Menstruation: detachment of the endometrium

Menstruation or menstruation occurs when there is no pregnancy and, therefore, the endometrium is shed. This bleeding appears periodically every month, which is why it is also known as a period.

If fertilization of the egg and subsequent embryo implantation were to occur, the endometrium would not detach and menstruation would not occur. The end of the period is the beginning of a new menstrual cycle, in which the maturation of a new ovarian follicle occurs.