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).
Sexual attraction
Research has also found that menstrual cycles affect sexual behaviour frequency in pre-menopausal women. For example, women who had weekly sexual intercourse with men had menstrual cycles with the average duration of 29 days, while women with less frequent sexual interactions tended to have more extreme cycle lengths.