Menstruation is a very complicated process involving many different hormones, the woman's sex organs and the brain. A woman's internal sex organs consist of two ovaries, the Fallopian tubes, the uterus (womb) and the vagina. The ovaries contain the eggs with which the woman is born and, during each period, a single egg will usually ripen and mature due to the action of hormones circulating in the bloodstream. When the egg is mature it bursts from the ovary and drifts through the Fallopian tube down into the uterus. The lining of the uterus - the endometrium - has been thickened by the action of hormones and made ready to receive the fertilised egg. If the egg is fertilized and the woman becomes pregnant, it will fasten itself onto the endometrium. If the egg is not fertilized, however, resultant hormonal changes cause the endometrium to slip away and menstruation begins. Menstrual discharge is composed of the endometrium itself, together with a little fresh blood caused by the breaking of very fine blood vessels within the endometrium as it detaches itself from the inside of the uterus. The amount of blood lost due to the normal monthly period is usually less than 80ml.