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).
Edward R. Dewey
Volume IV of the Cycles Classic Library Collection contains 1380 reports of cycles period determinations by scientists, doctors, economists and cycles researchers. In these reports there is a tendency for certain periods of cycles to be reported more commonly. These common periods include the underlined periods above and some other periods such as 9.6 years, found in the 9.6 year cycle of lynx abundance and 3.39 years (40.68 months), found in the US stock market, which Dewey says is the most commonly found period. Dewey further said that: