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The menstrual flow itself can last anywhere from two days to a week, with the average being four days. Spotting that occurs before your period generally does not count as part of the menstrual flow; only actual bleeding counts. It's normal for women in their teens and 20s to have slightly longer cycles, for women in their 30s to have shorter cycles, and for women in their mid-40s to 50s to have shorter cycles still. If yours varies wildly from month to month and you've had your period for longer than two or three years, it would be a good idea to see a doctor to make sure that you're not suffering from hormonal imbalances. Count up the number of days between the first day of your period and the first day of the subsequent period. That number is your cycle length. For most women, it's 28 days, but a normal cycle can range from 25 to 35 days. Note the first and last days of your period on a calendar. This way, you can estimate when your next period might come. Most women's periods come every 28 days, but if you keep track of your periods, you can determine what length your own cycle is. Consider using an online application like MyMonthlyCycles, MyMenstrualCalendar, or an app on your phone like Period Tracker. This kind of technology is great for helping your keep track of your period from the ease of your mobile phone. Set up a Google calendar event and send yourself a reminder around the time your next period is scheduled. That way, you can write it down in the calendar when your period actually comes and compare the two dates. This will help you learn your body's normal cycle variances, as well as reminding you to be on the lookout for your period when it is scheduled to come.
Know what's normal. Count the days. Keep a record. Use an app. Use an online calendar/planning tool.