The antiprogesterone steroid, RU 486 (mifepristone).

RU 486 (Mifepristone) represents a major development in the field of hormone antagonists as the first effective antiprogestogen. A number of therapeutic roles for the drug are envisaged. It is already being used extensively around the world for the procurement of first trimester abortion--particularly in combination with prostaglandins. It also has been shown to be effective as a cervical ripening agent and for the induction of labour. Initial human studies have involved nonviable pregnancies and more work is needed in animal models before the drug can be deemed safe for use in viable human pregnancies in the third trimester.