The reason for the appearance of spider veins is not known, it is probably genetic. A lack of exercise, overeating, etc. do not play a significant role in their development, but hormonal changes such as pregnancy, taking the pill or substituting hormone deficiencies (menopause) promote their development. The formation of spider veins is often associated with "poor connective tissue", which tends towards cellulite and increased subcutaneous fat deposits on the lower legs(en/lipedemaa) and thighs.

Spider veins consist of an expansion of the finest skin capillaries, whereby the visible part - which is aesthetically disturbing - only represents a small area of the entire expanded network. The cause is a connection between the smallest arteries and veins, the arteries "inflate" the venous capillaries through their pressure.

Depending on the extent of the arterial part, the spider veins tend to appear red, with little arterial influence, the majority appear blue. It is not uncommon for spider veins to develop in the congestion area of diseased (insufficient) large venous trunk veins, which lead to the dilation of these capillaries due to the pressure of the misdirected blood flow (so-called "internal varicose veins").