Rhythm method of birth control

Calendar Method. The calendar (rhythm method) is considered the least reliable of fertility awareness methods. Women who have very irregular periods may have even less success with this method. In the calendar method, the woman first keeps a record of her menstrual periods for about 6 - 12 months. She then subtracts 18 days from the shortest and 11 days from the longest of the previous menstrual cycles. For example, if a woman's shortest cycle was 26 days and her longest cycle was 30 days, she must abstain from intercourse from day 8 through day 19 of each cycle.