What are the main causes of spider veins?

Heredity: We inherit the gene for weak vein walls, thus spider or varicose vein or weak vein valves, sometimes later in life after environmental or work issues affect us - like prolonged sitting, standing, heavy weight lifting, pregnancies, hot tubs, trauma to legs, hormones. Wearing medical support stockings, esp for work or airline travel and weight lifting and pregnancies can slow the progress.

UV or inherited: Some people hjave a tendency to develop spider veins. In addition, sun exposure may injure collagen and lead to spider veins. Occasionally spider vein may develop after trauma.

Heredity & hormones: Heredity and the hormones are the most likely causes. If you are taking a birth control pill, that can play a role as well. It probably doesn't have much to do with standing, inactivity, what shoes you wear, and your weight. Crossing your legs when you sit down doesn't play a role at all.