Emergency Contraception

Mechanism: Multiple mechanisms are likely to contribute to the efficacy of these agents, but their precise contributions are unknown Some studies have shown that ovulation is inhibited or delayed, but additional mechanisms thought to play a role include alterations in endometrial receptivity for implantation; interference with functions of the corpus luteum that maintain pregnancy; production of a cervical mucus that decreases sperm penetration; alterations in tubular transport of sperm, egg, or embryo; or effects on fertilization. However, emergency contraceptives do not interrupt pregnancy after implantation.

Regimens: High doses of diethylstilbestrol and other estrogens once were used for postcoital contraception (the "morning-after pill") but never received FDA approval for this indication. The FDA has now approved two preparations for postcoital contraception. PLAN-B is two doses of the "minipill" (0.75 mg levonorgestrel per pill) separated by 12 hours. PREVEN is two 2-pill doses of a high-dose oral contraceptive (0.25 mg of levonorgestrel and 0.05 mg of ethinyl estradiol per pill) separated by 12 hours. This is sometimes referred to as the "Yuzpe" method after the Canadian physician who pioneered its use.

The first dose of such preparations should be taken anytime within 72 hours after intercourse, and this should be followed 12 hours later by a second dose. This treatment reduces the risk of pregnancy following unprotected intercourse by approximately 60% for the Yuzpe method and 80% for levonorgestrel alone. With either preparation, effectiveness appears to increase the sooner after intercourse the pills are taken

Adverse Effects: Nausea and vomiting are the main untoward effects, No changes in clotting factors have been reported for the combined regimen, but based on concerns with combination oral contraceptives, levonorgestrel alone might be considered for women who smoke or have a history of blood clots. Emergency contraceptive pills are contraindicated in cases of confirmed pregnancy.