Varicose veins form when the one-way valves inside your leg veins stop working properly, allowing blood to flow backward and pool instead of traveling up toward your heart. They affect roughly 25% of women and 7% of men, though estimates vary widely depending on the population studied. The underlying causes range from genetics and aging to lifestyle factors like prolonged standing and carrying extra weight.
How Blood Pools in Your Legs
Your leg veins have a tough job. They push blood upward against gravity, relying on tiny one-way valves that open to let blood through and snap shut to prevent it from sliding back down. When those valves weaken or fail, blood reverses course and collects in the vein. This backward flow, called reflux, stretches the vein wall outward over time, creating the bulging, rope-like appearance you can see under the skin.
There are two ways this process starts. The valves themselves may develop structural flaws that make them leak, and the resulting backward pressure gradually damages the vein wall. Or the vein wall weakens first, dilating near valve junctions until the valve leaflets can no longer meet in the middle. Both pathways end in the same place: a stretched, twisted vein that can no longer efficiently return blood to your heart. In many people, both processes happen at once.
Genetics Play a Significant Role
If your parents or siblings have varicose veins, your odds of developing them go up substantially. A large heritability study estimated the genetic component of chronic venous disease at about 17%, which researchers described as high enough to confirm a notable genetic contribution to the condition. By comparison, the two biggest non-genetic risk factors, age and sex, together accounted for roughly 11% of disease risk in the same analysis.
What you inherit isn’t the varicose veins themselves but the structural characteristics that make them more likely: thinner vein walls, fewer elastic fibers, or valves that are prone to early wear. These traits set the stage, and environmental factors determine whether and when visible veins actually appear.
What Aging Does to Your Veins
Vein walls are built from two key structural proteins: elastin, which lets the wall stretch and snap back, and collagen, which provides strength. As you age, both materials deteriorate. Elastin content drops because your body produces less of its precursor and less of the enzyme that cross-links mature elastic fibers. At the same time, the elastic membranes that give veins their resilience fragment and lose contact with the smooth muscle cells that help veins contract.
Collagen doesn’t simply disappear. It accumulates in a disorganized way while the small proteins that normally guide collagen fiber assembly decline. The result is a vein wall that’s stiffer yet paradoxically weaker, less able to contract and push blood upward. In older adults, elevated elastase activity (an enzyme that breaks down elastin) accelerates the process even further. This is why varicose veins become increasingly common after age 40 and are most prevalent between ages 40 and 79.
Pregnancy Creates a Triple Burden
Pregnancy stacks three separate pressures on your venous system at the same time. First, blood volume increases significantly to support the growing fetus, which means more fluid pushing against vein walls that haven’t gotten any stronger. Second, rising progesterone levels relax the smooth muscle in vein walls, making them more prone to stretching. Third, the growing uterus presses directly on the pelvic veins that drain your legs, creating a physical bottleneck for returning blood.
All three forces work together to slow blood flow from the legs back to the heart, and veins that were borderline healthy before pregnancy may swell and twist under the added load. Varicose veins that appear during pregnancy sometimes improve in the months after delivery as blood volume drops and pelvic pressure resolves, but they don’t always disappear completely, especially after multiple pregnancies.
How Hormones Affect Vein Walls
The higher prevalence of varicose veins in women points to hormonal influence, but the relationship is more complicated than “estrogen causes varicose veins.” Estrogen can trigger blood vessels to dilate through the release of nitric oxide, which may stabilize vein walls in some contexts but promote the kind of chronic dilation that leads to varicose veins in others. It also allows for turnover of the structural matrix inside vein walls, which can be either helpful remodeling or harmful degradation depending on the circumstances.
Progesterone has its own dual nature. It can suppress inflammatory signals that damage vein walls and increase vascular tone, which sounds protective. But it also inhibits certain enzymes involved in vein wall remodeling, potentially preventing the structural maintenance veins need to stay healthy. Neither hormone is clearly protective or clearly harmful on its own. The net effect depends on concentration, timing, and individual biology.
Excess Weight and Prolonged Standing
Carrying extra body weight raises the pressure inside your abdomen, which in turn increases the pressure that blood in your leg veins has to push against to reach your heart. Research comparing obese and non-obese patients with similar patterns of valve dysfunction found that the obese group had more advanced venous disease. The extra abdominal pressure leads to wider vein diameters, greater reflux, and higher venous pressures in the legs, even when the underlying valve problem is identical.
Prolonged standing works through a related mechanism. When you stand still for hours, gravity pulls blood downward and your calf muscles (which normally act as a pump to push blood upward when you walk) aren’t contracting enough to counteract it. Over years, the sustained pressure on valves in the lower legs can wear them out. Jobs that require long hours on your feet, like nursing, teaching, retail, or factory work, are consistently linked to higher rates of varicose veins.
Early Signs Before Veins Become Visible
Varicose veins don’t appear overnight. The earliest stage of venous insufficiency produces no visible signs at all. You may notice your legs feel achy, heavy, or unusually tired by the end of the day, especially after standing. Nighttime leg cramps, a burning or tingling sensation, and a “pins and needles” feeling in the lower legs are all common early symptoms that can precede any visible changes by months or years.
The next visible sign is often spider veins, which are smaller, flatter networks of red or purple vessels near the skin surface. Spider veins themselves are mostly cosmetic, but they can signal that deeper valve problems are developing. As the condition progresses, the larger, raised, twisting veins that most people picture when they hear “varicose veins” become apparent.
What Happens if Varicose Veins Go Untreated
For many people, varicose veins remain a cosmetic concern and a source of mild discomfort. But untreated venous insufficiency can progress. The ongoing high pressure in affected veins gradually damages surrounding skin and tissue, leading to changes like darkening or hardening of the skin around the ankles, chronic swelling, and persistent pain.
The most serious complication is venous ulceration, an open wound that develops when damaged skin breaks down, typically near the inner ankle. Venous ulcers affect between 0.25% and 0.5% of the population in European studies and are notoriously slow to heal. Varicose veins can also develop superficial blood clots, a condition called superficial thrombophlebitis, which causes localized redness, warmth, and pain along the vein. In rare cases, a clot in the main superficial vein of the leg can extend into the deeper venous system, raising the risk of a more dangerous deep vein clot.
Varicose veins can also bleed if the stretched, thin-walled vein is close to the skin surface and gets bumped or scratched. Because the pressure inside these veins is elevated, even a small break in the skin can produce surprisingly heavy bleeding that usually stops with firm pressure and leg elevation.