Mode of action of medical methods of abortion

Several pharmacologic agents capable of inducing abortion have become available in the last 20 years. Prostaglandins cause powerful contractions of the uterus that lead eventually to expulsion of the fetal or embryonic tissue, but their effects on smooth muscle elsewhere in the body lead to such troublesome side effects as vomiting and diarrhea. Antimetabolites such as methotrexate inhibit DNA synthesis in rapidly dividing cells such as the trophoblast, resulting in release of prostaglandins and cytokines, extravasation of blood into the decidua, uterine contractions, and expulsion of embryonic or fetal tissue. Drugs that inhibit the synthesis of progesterone (epostane) or block its receptor (mifepristone) reverse the dominant influence of progesterone during pregnancy. As a result, a cascade of events is initiated, including influx of leukocytes and red blood cells into the decidua, release of prostaglandins and cytokines, and uterine contractions. Addition of uterotonic agents such as prostaglandins results in powerful uterine contractions, which supplement those induced by the withdrawal of progesterone. Because these methods reproduce many of the same physiological changes, clinical management of medical abortion is similar to that of spontaneous abortion. These methods provide a useful alternative to surgical abortion in early pregnancy.