Can sitting for longer periods (especially: cross-legged or with one leg tucked under) lead to increased risk of formation of spider veins and varicose veins?

Yes: Spider veins and varicose veins on the legs can be caused by genetics, inactivity, and injury. When the blood can't flow properly, it pools in the leg vein and causes the vein to distend and make it visible thru the skin. In addition, when the blood can't flow properly due to the leg position, the vein can react in the same way.

Possibly: Varicose veins and, frequently, spider veins are due to underlying leaking(refluxing) valves usually in the saphenous system. Stasis of blood in the veins occurs when the calf muscle pump is not being used such as with standing or sitting in place for long periods of time. Stasis can lead to an increased risk of blood clotting but should not lead to increased varicose or spider veins.

Probably not: In all of the 13 major studies that have looked at factors associated with the development of varicose veins, none have shown that crossing your legs when sitting plays any role in the formation of varicose veins. As dr. Landi said, varicose veins are due to leaky valves not venous stasis.