Menstruation is the time during a woman's natural cycle when bleeding occurs from the vagina. Menstruation usually lasts between three and seven days.
Menstruation occurs when one of the eggs in the ovaries is released.
The egg travels down the fallopian tube towards the uterus. Once the egg is released, estrogen encourages the uterus to build up a thick lining to prepare for pregnancy . If sperm from a male fertilizes the egg, the egg implants itself into the uterine wall and begins to grow a fetus. If pregnancy does not occur, the uterus sloughs off the lining and excretes it through the cervix, or opening of the mouth of the uterus. This excreted uterine lining is the menstrual flow.