Periods, or menstrual cycles, occur in early adolescence when girls experience changes in their hormone levels.  Typically starting two years after breast budding (when nipples first become swollen or tender), girls will experience episodes of bleeding from their vagina about once a month that typically last 4 to 7 days at a time.  Patterns can be irregular for the first one to two years, but usually stabilize by the third year. Our menstrual cycles are like a “vital sign” and let us know our bodies are healthy and capable of reproduction. If a young woman is not having periods at least once every two to three months, there may be something wrong that needs medical attention (for example, hormone imbalance, inadequate nutrition, or pregnancy).