Epidemiologic evaluation of menstrual cycle is an important issue because ovarian endocrine environment is closely associated with women’s health. These precise data are necessary for women and clinicians to distinguish potentially pathologic alterations from short-term aberrations.  From the 1920’s, there have been reports concerning the normal human menstrual period.  However, since there is considerable variation in menstrual cycles among women, it is difficult to precisely determine the normal length of menstrual cycles in individuals with cyclic variability. When researchers strictly exclude women with irregular menstrual cycles, they tend to rigidly select the group with a narrow range of menstrual cycles as normal menstrual intervals from 25-28 to 32 days. In contrast, if researchers want to strictly select the group with irregular menstrual cycles, they will define the normal spectrum as a rather broader range, frequently 20 to 40 days, showing that there is no constant definition of regular or normal menstruation.  Therefore, the optimal range of regular or normal menstruation tends to be defined according to the aims and subjects of the study.