Hello, in menstruation your body sheds the lining of the uterus and here will be a blood flow from the uterus through the cervix that passes out through the vagina and lasts normally for 3- 5 days. These periods come regularly once a month and are known as menstrual cycle, provided your body is working normally. It provides hormones to keep you healthy and prepares your body for pregnancy. A cycle is counted from the 1-st day of one period to the first day of the next period. The average menstrual cycle is 28 days (21- 35 days in adults and from 21-45 days in young teens). The rise and fall of levels of hormones during the month control the menstrual cycle .A condition known as Amenorrhea is lack of menstrual period which is seen in young women who have not started menstruating by the age of 15 and could be due to many causes like extreme weight loss, eating disorders, excessive exercising, stress and serious medical conditions. Usually the first period starts about 2 years after breast development. If you have not had your periods even after the age of 15 or if it has been more than 2-3 years since breast growth started, I would suggest you to consult your doctor for a complete evaluation. Thank you.