Menstruation
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).
Tzolk'in
Another theory is that the 260-day period is the length of human pregnancy. This is close to the average number of days between the "first missed" menstrual period and birth, unlike Naegele's rule which is 40 weeks (280 days) between the "last" menstrual period and birth. It is postulated that midwives originally developed the calendar to predict babies' expected birth dates.