Antiprogestin alone is not sufficiently effective in terminating early pregnancy to be clinically useful. The only exception seems to be immediate post-ovulatory administration which inhibits endometrial development to an extent that prevents implantation of the fertilized ovum. During early pregnancy the uterus is inactive. Treatment with antiprogestin with result in an increased uterine contractility and a significant increase of myometrial sensitivity to prostaglandin. The effect is probably mainly due to the release of the inhibitory effect of progesterone. Antiprogestin not only activates the uterus, it also causes a ripening of the cervix. The combination of RU486 and either vaginal administration of gemeprost or i.m. injections of nalador provide a safe and effective medical abortion in the first 8 weeks of pregnancy. Recent clinical studies indicate that it may be possible to replace the prostaglandin analogues in current use by the orally active analogue misoprostol. Misoprostol is inexpensive and stable at room temperature and would facilitate the provision of medical abortion with mifepristone. Experimental data also indicate that a combination of RU486 and misoprostol may be developed into an effective once-a-month late luteal method to regulate fertility. Pre-treatment with RU486 is also useful in later stages of gestation. A combination of RU486 and the vaginal administration of gemeprost is a highly effective, safe and simple non-invasive method for terminating both early and late second trimester pregnancy.
