Menstrual cycle The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be normal. The average age of the first period is generally later in the developing world and earlier in developed world. 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 35 days in adults (an average of 28 days). Menstruation stops occurring after menopause which usually occurs
Tzolk'in agricultural cycle of highland Guatemala, which is also about 260 days. Aveni notes that "the average duration between successive halves of the eclipse season, at 173 ½ days, fits into the tzolkin in the ratio of 3 to 2." This may seem contrived, but the Maya did employ the tzolkin to predict positions of Venus and eclipses. 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