Progesterone was first introduced for medical use in 1934 and the first progestin, ethisterone, was introduced for medical use in 1939. More potent progestins, such as norethisterone, were developed and started to be used in birth control in the 1950s. Around 60 progestins have been marketed for clinical use in humans or use in veterinary medicine. These progestins can be grouped into different classes and generations. Progestogens are available widely throughout the world and are used in all forms of hormonal birth control and in most menopausal hormone therapy regimens.