There are two sets of veins within the leg, deep or femoral veins near the bones within the muscles and superficial or saphenous veins which are just under the skin.
The saphenous veins join the femoral veins at various points along the leg. Most of the blood flowing back towards the heart is carried by the femoral veins with very little blood flowing in the superficial veins.
When a person is standing the blood must therefore flow uphill towards the heart. As muscles in the legs contract, they squeeze the femoral veins and thus push the blood up the leg. As the muscles relax, blood is prevented from flowing back down the leg by the action of valves within the veins. The combined action of the muscles and the valves keeps blood flowing up the leg.
Venous reflux
Problems with these valves can lead to venous reflux and varicose veins. Venous reflux is where the valves in the veins and at the junctions between veins have failed and are not functioning properly, thus allowing blood to fall the wrong way down the leg, i.e. back down towards the feet. This back pressure damages valves further down the leg and stretches the veins under the skin; manifesting as varicose veins.
Left untreated venous reflux can cause aching in the legs, swelling of the ankles, thrombophlebitis, venous bleeding, venous eczema, lipodermatolsclerosis, haemosiderin deposition and eventually leg ulceration.
Venous reflux problems are also somewhat hereditary, in that if you have one parent with a history of venous problems, then you have a 50% chance of suffering yourself, two parents and the likelihood rises to 80%. Additionally men tend to have more vascular problems than women. Pregnancy and weight gain can indeed make vascular problems appear worse but they are generally not factors for actually making the problem worse in itself.