Emergency contraception prevents pregnancy after unprotected sexual intercourse. Emergency contraception (EC) does not protect against sexually transmitted infections. The emergency contraceptive formulations available in the United States include combined oral contraceptive tablets, two levonorgestrel 750mcg tablets (Plan B), and the Copper "T" IUD (133-135). The combination EC formulation (Preven) is no longer available in the US. The current treatment schedule for emergency contraceptive pills (ECPs) is one dose within 72 hours of unprotected intercourse and the second dose can be given at the same time as the first (136), or 12 hours after the first dose. Combined and progestin-only ECPs reduce the risk of pregnancy by about 75-88% (137, 138). Effectiveness declines with increasing delay between unprotected intercourse and initiation of treatment (139).
Mechanism of action: Emergency contraception likely inhibits or delays ovulation (140). Some investigators have shown histological alterations in the endometrium suggesting impairment of endometrial receptivity to implantation (141) while others have found no such effects (142, 143). Other possible mechanisms include interference with corpus luteum function, thickening of cervical mucus, and alterations in tubal transport of sperm, egg, or embryo (144). Emergency contraceptives do not interrupt an already established pregnancy.
Progestin-only emergency contraceptives are more effective and associated with significantly less nausea and vomiting than combined emergency contraceptive pills (145). The only absolute contraindication to the use of emergency contraceptive pills is a confirmed pregnancy. The absence of contraindications is likely due to the very short duration of exposure and low total hormone content. There are no conclusive studies of women who were already pregnant when they took emergency contraceptive pills or of women pregnant after failed emergency contraception. However, there is no epidemiologic evidence that progestins are teratogenic and observational studies provide reassurance regarding birth defects (146).
The FDA has not granted approval for the over the counter use of EC for prevention of pregnancy (147). Evidence has been reported that making ECPs widely available does not increase risk taking behavior or increase the incidence of unintended pregnancy (148). Additionally, it has been demonstrated that women most likely to seek emergency contraception are those already concerned about or using contraception (149).
Copper IUD's can be inserted up to five to seven days after ovulation to prevent pregnancy. Insertion of a Copper IUD is significantly more effective than the use of hormonal emergency contraception. The use of a Copper IUD can reduce the risk of unintended pregnancy by more than 99% (150).