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 one menstrual onset for each woman in a couple, they calculated the degree of synchrony. They reported that more than half of the couples had synchronized within a two-day timespan of each other. Trevathan, Burleson, and Gregory also conducted a study with 29 lesbian couples (22 to 48 years of age), but they incorporated the methodological critique of Wilson into the design of their study. In particular, Wilson emphasized the importance of using actual menstrual cycle lengths with their inherent variability. The lesbian couples were drawn from a larger sample of women who had kept daily records of their menstrual