Progesterone changes considerably over a lifetime, and this chapter follows it stage by stage. It starts with the small role the hormone plays before puberty, then explains how it becomes central to the menstrual cycle once periods begin. A page on the luteal phase describes the two weeks after ovulation, when the corpus luteum produces progesterone to thicken the uterine lining, and what happens when pregnancy does not occur. The chapter then covers pregnancy, when rising progesterone helps relax the uterus and prepares the breasts for feeding, before ending with the natural decline around menopause and symptoms such as hot flushes and disrupted sleep.