Varicose veins affect humans because we stand upright rather than run around on all fours. This means that gravity puts pressure on the veins in the legs, and they have to work harder to pump blood back upwards to the heart. Sometimes this pressure can break the valves in the veins which should only let the blood go one way - towards the heart. This means blood can collect in pools in the veins. They are made worse by standing for long periods, by being constipated or overweight, and by pregnancy.
Women suffer from varicose veins more often than men.
You may find that varicose veins appear, or get much worse, when you are pregnant. You will usually see varicose veins before you get any other problems from them. They are like wriggly lines above the surface of the skin on the legs. You may find that they start to ache as the day goes on. They can also get sore and inflamed, causing redness of the skin around them.