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 study has ever found that menstrual cycles become strictly concordant, nevertheless "menstrual synchrony" is used to refer the phenomenon of menstrual cycle onsets becoming closer to each other over time. In a recent systematic review of menstrual synchrony, Harris and Vitzthum concluded, "In light of the lack of empirical evidence for MS [menstrual synchrony] "sensu stricto", it seems there should be more widespread doubt than acceptance of this hypothesis" (pp. 238–239). The experience of synchrony may be the result of the mathematical fact that menstrual cycles of different frequencies repeatedly converge and diverge over time and not due to a