Menstruation is a periodical exfoliation of uterus epithelium; it usually appears every 21-35 days and lasts for 3-6 days. The period between menstruations is called a menstrual cycle, in which there are fundamental changes in woman’s organism related with maturation of an egg cell, ovulation and a yellow body (corpus luteum) functions. The moment of releasing of an egg cell from an ovary – ovulation – divides the menstrual cycle into a preovulatory (follicular) phase and a postovulatory (luteal) phase. The ovulation itself usually appears 14 days before menstruation. In case of absence of fertilization during the ovulation, the egg cell and the yellow body would demise. This causes a fall of ovary hormones level and, as a result, the uterus sheds its lining, which is then passed out of the body with blood – a menstruation appears.