Progesterone is made in the ovaries and regulates many things such as the menstrual cycle, pregnancy and sexual desire. A woman has most progesterone in her body during ovulation (release of egg) and its job is to thicken the lining of the womb to receive and nourish the egg. Some family planning methods (like injections and some pills) use progesterone to change the lining of the womb and cervix to prevent pregnancy.
Attribution: Marie Stopes