If the menstrual cycle always comes once every 21 days, premature arrival at perimenopause may occur. Women ovulate 400-500 times in their lifetime, and if they have a 21-day menstrual cycle, they will experience premature ovarian failure. Because normal menstruation is a cycle of 28-30 days, a menstrual volume of 30-80 ml is normal, and it is normal for menstruation to be 7 days early or 7 days late. Therefore, if you have a 21-day menstrual cycle, you must go to the hospital to check whether there are organic lesions, such as uterine fibroids, ovarian tumors, ovarian corpus luteum atrophy, incompetence, or due to mental factors, excessive stress, and frequent late nights, so you should relax and combine work and rest.