• Emergency contraceptive pills use the same ingredients as regular oral contraceptives.
• The pills, or other oral approaches under study, should be initiated ideally within three days (72 hours) of unprotected coitus. (Recent research indicates some protection may be provided up to five days.)
• Emergency contraceptive pills should be taken in two doses 12 hours apart.
• In addition to pills that are packaged for emergency contraceptive use, regular oral contraceptives can be used, with the number of pills per dose based on the brand involved.

Levonelle-2, NorLevo Plan B, Postinor-2, Vikela (packaged and labeled for emergency contraception)

20 tablets per dose: Each tablet contains 0.0375 mg levonorgestrel.

Microlut, Microval, Norgestron

25 tablets per dose: Each tablet contains 0.03 mg levonorgestrel.

Two tablets per dose: Each tablet contains 50 µg ethinyl estradiol and either 0.25 mg or 0.50 mg levonorgestrel.

Four tablets per dose: Each tablet contains 30 µg ethinyl estradiol and either 0.15 mg or 0.30 mg levonorgestrel.

Copper T and others

Insertion within 120 hours (five days) of unprotected coitus.

Under study

10 mg is effective as a single dose and causes less menstrual delay.

Under study

Two tablets per dose: Each tablet contains 50 µg ethinyl estradiol and 1.0 mg norethindrone.

What are emergency contraceptive pills?

These are oral contraceptive pills that a woman can take within 72 hours of unprotected intercourse to reduce her risk of becoming pregnant. They contain the active ingredients in regular birth control pills, except in higher doses. Recent research suggests the pills may also be effective if taken within 120 hours.

When should emergency contraceptive pills be used?

They are intended for use after sexual intercourse when no contraception is used, when a couple’s regular contraception does not work properly (as when a condom breaks or slips) or if a woman is sexually assaulted. The pills may be appropriate for adolescent women.

How do the pills work?

Depending upon when the pills are taken during the woman’s menstrual cycle, they may:

• prevent or delay ovulation, the release of an egg from the ovary
• prevent fertilization
• stop a fertilized egg from attaching to the uterus

The pills will not work if taken after pregnancy has started. While studies indicate the pills prevent ovulation, more research is needed to show conclusively that they prevent fertilization or stop a fertilized egg from attaching to the uterus.

How effective are the pills?

A woman should begin the pills as soon as possible, since effectiveness declines as time passes. If a woman uses them within three days (72 hours) after sex, progestin-only pills lower the chance of pregnancy by about 85 percent. Combined pills are about 75 percent effective if used within three days.

What if a woman had unprotected sex more than three days ago?

The pills may still work, but the risk of pregnancy increases with time. Another option after three days is the insertion of an intrauterine device (IUD), considered to be effective within five days of unprotected sex, but usually recommended for women who would then continue using an IUD as their routine family planning method.

Do the pills cause side effects?

The pills sometimes cause nausea, vomiting, headaches, dizziness, cramping, fatigue or breast tenderness. If vomiting occurs more than one hour after taking the pill, the woman need not worry because the medication is already in her system.

The pills also may cause irregular bleeding until a woman menstruates again, and menstruation may begin early or late.

What should a woman do after using the pills?

If a woman’s menstruation is more than a week later than expected, she could be pregnant and may want to see a health care provider. If she is pregnant, all available evidence indicates that use of emergency contraceptive pills will not have harmed the pregnancy.

Can a woman use these pills every time she has sex?

Emergency contraceptive pills should not be used routinely to prevent pregnancy because they are less effective than other family planning methods such as condoms, regular oral contraceptives, injectables, intrauterine devices and sterilization. Also, they have more side effects than other methods.

Do the pills protect against sexually transmitted infections?

No. They provide no protection whatsoever. Latex condoms provide the best protection against sexually transmitted infections, including HIV.

Is the emergency contraceptive pill the same as the "morning-after pill"?

Yes, but the term "morning-after pill" can be misleading. Women might think they must wait to begin treatment until the morning after unprotected sexual intercourse. Or, they might think incorrectly that it would be too late to use this method if they cannot obtain treatment until the afternoon or evening after unprotected intercourse, or two days after unprotected sex.

Source: Consortium for Emergency Contraception. Expanding Global Access to Emergency Contraception. Seattle, WA: Consolidated Printers, 2000.

The primary mechanisms through which emergency contraceptive pills operate appear to occur prior to fertilization.

Research has demonstrated that emergency contraceptive pills can prevent or delay ovulation. Depending on when pills are taken during the menstrual cycle, the pills may also inhibit fertilization by affecting tubal transport of the ovum or, after fertilization, they may interfere with implantation of the fertilized egg in the uterus.¹

Pills cannot disrupt an established pregnancy -- the pills have no effect after implantation has been established.

In a study of 12 women taking the combined pill regimen, with the first dose taken just before their predicted time of ovulation and the second dose 12 hours later, blood samples showed diminished levels of luteinizing hormone (LH) and the steroid hormones, estradiol and progesterone. LH triggers ovulation, the release of the egg from the ovary.

