Menarche ( /mɨˈnɑrkiː/ mə-NAR-kee; Greek: μήν moon + αρχή beginning) is the first menstrual cycle, or first menstrual bleeding, in female human beings. From both social and medical perspectives it is often considered the central event of female puberty, as it signals the possibility of fertility.

Girls experience menarche at different ages. The timing of menarche is influenced by female biology, as well as genetic and environmental factors, especially nutritional factors. The average age of menarche has declined over the last century but the magnitude of the decline and the factors responsible remain subjects of contention. The worldwide average age of menarche is very difficult to estimate accurately, and it varies significantly by geographical region, race, ethnicity and other characteristics. Various estimates have placed it at 13.0 Some estimates suggest that the median age of menarche worldwide is 14, and that there is a later age of onset in Asian populations compared to the West. The average age of menarche is about 12.5 years in the United States, 12.72 in Canada, 12.9 in the UK and 13.06 ± 0.10 years in Iceland. A study on girls in Istanbul, Turkey, found the median age at menarche to be 12.74 years.