A woman usually has her period for 3-7 days, less than 3 days is called a short period and more than 7 days is called a prolonged period. Menstruation is the periodic peeling and bleeding of the endometrium that accompanies the cyclical changes of ovarian hormones. The normal menstrual cycle is 21-35 days, less than 21 days is called frequent menstruation and more than 35 days is called scanty menstruation. The normal menstrual volume is 5-80 ml, less than 5 ml is called too little menstruation and more than 80 ml is called too much menstruation. The normal menstrual cycle varies little, less than 7 days in a year, more than 7 days is called irregular menstruation, if no menstruation for more than 6 months is called amenorrhea.