If the egg is not fertilised, it lives for about one day. The uterus also prepares itself every month to receive a fertilised egg. Thus its endometrial lining becomes thick and spongy. (This is for nourishing the embryo if fertilisation had taken place.) Now, however, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood, tissue and mucous. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about 3-7 days. The menstrual blood passes out of the body through the vagina.