Increase in the diameter of a vein results in the malfunction of valves inside of the vein that are designed to prevent blood from flowing backward in the vein. An inherited weakness of the vein wall (genetic) and circulating hormones that relax the smooth muscle of the vein wall are the most common causes. Females have high levels of such a hormone, the hormone progesterone. Pregnancies, obesity, hormone replacement therapy, the use of female oral contraceptives as well as occupations that require a standing position are additional causes of varicose veins. Since females usually have multiple risk factors for the development of varicose veins, about three fourths of the persons who have varicose veins are women and 25% are men.