EC Methods). The WHO study (1998) also indicated that the onset of menses after taking EC might be early or late by up to 1 week, but that duration of menses was within the norm of 4-7 days. The use of EC only affects the cycle the woman is currently in, not future cycles. No significant or medically dangerous side effects have been reported during the many years women have been using EC to prevent unwanted pregnancy.

There was initially some concern that the use of EC might increase a woman's risk for the development of an ectopic pregnancy, but a recent large scale review of the literature indicates that of over 33,000 women who used EC included in the studies reviewed, only 5 ectopic pregnancies were reported, which is the same or lower than the expected rate of ectopic pregnancy in the general population (Cheng et al., 2004; Farquhar, 2005).

There are no contraindications to multiple uses of EC. Many women who have medical contraindications to the use of estrogen in combined OCPs can safely use Plan B because it does not contain estrogen (Abuabara et al., 2004). Plan B is only contraindicated in women with a prior allergic reaction to Plan B or its components, known or suspected pregnancy (because it will not work, not because it will cause teratogenic effects), or undiagnosed abnormal vaginal bleeding (Barr Laboratories, Inc., 2005).


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