This chapter breaks the menstrual cycle into its four phases: menstruation, the follicular phase, ovulation and the luteal phase. For each one it explains what is happening to hormone levels and in the ovaries and uterus. Readers learn why oestrogen and progesterone are low during a period, how the follicular phase overlaps with menstruation and begins with signals from the hypothalamus and pituitary gland, and why usually only one follicle matures. It describes ovulation as the release of a mature egg, how the follicle then becomes the corpus luteum, and how falling hormone levels when pregnancy does not occur trigger the next period.