Varicose veins develop when one-way valves inside your leg veins stop working properly, allowing blood to flow backward and pool instead of returning to the heart. This pooling increases pressure inside the vein, stretching the walls outward until they bulge and twist into the ropy, bluish cords visible beneath the skin. The underlying reasons those valves fail in the first place involve a combination of genetics, lifestyle, hormonal changes, and physical forces acting on the venous system over time.
How Venous Valves Fail
Your leg veins contain a series of small, two-flap (bicuspid) valves spaced along their length. These valves open to let blood move upward toward the heart, then snap shut to prevent it from sliding back down under gravity. Each valve cycle has four phases: opening, equilibrium, closing, and closed. The closing mechanism depends on swirling blood flow patterns that push the valve flaps outward against the vein wall, raising pressure behind the flaps until they seal shut.
When those valves weaken or become damaged, they no longer close completely. Blood leaks backward, a process called reflux, and the column of blood below the faulty valve bears extra hydrostatic pressure. Over time this pressure dilates the vein further, which stretches the next valve downstream and causes it to fail too. This cascading effect explains why varicose veins tend to worsen gradually rather than appearing all at once.
There’s also a less obvious pathway. Connector veins called perforators link the deep veins (surrounded by muscle) to the superficial veins (just under the skin). If the valves in these perforators fail, the powerful pressure generated by your calf muscles during walking gets shunted outward into the superficial system, forcibly dilating those thinner-walled veins from the inside.
Genetics and Vein Wall Structure
Family history is one of the strongest predictors of varicose veins, and researchers have begun identifying the specific genes involved. Mutations in a gene called FOXC2 on chromosome 16 are strongly linked to primary valve failure in both superficial and deep leg veins. In one study, every participant carrying a FOXC2 mutation showed reflux in the great saphenous vein (the long vein running from ankle to groin), compared with just 1 out of 12 people without the mutation. FOXC2 appears to be critical for the normal development and maintenance of venous and lymphatic valves.
Beyond specific gene mutations, some people inherit structurally weaker vein walls. Studies comparing varicose saphenous veins to healthy ones have found measurable differences in their protein composition. Varicose veins contain higher proportions of certain collagen types and altered ratios of elastin to collagen, which changes how the wall stretches and recoils. A vein wall that stretches too easily under pressure is more likely to dilate to the point where its valves can no longer close.
Prolonged Standing and Sitting
Gravity is the main force your leg veins work against, so any position that keeps blood in your legs for extended periods adds stress to the valves. Occupations that require standing in one place carry a measurably higher risk. A large study adjusting for age, social class, and smoking found that men who worked mostly standing had 1.85 times the risk of being hospitalized for varicose veins compared to other men. For women in standing jobs, the risk was even higher at 2.63 times.
Prolonged sitting isn’t harmless either, especially with legs bent at the knee or crossed, which can compress veins and slow return flow. The key factor in both cases is the absence of regular calf muscle contractions. When you walk, your calf muscles squeeze the deep veins and pump blood upward. Standing still or sitting for hours removes that pump action, leaving blood to stagnate and pressure to build.
Pregnancy
Pregnancy increases varicose vein risk through several overlapping mechanisms. Blood volume rises by roughly 40 to 50 percent to support the growing fetus, placing greater demand on veins that weren’t designed for that load. Hormonal shifts, particularly rising progesterone, relax the smooth muscle in vein walls, making them more prone to stretching. And as the uterus enlarges, it compresses the large veins in the pelvis, partially obstructing blood returning from the legs.
The good news is that pregnancy-related varicose veins are often temporary. A prospective study at a tertiary care center found that 72.1% of women saw their varicosities become asymptomatic or invisible within six months after delivery. Each subsequent pregnancy, however, tends to increase the severity and reduce the likelihood of full resolution, because the vein walls and valves accumulate damage with each round of stretching.
Obesity and Abdominal Pressure
Excess body weight, particularly fat stored around the abdomen, increases intra-abdominal pressure. That elevated pressure acts like a partial dam on the veins draining the legs, reducing the pressure gradient that normally pulls blood upward. Research has confirmed that obesity is associated with increased lower extremity venous pressure and decreased shear stress on vein walls. Over time, these forces damage the tiny valves in the skin’s microvascular system and contribute to larger vein dilation. The combination of subcutaneous and visceral abdominal fat appears to have both a mechanical effect (physically compressing veins) and a metabolic one (promoting inflammation in vein walls).
Blood Clots and Vein Damage
A history of deep vein thrombosis (DVT) can cause varicose veins through a different route than the typical valve-weakening process. When a blood clot forms in a deep leg vein, it can scar and permanently damage the valves in that segment. Even after the clot dissolves or is treated, the valve damage persists. Blood pools in the affected deep vein, raising pressure that gets transmitted outward through perforator veins into the superficial system. This condition, called post-thrombotic syndrome, frequently produces new varicose veins along with swelling, skin changes, and chronic discomfort in the affected leg.
Age and Other Contributing Factors
Aging is an independent risk factor simply because vein walls and valves accumulate wear over decades. Collagen and elastin in the vein walls degrade with time, reducing the structural support that keeps veins from dilating. Hormonal factors also play a role outside of pregnancy: estrogen and progesterone fluctuations throughout life can soften vein walls, which is one reason varicose veins are more common in women than in men.
A sedentary lifestyle compounds many of the risks above by weakening the calf muscles that serve as the leg’s venous pump. Smoking may also contribute by damaging blood vessel walls and impairing circulation, though its effect on varicose veins specifically is less dramatic than its impact on arteries.
How Varicose Veins Progress
Clinicians grade chronic venous disease on a scale from C0 (no visible signs) through C6 (active skin ulcers). Varicose veins visible to the eye fall at C2 on this scale. Left unmanaged, the ongoing high pressure in the veins can progress through stages: swelling (C3), skin discoloration and eczema (C4), and eventually open sores near the ankle that are slow to heal (C5 and C6). Not everyone progresses through all stages, and many people with varicose veins remain at C2 indefinitely, but the trajectory helps explain why addressing the underlying causes early, whether through compression, exercise, weight management, or medical procedures, can prevent complications down the line.
The causes of varicose veins rarely act alone. Most people who develop them have some inherited vulnerability in their vein walls or valves, combined with one or more triggers like prolonged standing, pregnancy, or weight gain that push those veins past their structural limits. Understanding which factors apply to you can help you focus on the ones you can actually modify.