A phase of the endometrial cycle in which the endometrial tissue is broken down by bleeding that occurs in the absence of fertilisation. This process is manifested by the shedding of the endometrium in response to the deprivation of oestrogen and progesterone following the regression of the corpus luteum. With an average periodicity of about 28 days, menstruation typically lasts between 3 and 5 days, with a volume that can vary between 30 and 100 ml. It is also known as period.