Menstruation refers to regular uterine bleeding in women of childbearing age, a manifestation of the normal activity of the reproductive function of women. Normal menstruation occurs once a month. But sometimes under normal conditions menstruation may occur once every other month known as bimonthly menstruation or once three months known as tri-monthly menstruation, or even once a year known as yearly menstruation. If menstruation never occurs in the whole life of a woman, it is called latent menstruation. In some cases menstruation may occur regularly in the first three months of pregnancy without affecting the fetus, it is called menstruation in pregnancy. Such changes in menstruation are regarded as normal phenomena, not morbid. 

Menstruation is discharged from the uterus, but its production is in close relation with the normal functions of the viscera, exuberance of qi and blood as well as smooth circulation of meridians. Among these factors, blood is one of the substantial bases fur menstruation. But blood is produced from the cereal nutrient transformed by the viscera and transported to the uterus by the meridians. 

The production of menstruation is related to the viscera, meridians, qi and blood, especially to the sufficiency of kidney qi and normal functions of the thoroughfare and conception vessels. Kidney qi is key to the physiological development of women all through the life. According to Neijing, at the age of 7 female is gradually rich in kidney qi, starting to change teeth and growing long hair; at the age of 14, reproductive substance has well developed, conception vessel is smooth in circulation, thoroughfare vessel is in predomination, menstruation occurs regularly and pregnancy is possible. 

At the age of 27, her kidney qi is sufficient and she has reached the age of bearing baby; at the age of 28, her body has perfectly developed; but at the age of 35, yangming meridian begins to decline and her face starts to change and her hair begins to lose; at the age of 42, her body declines further, her face becomes withered and her hair starts to turn grayish; at the age of 49, the conception vessel becomes deficient, the thoroughfare vessel declines, reproductive substance has been exhausted and menstruation stops. At this period, woman is changed in physical building, declines in sex glands and loses reproductive ability.