Morning-After Pill

History

Though the first trials of emergency contraception occurred in the 1930s, EHC was not widely used until the early 1970s. Initially, the Yuzpe method employed COCs “off-label” by taking several pills together. In 1998, a World Health Organization (WHO) study found that POPs were more efficacious and caused fewer side effects than COCs. POPs are preferred over COCs. However, when approved EHC is not accessible, the Yuzpe method can be used.

Mechanisms

When taken before ovulation, EHC inhibits ovulation primarily by blocking the luteinizing hormone (LH) surge. Thickened cervical mucus acts as a barrier to sperm. Thinning of the uterine lining creates an inhospitable environment for sperm. Fallopian tube motility may be impaired by EHC, decreasing the ability of sperm and ova to move along the tube.

When taken after ovulation, EHC has little effect on ovarian hormonal production and uterine lining. However, thickened cervical mucus is still present. Though unproven, EHC used after ovulation may prevent the implantation of a fertilized egg.

Pregnancy, as defined by WHO; U.S. Food and Drug Administration; and U.S., U.K., and international obstetric and gynecological organizations, begins when a fertilized egg implants into the uterine wall. Progesterone is required to maintain pregnancy. Progestins, synthetic progesterone, are “pregnancy-like hormones.” EHC, which relies on the effect of progestin, does not cause abortion and does not harm a pregnancy.

Effectiveness

Failure rate can be calculated by dividing the number of pregnancies in a sample by the sample size. However, not every act of unprotected sex results in pregnancy. An alternative calculation divides the [Page 582]number of pregnancies by the number of expected pregnancies. Expected pregnancy is dependent on age, day of menstrual cycle, and other factors. Both calculations are useful in determining the effectiveness of EHC.

Progestin-Only Pills

A postcoital dosing regimen of POPs must contain at least 1.5 mg of norgestrel or 0.75 mg of levonorgestrel, the active stereoisomer of norgestrel. Most packages of progestin-only EHC contain two pills. The first pill is taken within 120 hours of unprotected sex, and the second pill is taken 12 hours later. If both pills are taken together within 120 hours of unprotected intercourse, there is increased efficacy with no increase in side effects. Of women taking POPs less than 12 hours after sex, 0.4 percent will become pregnant. Of women taking POPs at 120 hours, 2.7 percent will become pregnant. The average rate of pregnancy is 1.1 percent. POP prophylactic contraceptives can be used off-label as emergency contraception if the dose ingested contains at least 0.75 mg of levonorgestrel.

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