Varicose veins are dilated, thickened, elongated and twisted blood vessels that don't control blood flow as they should. In some cases, they can be small spider veins and even appear thread-like. In other cases they may appear as large grape-like clusters under the skin. Varicose veins occur more often in women than men, especially during pregnancy, and in people who have a job for which they must stand for long periods of time.