In the 1950s, scientists (such as Carl Djerassi, George Rosenkranz and Alejandro Zaffaroni) realized that the hormone progesterone stopped women from making eggs (stopped ovulation). After this discovery, the combined oral contraceptive pill was created. The combined oral contraceptive pill has two female hormones: estrogen and progesterone. Some oral contraceptive pills have only progesterone. These are often called the "minipill".