The first day of your last period is considered Day 1 of your cycle.
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Cycle length is measured from the first day of bleeding (Day 1) to the day before your next period begins. The average cycle length is 28 days, but often varies between 21 and 40 days. If your cycle length is irregular, that is, if it varies by more than a few days each month, use the average number of days for the last 3 months. If your cycle is regularly less than 21 days long or greater than 40 days long, please consult a physician to determine when you should begin testing for ovulation.