Informed consent is a crucial concept connected with the prescription of any medical therapy. When it comes to the practice of emergency postcoital contraception, discussions about the expected outcomes, the potential side effects and the possible mechanisms of action are essential for patients to make an informed decision about its use. Without accurate information presented before prescribing, patients may experience emotional distress from an unanticipated result, an unforeseen side effect or the later discovery of a mechanism of action that is in conflict with their value system.

Discussion with patients about expected results should include the risk of pregnancy at the current stage of their menstrual cycle and the reduction of that risk. The likelihood of pregnancy with treatment is implied when discussing the topic of risk reduction. In an article in this issue of American Family Physician, Wertheimer¹ cites a pregnancy rate of 8 percent as a result of intercourse in the second or third week of the menstrual cycle. The risk reduction rate after emergency contraception treatment is approximately 75 percent; that is, the pregnancy rate is reduced from 8 percent to 2 percent. The number needed to treat (NNT) is a valuable concept to use in patient communications. With approximately 6 percent of treated women achieving an antigestational effect, the NNT means that 17 women would need to be treated to prevent one ensuing pregnancy. Likewise, one in 50 treated women can expect pregnancy despite treatment.

Wertheimer¹ mentions that the most common side effects of emergency contraception are nausea and vomiting. Mastalgia, fatigue and dizziness are also noted as adverse reactions.^2,3 Delayed onset of menstruation is more likely with mifepristone treatment.⁴ Headache is the symptom most commonly reported either with estrogen plus progestin treatment or with mifepristone alone.⁴ Pelvic infection is a risk with insertion of an intrauterine device (IUD). Discussion of these reactions constitutes a part of informed consent.

How these methods exert their antigestational effect is not known with certainty. Several mechanisms of action have been proposed for the various agents used to prevent the possible progression to fertilization and pregnancy after intercourse.² These mechanisms are summarized by Wertheimer.¹ Some of these proposed mechanisms involve effects that make fertilization less likely, such as inhibition of ovulation and toxicity to sperm. Others involve effects after fertilization, like reduction in progesterone secretion in the corpus luteum and inhibition of implantation. Effects on fallopian tube transport may occur before or after fertilization.

I would like to comment on the evidence for postfertilization effects. With regard to the corpus luteum, estrogen combined with progestin or high-dose estrogen can lower progesterone levels during the luteal phase. Whether the levels decrease to the point at which pregnancy is not sustained is not yet known. Mifepristone “induces regression of the corpus luteum in about 50 percent of women.”² IUD insertion or mifepristone administration following ovulation may cause endometrial changes that could impede implantation of the conceptus in the endometrium.

Glasier² suggests that the changes noted in the endometrium after postovulatory use of high-dose estrogen or estrogen plus progestin “may not be sufficient to inhibit implantation.” She also notes, however, that “the postovulatory administration of estrogen or levonorgestrel inhibits implantation in some animals.”² Additionally, high doses of postcoital estrogen may lead to an “increased incidence of ectopic pregnancy.”² This observation implies that some effect occurs after fertilization, because prefertilization mechanisms would be expected to correspond with an equal decrease in both tubal and intrauterine pregnancy rates. A proportionately increased rate of tubal pregnancy would be explained by decreased tubal motility or by inhibition of intrauterine implantation, or both.

Another observation relates to postfertilization action. As previously noted, postcoital use of mifepristone often causes a delay in the subsequent menstrual period, which is indicative of its suppression of ovulation.⁴ A delay was much less likely with high-dose estrogen and progestin (13 percent versus 42 percent). This effect would suggest that inhibition of ovulation is less a factor with the estrogen-progestin combination and that other mechanisms of action (e.g., postfertilization effects) are more responsible for the results of these particular agents. The evidence is certainly indicative of postfertilization effects at least some of the time.

For those who believe that human life begins with fertilization, these postfertilization effects are certainly of concern. The case has already been made—in detail—that adequate informed consent for users of oral contraceptives includes discussion of their postfertilization effects.⁵ Likewise, information about the antigestational effects after fertilization should be part of the information that is given to any patient who is considering the option of postcoital contraception. Patients will then be able to make informed choices that align with their values.

Attention should also be paid to the language used in such discussions with patients. The terms “conception,” “contraception,” “abortion” and “beginning of pregnancy” may have different meanings to patients compared with the definitions that have evolved in medical practice. “Conception” in current medical usage is sometimes used as a synonym for fertilization. However, more often it is used to indicate implantation. This trend is not necessarily reflected in the general public's usage and understanding of this term.

Many lay persons would consider the terms “conception” and “fertilization” to be synonymous. For many lay persons, “contraception” would be thought of as something that prevented fertilization; abortion would refer to any interruption after fertilization; and the beginning of pregnancy would be at fertilization. Wertheimer¹ acknowledges this in the statement, “Some individuals may consider these hormones to be abortifacients if they interfere with implantation.” Effective and truthful communication with our patients depends on language that is mutually understood. Such communication is necessary for informed consent and is one of the foundations of quality medical practice.