On day one, the menstrual period begins. Every month, your body builds up a fresh lining of tissue in the uterus in response to increasing amounts of the reproductive hormones estrogen and progesterone. If you are not pregnant, the levels of these two hormones decline. When these hormone levels have fallen to their lowest point, your lining, which is known as the endometrium, is shed from the uterus through the cervix, vagina and out the vaginal opening. An average period lasts between 3 and 7 days.

Phase One
On day one of the cycle, your menstrual period begins. Every month, the uterus in your body builds up a fresh new lining of blood and tissue. The purpose of this process is to help nourish a developing baby if you are pregnant. When this lining, called the endometrium, is not needed to nourish a baby, it leaves the uterus, travels through the cervix and the vagina, and trickles out of the vaginal opening. This menstrual blood, called the period, may be bright red, light pink or even brown. A period usually lasts about three to seven days. The normal amount of menstrual flow is usually about ¼ cup.