Emergency contraception: how does it work?

Emergency (or post-coital) contraception is any substance or device that is used to prevent pregnancy after unprotected intercourse. Currently used hormonal methods of emergency contraception (high-dose combined oral contraceptive pill or levonorgestrel) prevent about 50-80% of pregnancies. Research has demonstrated that these methods inhibit the midcycle surge of LH from the pituitary and, if given at least 2 days before ovulation, ovulation is delayed or prevented. Ovulation still occurs if administration is delayed until ovulation is imminent. Biological data that suggest that the most likely mode of action is by preventing fertilization are supported by the clinical observation that the greater the interval between coitus and administration the greater the chance of pregnancy. There are no data supporting the view that levonorgestrel can impair the development of the embryo or prevent implantation. In contrast, other very effective methods of emergency contraception, such as mifepristone and intrauterine devices, can also inhibit implantation.