During early adolescence, it is common for menstrual periods to be irregular, at least for the first 18 months after the first period (menarche). It is also common for menstrual periods to be irregular as you approach
(usually between the ages of 40-58, sometimes slightly earlier or later). Menstrual periods also stop during pregnancy.
If you are not pregnant or entering menopause, you may have secondary amenorrhea. This may be the case if you had normal menstrual periods, but they have stopped for at least three consecutive months.