Tiangui refers to women's menstruation in ancient times, which first appeared in Su Wen's Theory of Ancient Naivety. Menstruation refers to regular and periodic uterine bleeding, which is regular and constant every month, so it is called "monthly letter", "monthly event" and "monthly water", which means that menstruation is periodic with "monthly rhythm".

Menstruation is a very familiar "friend" for women. It visits once a month, and whether menstruation is normal or not also represents women's health to some extent.

A normal "period" has four rules to follow.

1, cycle frequency: the normal menstrual frequency is 2 1~35 days, less than 2 1 day is frequent menstruation, and more than 35 days is rare menstruation.

2, cycle regularity: the difference between the days of adjacent menstrual cycles is not more than 7 days.

3, menstrual length: the normal menstrual period should be 3~7 days.

4, menstrual bleeding: the normal menstrual flow is 5~80ml, less than 5ml is called oligomenorrhea, and more than 80ml is called menorrhagia.