Unavoidable underlying causes of chronic venous insufficiency that can lead to varicose veins and spider veins include an inherited genetic predisposition and the normal aging process. Any condition that puts more pressure on leg veins – including standing for long periods of time, being overweight, or pregnancy – can also cause varicose veins or spider veins. Women are at greater risk than men due to hormonal changes that relax vein walls during pregnancy, pre-menstruation or menopause. Birth control pills and hormone replacement therapy may also increase the risk, as do a history of blood clots and conditions that increase pressure in the abdomen, such as tumors, constipation and tight garments like girdles. Other factors include previous venous surgery and exposure to ultraviolet rays.