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To get an accurate depiction of your menstrual cycle, start counting on the first day of your period. Make a note on your calendar or in a menstrual cycle monitoring app when your period begins. Smartphone apps such as Clue, Glow, Eve, and Period Tracker are designed to help you monitor your menstrual cycle, ovulation, and other key points in your cycle. They can be an easy and data-driven means of monitoring your cycle length. Your count resets on day 1 of your menstrual cycle. This means that your count for each cycle should end on the day before your next period. Include a count for the day before your period starts, but do not include the start date of your period, even if it starts later in the day. If, for example, your cycle started on March 30 and your next period came on April 28, your cycle would be March 30 to April 27, and would total 29 days. The length of your menstrual cycle may vary from month to month. If you want an accurate depiction of your average cycle length, monitor your cycle for at least 3 months. The longer you monitor your cycle, the more representative your average will be. Find the average for the length of your cycle using the numbers you collected while counting your period. You can recalculate this every month to get a more accurate depiction of your general cycle length. Remember, though, that the average shows a trend—it doesn’t definitively represent the length of your next cycle.  To find the average, add the total number of days of your cycle for each month that you have monitored. Then, divide that total by the number of months you monitored. This will give you your average cycle length. For example, you had a 28 day cycle in April, a 30 day cycle in May, a 26 day cycle in June, and a 27 day cycle in July, your average would be (28+30+26+27)/4, equalling a 27.75 day average cycle. Keep tracking your cycle every month. Even if you pass a certain target, such as getting pregnant, keeping track of your cycle throughout your life can help you know when something is off. Medical professionals often ask for information about your cycle, as well. Monitoring your periods and cycle length will help you provide the most accurate information possible. If your doctor asks you the date of your last period, the answer is the first day of your last period, not the day it ended.

Summary:
Start counting on the first day of your period. Count up to the day before you start your next period. Monitor your cycle for at least 3 months. Calculate your average cycle length. Continue to track your cycle.