Varicose veins develop when small one-way valves inside your leg veins stop working properly, allowing blood to flow backward and pool instead of moving up toward your heart. This pooling increases pressure inside the vein, stretching the walls outward until they become the swollen, twisted, bluish-purple cords visible under the skin. The process is usually gradual, driven by a combination of genetics, hormones, aging, and lifestyle factors that weaken either the vein walls or the valves themselves.
What Happens Inside the Vein
Your leg veins have to work against gravity to push blood back to your heart. To manage this, they contain a series of tiny valves that open to let blood flow upward and snap shut to prevent it from sliding back down. When these valves fail, blood reverses direction and collects in the vein below. This backward flow creates a rising column of pressure that stretches the vein walls wider and wider over time.
There’s an ongoing debate about what fails first. One theory holds that the valves break down due to sustained pressure from above, like a chain of dominoes falling from top to bottom. A competing theory, sometimes called the “weak wall hypothesis,” suggests the vein wall itself deteriorates before the valve does. As the wall weakens and stretches just below a valve, the two flaps of the valve get pulled apart and can no longer meet in the middle to form a seal. Either way, the result is the same: blood pools, pressure builds, and the vein bulges outward.
Genetics Play a Major Role
If your parents or siblings have varicose veins, your chances of developing them are significantly higher. Researchers have identified several genes involved. One gene called FOXC2, located on chromosome 16, is strongly linked to primary venous valve failure. Variants near this gene appear to directly affect how well your valves form and function. Another gene, desmuslin, influences the smooth muscle cells that give vein walls their tone and structure. When this gene is less active, those muscle cells weaken, and the vein loses its ability to hold its shape under pressure.
A variant in a gene called SLC40A1 is associated with a four-fold increased risk of chronic venous disease and a five-fold increased risk of developing leg ulcers, one of the more serious complications. These aren’t rare mutations found only in a lab. They represent real variation in the general population that helps explain why some people develop severe varicose veins in their 30s while others never get them at all.
How Hormones Affect Your Veins
Estrogen relaxes the walls of veins, making it harder for valves to close tightly. Progesterone does something similar, loosening the smooth muscle in blood vessel walls and reducing valve function. When either hormone spikes, blood is more likely to flow backward and collect in the lower legs, causing swelling, heaviness, and eventually visible varicose veins.
This is a key reason varicose veins are more common in women, particularly during periods of hormonal change. Birth control pills and hormone replacement therapy both raise estrogen levels enough to weaken vein walls over time. The effect is compounded with years of exposure, which is why varicose veins often appear or worsen during perimenopause rather than showing up all at once.
Why Pregnancy Is a Common Trigger
Pregnancy creates a near-perfect storm for varicose veins. Hemodynamic changes begin as early as five weeks into gestation. Blood volume increases substantially to support the growing fetus, while the diameter of leg veins expands and blood flow velocity through those veins actually decreases. Slower-moving blood in wider veins puts more strain on already-stressed valves.
On top of that, the growing uterus presses on the major veins in the pelvis, partially blocking the return of blood from the legs. Combine this mechanical compression with the high progesterone and estrogen levels of pregnancy, and the valves face pressure from multiple directions at once. Symptoms like visible varicose veins, night cramps, and skin discoloration tend to become most noticeable in the third trimester. Many of these changes reverse after delivery, but with each subsequent pregnancy the veins are less likely to fully recover.
What Aging Does to Vein Walls
Your veins rely on two key structural proteins: elastin, which gives them flexibility, and collagen, which provides strength. With age, elastin gradually fragments and breaks down. Your body compensates by producing more collagen, but collagen is stiffer and less forgiving. The ratio of elastin to collagen drops steadily over the decades, leaving vein walls that are rigid in some places and weak in others. This makes them less able to bounce back after being stretched by blood pressure, and less effective at supporting the valves inside them.
This is why varicose veins become increasingly common after age 40 and continue to rise in prevalence through the 50s and beyond. The structural decline is slow enough that you won’t notice it happening, but the cumulative damage from decades of standing, walking, and pumping blood against gravity eventually catches up with the tissue.
How Weight and Daily Habits Contribute
Carrying excess weight increases the pressure inside your abdominal cavity, which in turn raises the pressure on the veins running through your pelvis and legs. Research published in the Journal of Vascular Surgery found that body weight correlated directly with wider vein diameters, higher resting venous pressure, and greater blood reflux in the legs. People with obesity had significantly higher foot venous pressure in every position measured, whether standing, sitting, or lying down. That constant extra pressure accelerates valve failure and vein stretching.
Prolonged standing or sitting works through a similar mechanism. When you stand still for hours, your calf muscles aren’t contracting to squeeze blood upward, so it pools in the lower legs. Occupations that require long periods on your feet carry higher risk. A study of teaching professionals found varicose vein prevalence of nearly 19% overall, with rates climbing among those with more years in the profession. The effect is cumulative: years of standing in the same position gradually overwhelm the valves’ ability to keep blood moving in the right direction.
Regular movement helps counteract this. Walking, cycling, or even shifting your weight and flexing your calves throughout the day activates the muscle pump that assists venous return. Elevating your legs above heart level when resting also reduces the pressure your valves have to fight against. These habits won’t reverse existing varicose veins, but they can slow progression and reduce symptoms like aching and swelling.
Putting the Risk Factors Together
Most people who develop varicose veins have more than one risk factor working against them at the same time. A woman with a family history of venous disease who goes through two pregnancies and works a job that requires standing has compounding pressures on her vein walls and valves from multiple directions: genetic vulnerability, hormonal shifts, mechanical compression, and gravitational stress. Each factor alone might not be enough to cause problems, but together they tip the balance.
Age acts as a multiplier for everything else. The same genetic predisposition that causes no symptoms at 25 can produce visible, symptomatic varicose veins by 45 as the structural proteins in vein walls degrade. Weight gain in middle age adds abdominal pressure on top of already-weakening tissue. This layering of causes is why varicose veins are so common in the general population and why they tend to worsen gradually rather than appearing overnight.