A typical menstrual “cycle” (the time from the first day of your period until the first day of your next period) is about 28 days. Although this is the average length, it is still completely normal to have a cycle that is as short as every 21 days or as long as 40 days in the first few years after starting your periods. The length of the cycle can also change from month to month. It is common for girls to have irregular periods during the first two years after they get their first period. Sometimes stress or increased exercise or weight loss can also change the timing of your period each month. If your cycles are less than 21 days or longer than 40 days or you are having heavy or more frequent bleeding, make an appointment with your primary care provider (PCP).