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
Menstrual synchrony to menstrual synchrony, has been reported in several other mammalian species. Menstrual or estrous synchrony has been reported in other species including Norway rats, hamsters, chimpanzees, and Golden lion tamarins. In non-human primates, the term may also refer to the degree of overlap of menstrual or estrous cycles, which is the overlap of estrous or menses of two or more females in a group due, for example, to seasonal breeding. However, as with early human studies on menstrual synchrony, non-human estrous synchrony studies also were criticized for methodological problems. Subsequent studies failed to find estrous synchrony in rats, hamsters, chimpanzees,