Mechanism of action of gestagens.

In the mid-1950s it was shown that ovulation can be inhibited by oral administration of progesterone or of synthetic progestogens such as norethynodrel. The idea that progestogens may interfere with fertility even when administered in doses so small, that they do not invariably inhibit ovulation was launched in the mid-1960s by a group of Mexican investigators. Since then a large number of studies have been conducted on the mechanism of action of gestagens when used as contraceptives. The objective of this communication is to review the information collected so far on this topic.