"The mechanism of action appeared to be antiovulatory in three subjects in whom both LH and steroids were suppressed," the study found. Eight of the other women showed varied hormonal patterns, and the remaining woman already had ovulated prior to beginning the regimen. If one assumes pregnancy had been prevented in all cases, the researchers concluded, the mode of action must involve other mechanisms besides suppression of ovulation.²

In a subsequent study by the same research team, the regimen was administered to 12 women at 36 and 48 hours after ovulation. Endometrial biopsies showed signs of altered binding properties for steroids in the endometrial tissue. "This temporary disturbance of early events in endometrial development is probably sufficient to prevent … successful implantation," the study concluded.³ Other studies that examined the endometrium after administering the combined pill regimen also found alterations that could inhibit implantation.⁴

Other studies have shown only limited impact on the endometrium. An FHI study administered the combined pill regimen to 19 women on the day of the LH surge. Endometrial biopsies and other procedures found no striking effects on the endometrium. The study also concluded that when administered at this time, the regimen does not affect ovulation, leaving "a puzzling gap in our understanding of the mechanism of action of this therapy."⁵

In a 1996 study, eight women took the combined pill regimen before the LH surge. The researchers reported a variety of hormonal patterns, ranging from fully suppressed LH levels to no significant effect on hormonal patterns. As with other studies, it showed that the combined pill regimen prevented ovulation among some women but not among others.⁶

This study also administered the combined pill regimen to another eight women two days after ovulation. Endometrial biopsies from women in this post-ovulatory group showed only minor changes in development, which the researchers did not consider sufficient to prevent implantation. Another study also found that the combined pill regimen did not result in a significantly altered endometrium, suggesting "emergency contraceptives may exert their effect through more complex mechanisms than endometrial cell surface changes."⁷

A recent review of mechanism of action concluded that the "most difficult parameter to assess with certainty is endometrial receptivity." Even if the endometrium is altered, "other steps that precede implantation may also be altered enough to interrupt the process at an earlier stage."⁸

A 1999 statistical analysis of the combined pill research studies concludes that preventing ovulation could not be the only mechanism of action. It examined the effectiveness rates from eight studies that reported the number of women treated on each cycle day, using five different reports of the probabilities of ovulation by cycle day.⁹ For example, thicker cervical mucus would inhibit sperm from reaching the egg. While no research on cervical mucus has been done regarding emergency contraceptive pills, progestins in regular oral contraceptive pills and injectables do cause cervical mucus to thicken, and this is considered to be a mechanism of action for those contraceptive products.¹⁰

Regarding progestin-only emergency contraception research, in an FHI-sponsored study of 45 women in Mexico, the levonorgestrel-only regimen was administered to three randomly assigned groups comprised of women at different stages of their menstrual cycle: day 10 of the cycle, immediately after the LH surge and 24 hours after the follicle rupture. Ultrasound was performed daily to monitor ovulatory function, and endometrial biopsies were performed nine days after the LH surge, the approximate day that a fertilized egg would be implanted. The pre-ovulatory group had significantly suppressed hormonal levels, although some did ovulate. The other two groups of women did not have altered ovulatory function. The study concluded that the mechanism of action for the post-ovulatory groups appeared to be at the endometrial level, suggesting that the pills can help prevent implantation.¹¹

Another study of the levonorgestrel-only regimen, involving 12 women in the United Kingdom, concluded that if taken immediately before ovulation, the pills delay or prevent ovulation. If taken after the LH surge, this regimen acts by other mechanisms, which need to be explored further.¹²

-- William R. Finger

References

1. Rivera R, Yacobson, I, Grimes D. The mechanism of action of hormonal contraceptives and intrauterine contraceptive devices. Am J Obstet Gynecol 1999;181(5):1263-69.
2. Ling WY, Robichaud A, Zayid I, et al. Mode of action of dl-norgestrel and ethinylestradiol combination in postcoital contraception. Fertil Steril 1979;32(3):297-302.
3. Ling WY, Wrixon W, Zayid I, et al. Mode of action of dl-norgestrel and ethinylestradiol combination in postcoital contraception. II. Effect of postovulatory administration on ovarian function and endometrium. Fertil Steril 1983;39(3):292-97.
4. Yuzpe AA, Thurlow HU, Jamzy I, et al. Postcoital contraception -- a pilot study. J Reprod Med 1974;13(2):53-58; Kubba AA, White JO, Guillebaud J, et al. The biochemistry of human endometrium after two regimens of postcoital contraception: a dl-norgestrel/ethinylestradiol combination or danazol. Fertil Steril 1986;45(4):512-16.
5. Raymond EG, Lovely LP, Chen-Mok M, et al. Effect of the Yuzpe regimen of emergency contraception on markers of endometrial receptivity. Hum Reprod 2000;15(11):2351-55.
6. Swahn MI, Westlund P, Johannisson E, et al. Effect of post-coital contraceptive methods on the endometrium and the menstrual cycle. Acta Obstet Gynecol Scand 1996;75(8):738-44.
7. Taskin O, Brown RW, Young DC, et al. High doses of oral contraceptives do not alter endometrial alpha 1 and alpha v beta 3 integrins in the late implantation window. Fertil Steril 1994;61(5):850-55.
8. Croxatto HB, Devoto L, Durand M, et al. Mechanism of action of hormonal preparations used for emergency contraception: a review of the literature. Contraception 2001;63(3):111-21.
9. Trussell J, Raymond EG. Statistical evidence about the mechanism of action of the Yuzpe regimen of emergency contraception. Obstet Gynecol 1999;93(5):872-76.
10. Guillebaud J. The Pill and Other Hormones for Contraception. Oxford: Oxford University Press, 1997.
11. Durand M, Durán O, Cravioto MC, et al. Mechanisms of action of levonorgestrel (LNG) as emergency contraceptive (EC). XVI FIGO World Congress of Gynecology and Obstetrics Book of Abstracts, Monday, September 4. (n.p.: International Federation of Gynecology and Obstetrics, 2000)23.
12. Hapangama D, Glasier AF, Baird DT. The effects of peri-ovulatory administration of levonorgestrel on the menstrual cycle. Contraception 2001;63(3):123-29.