Research on new methods of emergency contraception.

The ideal emergency contraceptive would be highly effective (surpassing the 75% effectiveness rate of the Yuzpe method), free of side effects, non-disruptive of the menstrual cycle, easily administered, interceptive, and affordable. Prevention of fertilization rather than ovulation would be an important advance since it would increase the window of time after unprotected intercourse that a drug can be administered. Recent research in this area has focused on levonorgestrel alone, danazol, anti-estrogens, gonadotropin-releasing hormone antagonists, and progesterone inhibition. Crucial to further advances is basic research on embryonic development and signaling before implantation, tubal transport and milieu, endometrial development before implantation, and the inter-relationships between the embryo and uterus during implantation. Until more knowledge is amassed about these physiological mechanisms, no new methods are likely to emerge. In the interim, an emphasis is being placed on the evaluation of anti-progestogens such as RU-486 that are easier to administer and associated with fewer side effects than the Yuzpe method.