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1 Record the number of days in each cycle, including the day you start your period. 2  The last day of each cycle is the day before your next period starts. 3  Keep tracking your cycle this way for eight to twelve cycles. 4  The more cycles you have to reference, the more accurate the calendar method will be.
Day 1 is the first day of your period. Since the length of your cycle can vary slightly from month to month, itâs best to keep track for a few months. Once you have an average, subtract 18 days from the length of your shortest cycle. This is the first day youâre likely to be fertile.