Any pregnant women can tell you about this. It occurs because the valves do not empty completely during muscle contraction due to incompetent emptying of the valves. Remember, in the venous system there is no pump to pump the blood back. What the heart does it pumps away and with the pressure what the does is pumps the blood to the extremities continuously and allows the blood to go back to venous system and to come back to the heart. In arteries there are no valves but in the veins there are one-way valves. Essentially what the valves need to empty during contraction but a lot of time they don’t. So what happens there is a back flow and the blood sits and that is the cause of varicosities.
People with varicose veins tell you that their legs absolutely aches from standing and the skin turn a little bit brown from the blood but most specifically what they tell you the most is that these veins got elongated and twisted.
The treatments are: if you are smoking, stop smoking, wear anti-embolic stockings, ambulate a lot because the more you ambulate the better blood returns to the heart, avoid constrictive clothing includes tight pants, vein stripping.