It represents the interval between the first day of a menstruation period and the first day of the next menstruation period, including the days with bleeding.
The medium length of the menstrual period is 28 days, but it can be much longer or shorter than that. The normal length of the menstrual period is between 21 and 39 days. The ovulation occurs about 14 days before the next menstruation.
The lack of the menstruation is usually the sign of pregnancy.

The menstruation represents the detachment of the tissue that lines the uterus, which regularly appears at the women between [menarche] (the beginning of the menstrual period) and the menopause (the end of the menstrual period). It causes a vaginal bleeding which lasts 3 to 7 days and appears every days (the length of the period).

Although many people consider the menstruation as the beginning of a new cycle (CALMENS does it too), it is in fact the end of the previous cycle. Being so easy to identify, it has symbolically become the beginning of a new period.

The endometrium is a tissue that lines the uterus. It is formed before the ovulation and awaits a fecundated ovule to implant. If the ovule is not fecundated, it wont be implanted; about 2 weeks later, the tissue that has been formed in the uterus detaches. The body will produce it again for the next ovulation.

Contrary to a wide spread belief, when a woman is in her menstruation period, she does not have a bleeding, although it looks this way. There is no loss of blood, excepting the one that has already been formed into the endometrium.