What those red veins could be
Red or purplish veins appearing or spreading on the thighs are usually tiny superficial blood vessels near the surface of the skin. They may look like fine lines, branches, or webs. They can be flat or slightly raised, sometimes painless, sometimes achy. Common causes include:

a. Spider veins (telangiectasias)
These are very small, thin veins (1 mm or less) just under the skin. They can appear red, purple, or blue and often form “spider-web” or “tree branch” patterns. They’re typically harmless but can be cosmetic concerns or mildly uncomfortable.

b. Early varicose veins
Varicose veins are larger, swollen, and rope-like. They result from weakened vein valves, leading to poor blood return from the legs. Sometimes red or purple capillaries appear before larger varicose veins develop.

c. Irritation or inflammation (superficial thrombophlebitis)
A small vein can become inflamed, sometimes after trauma, prolonged sitting, or a mild clot. This can cause a red, tender line or patch over the vein.

d. Skin irritation, allergy, or infection
Occasionally, what looks like veins are actually small red streaks or dilated capillaries from dermatitis, allergic rash, or mild infection.

e. Hormonal and lifestyle factors
Pregnancy, birth control, hormonal shifts, prolonged standing, or sitting can all stress leg veins and make these networks appear or worsen.

2. Why this happens