A traditional method of natural family planning. The fertile phase of the menstrual cycle is determined by calculating the length of at least six previous menstrual cycles. The beginning of the fertile phase is determined by subtracting 18 to 20 from the length of the shortest menstrual cycle. The end of the fertile phase is determined by subtracting 10 or 11 from the longest menstrual cycle. When used alone, the calendar method may be unreliable, especially for women with irregular menstrual cycles, and may be overly restrictive for some couples.