Most periods are 2-6 days long, with bleeding stopping naturally in an average of about 5 days. Menstruation is actually a cyclical process of repair and proliferation, secretion and exfoliation of the endometrium under the action of estrogen and progesterone. The duration of menstrual bleeding is abnormal if it is too short or too long, and is often closely related to the regulation of hormone levels in the body. The period between the first day of menstruation and the first day of the next menstrual period is called the menstrual cycle, most of which is 28-30 days, but there are shorter ones of about 21 days or longer ones of three months or six months. The menstrual cycle is divided into follicular phase, ovulation phase and menstrual phase. The amount of menstrual bleeding is related to the thickness of the endometrium. If the endometrium is damaged by various internal and external factors, or if any part of the gonadal axis is affected by disease, abnormal menstrual bleeding times may occur.