Spider veins on the legs develop when tiny blood vessels near the skin’s surface become dilated and visible, typically because of increased pressure in the venous system. The underlying cause is almost always some combination of faulty vein valves, hormonal influences, and sustained pressure on the legs. While they’re common and usually harmless, understanding what drives them can help you reduce new ones from forming.
How Vein Valves Create the Problem
Your leg veins contain one-way valves that keep blood moving upward toward the heart against gravity. When those valves weaken or fail, blood flows backward and pools in the vein below. This is called venous reflux, and it raises pressure inside the affected veins. That elevated pressure, known as venous hypertension, is the central mechanism behind spider veins.
The valve failure itself happens through physical deterioration: the tiny valve leaflets can thin out, tear, deform, or stick together over time. Once a valve stops closing properly, the column of blood above it pushes down on the veins below, stretching their walls. In the smallest superficial veins, this stretching produces the red, blue, or purple web-like patterns visible through the skin. Spider veins sit at the mildest end of a spectrum of venous disease that also includes reticular veins and the larger, bulging varicose veins.
Genetics Play an Outsized Role
If your parents or grandparents had spider veins, you’re far more likely to develop them yourself. Up to 90% of people with spider veins have a family history of them. That inherited component likely involves the structural quality of vein walls and valves. Some people are simply born with weaker connective tissue in their veins, making valve failure more probable as they age.
Hormones and Why Women Are More Affected
Estrogen and progesterone directly influence vein behavior, which helps explain why spider veins are significantly more common in women. Estrogen relaxes the smooth muscle in vein walls by interfering with the calcium-dependent contraction mechanism that keeps veins firm. Under high estrogen levels, veins become more distensible, meaning they stretch more easily under pressure. Progesterone has similar relaxing effects.
Research on women with varicose veins has found that their vein walls contain substantially more hormone receptors than healthy veins do. Women with varicose veins had roughly twice the number of progesterone receptors compared to women without them, and this increase was spread throughout every layer of the vein wall. The greater density of receptors may amplify the effects of normal hormone levels, stimulating the cell changes responsible for vein dilation and the twisting, enlarged shape characteristic of damaged veins. This is why spider veins often appear or worsen during hormonal shifts: puberty, pregnancy, use of hormonal birth control, and menopause.
What Pregnancy Does to Leg Veins
Pregnancy combines nearly every risk factor for spider veins at once. Blood volume increases dramatically to support the developing baby, which raises pressure throughout the venous system. At the same time, estrogen and progesterone levels surge, relaxing vein walls and making them more prone to stretching. The growing uterus also compresses the large veins in the pelvis, slowing blood flow from the legs back to the heart.
Together, these changes create higher venous pressure and reduced flow rates in the deep leg veins, predisposing pregnant women to both temporary and lasting venous problems. Spider veins that appear during a first pregnancy sometimes fade afterward, but women who go through multiple pregnancies are more likely to develop chronic venous insufficiency, where the damage becomes permanent.
Standing and Sitting for Long Periods
Gravity is the constant enemy of leg veins. When you’re standing upright and still, the blood pressure in your foot veins reaches roughly 90 mmHg, significantly higher than when you’re lying down, when gravitational effects essentially disappear. Your calf muscles act as a pump: each time they contract during walking, they squeeze the deep veins and push blood upward. When you stand or sit without moving, that pump shuts off, and about 500 ml of blood shifts into the veins of your lower body.
This pooling doesn’t mean blood sits motionless. It means blood moves through those veins much more slowly, spending more time pressing against the vessel walls. Over months and years, that sustained pressure weakens valves and stretches the smallest veins near the skin’s surface. Prolonged standing also pushes fluid out of the capillaries and into surrounding tissue, contributing to swelling that further strains the venous system. Jobs that require hours of standing or desk work without movement are a well-recognized risk factor.
Excess Body Weight
Carrying extra weight raises pressure on your leg veins through a straightforward mechanical pathway. Obesity increases intra-abdominal pressure, which creates a functional obstruction to blood flowing back from the legs. This effect has been measured directly: people with obesity have wider femoral veins (the large veins in the upper thigh) with higher internal pressures and slower blood flow compared to people at a healthy weight. Those differences persist whether the person is standing, sitting, or lying down.
The result is the same cycle that drives other causes of spider veins. Chronically elevated venous pressure leads to valve damage, reflux, inflammation, and eventually visible changes in the superficial veins and skin.
Aging and Skin Changes
Spider veins become more common with age for two reasons that reinforce each other. First, vein valves accumulate wear over decades of use, making reflux more likely in older adults. Second, the skin itself changes in ways that make small dilated veins more visible.
As skin ages, collagen production drops while collagen breakdown increases. The well-organized, tightly packed collagen fibers of younger skin become fragmented and sparse. The dermis thins, loses elasticity, and provides less structural support to the tiny blood vessels embedded within it. Fibroblasts, the cells responsible for maintaining this scaffolding, shrink and become less active. The combined effect is thinner, less resilient skin overlying veins that are themselves under more pressure than they were years earlier.
Sun Exposure
Ultraviolet radiation, particularly UVA rays that penetrate deeper into the skin, damages the endothelial cells lining small blood vessels. Over time, this damage causes superficial blood vessels to dilate permanently, a process grouped under the broader category of photoaging alongside deep wrinkles, leathery texture, and dark spots. Sun-related spider veins are more common on areas with chronic exposure, but UV damage to leg skin (especially in people who spend time outdoors in shorts or swimwear) can contribute to their development there as well.
Reducing Your Risk
You can’t change your genetics or stop aging, but several modifiable factors make a real difference. Moving regularly throughout the day activates your calf muscle pump and prevents blood from pooling. If your job requires long periods of standing or sitting, even brief walking breaks or calf raises help. Compression stockings provide external pressure that supports blood flow back toward the heart, which is why they’re commonly recommended for people on their feet all day or during pregnancy.
Maintaining a healthy weight reduces the abdominal pressure that obstructs venous return. Protecting your skin from UV damage with sunscreen or clothing preserves the structural integrity of both the skin and the small vessels within it. Elevating your legs when resting, even for 15 to 20 minutes, drops venous pressure in the feet and calves close to zero by removing the gravitational load. None of these strategies will eliminate spider veins that already exist, but they slow the progression and reduce new ones from appearing.