You noticed a rope-like vein on your calf that was not there last summer. Your legs feel heavy by the end of a shift. Your mother had the same veins, and now you are wondering whether this was always going to happen to you.
What causes varicose veins is a mechanical failure inside your leg veins. Tiny one-way valves that are supposed to push blood upward toward your heart stop closing properly. Blood then flows backward and pools in the vein, a process called venous reflux. That pooled blood raises pressure inside the vein, and the vein wall stretches, twists, and bulges until you can see it through your skin.
Genetics decides how likely your valves are to fail. Your daily life decides how fast it happens. Understanding both is the difference between watching your legs get worse every year and doing something about it early.
At Vein and Knee Pain Vascular Group in New York, we evaluate this every day with duplex ultrasound, and the underlying pattern is remarkably consistent. Here is what is actually going on inside your legs.
What Causes Varicose Veins? The Direct Answer
Varicose veins are caused by failed vein valves that allow blood to flow backward and pool in the leg, which raises pressure inside the vein and forces the vein wall to stretch and bulge. Genetics is the strongest predictor of valve failure. Age, pregnancy, prolonged standing, and excess weight accelerate it by increasing the pressure those valves must hold back.
The chain of events runs in this order:
- Vein valves weaken from inherited vein wall weakness, age, or hormonal changes.
- Blood refluxes backward instead of moving up toward the heart.
- Blood pools in the superficial leg veins.
- Venous pressure rises inside the vein, a state called venous hypertension.
- The vein wall stretches and twists until it bulges visibly under the skin.
Everything below explains why this happens to some people and not others, and what you can actually do about it.
How Healthy Leg Veins Are Supposed to Work
Leg veins move blood uphill against gravity, which is a demanding job that depends on two systems working together. When either system fails, blood begins to collect in the lower leg.
Your legs contain three types of veins, and only one of them becomes varicose:
| Vein type | Location | Carries | Becomes varicose? |
|---|---|---|---|
| Deep veins | Inside the leg muscle | About 90% of blood returning to the heart | Rarely |
| Superficial veins | Just under the skin (includes the great saphenous vein) | About 10% of return flow | Yes, this is where varicose veins form |
| Perforator veins | Connect superficial to deep | Redirects flow inward | Can fail and feed varicose veins |
The Calf Muscle Pump
Your calf is a pump. Every time you take a step, the calf muscle contracts and squeezes the deep veins, pushing a column of blood upward toward the heart. Vascular specialists sometimes call the calf the peripheral heart for this reason.
When the muscle relaxes, gravity tries to pull that blood straight back down. This is where the valves matter.
What Happens When Vein Valves Fail
Vein valves are thin, flexible flaps that snap shut the instant blood tries to move backward. In a healthy vein, they close completely and blood only travels one direction.
When a valve weakens, stretches, or is damaged, the flaps no longer meet in the middle. Blood leaks backward through the gap. That backward flow is venous reflux, and it is the single event that starts varicose vein disease.
The problem compounds. Refluxing blood pools above the next valve down, raising pressure on that valve. That valve then fails, sending more blood downward onto the next one. A single incompetent valve at the top of the great saphenous vein can eventually take out an entire chain of valves below it.
This cascade is why varicose veins rarely stay the same size, and why bulging veins in your legs that are not varicose still deserve a proper evaluation before you assume they are harmless.
Chronic Venous Insufficiency: The Disease Behind the Bulge
Chronic venous insufficiency is the medical condition that produces most varicose veins, and the visible vein is only the surface sign of it. When reflux becomes persistent, the elevated pressure inside the leg veins is called venous hypertension, and it affects far more than appearance.
Sustained venous hypertension pushes fluid and inflammatory cells out through the vein wall into surrounding tissue. That is why patients report:
- Ankle and lower leg swelling that worsens through the day
- Aching, throbbing, or a heavy sensation in the legs
- Itching or dry, scaly skin over the vein
- Brown or reddish skin staining above the ankle
- Restless legs and night cramping
- In advanced cases, open venous ulcers near the ankle
The bulging vein you can see is the visible tip of chronic venous disease happening in the tissue you cannot see.
The CEAP Stages of Chronic Venous Disease
Vascular medicine uses the CEAP classification to stage this progression. The reason vein specialists take early varicose veins seriously is that CEAP is a progression, not a set of separate conditions.
| CEAP stage | What it means |
|---|---|
| C0 | No visible or palpable signs of venous disease |
| C1 | Spider veins and reticular veins |
| C2 | Varicose veins |
| C3 | Leg and ankle swelling (edema) |
| C4 | Skin changes: pigmentation, eczema, lipodermatosclerosis |
| C5 | Healed venous ulcer |
| C6 | Active venous ulcer |
Left alone, C2 tends to become C3 and C4 over the years. That trajectory, not the appearance of the vein, is what drives the recommendation to evaluate early.
What Are Varicose Veins Caused By at the Tissue Level?
