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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.
Talk to your doctor if you're unsure.) When your next cycle begins, that is the next Day One. Your menstrual cycle length is calculated by counting the days that occur between Day One of one cycle and the next. Your period is irregular if. it comes more frequently than 21 days. you go 35 days or more between periods.