How is a menstrual cycle’s length calculated?
The first day of the period is Day 1 of the cycle and the last day of the cycle is the day before the next period. The length of a menstrual cycle varies between 21 and 35 days. A woman’s cycle is considered regular when the length of the cycles is stable or varies of only a few days. In the teenage years and nearing menopause, the menstrual cycle can be much more unpredictable.