Menstruation menarche has changed little in the United States since the 1950s. Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days). Perimenopause is when fertility in a female declines, and menstruation occurs less regularly in
Estrous cycle almost any arbitrary moment (comparable with humans, who, however, have a menstrual cycle in place of an estrous cycle). Generally speaking, the timing of estrus is coordinated with seasonal availability of food and other circumstances such as migration, predation etc., the goal being to maximize the offspring's chances of survival. Some species are able to modify their estral timing in response to external conditions. The female cat in heat has an estrus of 14 to 21 days and is generally characterized as an induced ovulator, in that coitus induces ovulation. However, various incidents of spontaneous ovulation have been documented in