How menstruation changes across the stages of a woman's life. The chapter starts with menarche, the first period, typically around the age of 12, and the physical changes of puberty that come with it. It then covers the reproductive years, noting how stress, lifestyle and conditions such as PCOS can lead to irregular periods for some people. Readers learn why periods stop during pregnancy and may be delayed afterwards, and how irregular cycles can signal perimenopause, which usually begins in the 40s. The chapter ends with menopause, defined as 12 consecutive months without a period, and explains what postmenopausal bleeding is.