At the microscopic level, varicose veins are caused by remodeling of the vein wall itself, not just by valve mechanics. Research into the molecular pathology of varicose veins describes a sequence: abnormal shear stress from high venous pressure disrupts the endothelial lining, immune cells infiltrate the vein wall, local inflammation follows, and the extracellular matrix that gives the vein its strength is broken down and rebuilt incorrectly.
The practical consequence is important. A vein that has remodeled this way does not regain its strength. Compression stockings and leg elevation control symptoms and slow progression, but they do not restore a vein wall that has already lost its structural collagen and elastin. That biological reality is why definitive treatment closes the failed vein rather than trying to repair it.
Genetics: The Strongest Risk Factor for Varicose Veins
Family history predicts varicose veins better than any lifestyle factor, because the strength of your vein walls and the architecture of your valves are inherited traits. If you have wondered whether varicose veins are genetic, the evidence is unusually strong for this condition.
What the genetic research shows:
- Genome-wide association studies have identified more than 30 genetic loci linked to varicose vein risk.
- Larger multi-ancestry analyses describe a polygenic architecture, meaning many genes each contribute a small amount rather than one gene causing the condition outright.
- Several of these loci overlap with genes governing connective tissue and vascular development, which fits what surgeons observe in the operating field: some patients simply have thinner, more distensible vein walls.
- Vein specialists commonly cite that risk rises sharply when both parents are affected, with an inheritance pattern consistent with autosomal dominant transmission and variable penetrance.
Variable penetrance explains a pattern our New York patients describe constantly: one sibling has severe varicose veins at 32, another has none at 50, and both have the same parents.
Why Genetics Alone Does Not Decide Your Outcome
Inheriting weaker veins sets your baseline risk. It does not set your timeline. Two people with identical genetic risk can have completely different outcomes depending on how much venous pressure their daily life generates over 30 years.
In practice, this is the most useful thing a patient with a strong family history can understand. You cannot change the vein wall you inherited. You can change how many hours a day it is asked to hold back a standing column of blood.
Varicose Vein Risk Factors at a Glance
Varicose vein risk factors fall into two groups, and separating them tells you where your effort is worth spending. Fixed factors set your baseline risk. Modifiable factors control how fast that risk turns into visible, symptomatic veins.
| Risk Factor | Type | How It Causes Varicose Veins |
|---|---|---|
| Family history | Fixed | Inherited vein wall weakness and valve architecture |
| Age | Fixed | Loss of elastin; valve leaflets thin and stiffen |
| Female sex | Fixed | Estrogen and progesterone increase vein distensibility |
| Pregnancy | Fixed | Blood volume rise, hormones, and pelvic vein compression |
| Height | Fixed | Longer blood column raises hydrostatic pressure at the ankle |
| Prior DVT | Fixed | Scarred deep valves force blood into superficial veins |
| Prolonged standing | Modifiable | Calf pump stays idle while pressure keeps building |
| Prolonged sitting | Modifiable | Hip and knee flexion restricts venous outflow |
| Excess body weight | Modifiable | Raised abdominal pressure impedes venous return |
| Inactivity | Modifiable | Weak calf muscle means weak pump capacity |
| Smoking | Modifiable | Endothelial damage impairs vein wall repair |
| Chronic straining | Modifiable | Repeated intra-abdominal pressure spikes |
Risk Factors You Cannot Control
Several established varicose vein risk factors are fixed, and knowing which ones apply to you tells you how closely to monitor your legs.
Aging Veins
Vein walls lose elastin with age, and valve leaflets thin and stiffen. Prevalence of varicose veins rises steadily with each decade, and estimates suggest roughly 25 million American adults are affected overall. Aging veins do not cause varicose veins on their own, but they lower the pressure threshold at which valves give way.
Sex and Hormones
Women develop varicose veins at higher rates in most population studies, and hormones are the reason. Estrogen and progesterone both relax smooth muscle in the vein wall, which increases venous distensibility. Progesterone in particular allows veins to stretch more under the same pressure.
Common hormonal trigger points include:
- Starting combined oral contraceptives
- Pregnancy and the postpartum period
- Perimenopause and menopause
- Hormone replacement therapy
- The premenstrual week, when many women notice symptoms worsen
Pregnancy
Pregnancy raises varicose vein risk through three separate mechanisms at once, which is why it is such a powerful trigger:
- Blood volume increases by roughly 40 to 50 percent, placing more load on every valve.
- Progesterone rises steeply, relaxing vein walls and increasing distensibility.
- The growing uterus compresses the pelvic veins, obstructing outflow from both legs.
Varicose veins in pregnancy often appear in the first trimester, before the uterus is large enough to compress anything, which tells you the hormonal effect arrives first. Many pregnancy-related veins improve in the months after delivery. Veins that persist beyond about six months postpartum usually reflect valve damage that will not reverse on its own, and each subsequent pregnancy tends to leave more behind.
Height and Build
Taller people carry a longer vertical column of blood, meaning the valves at the ankle bear more hydrostatic pressure with every hour spent upright. Genetic analyses have found height itself associates with varicose vein risk independently of other factors, partly because the genes influencing height also influence connective tissue.
Risk Factors You Can Control
Modifiable risk factors work by one common pathway: they increase how much pressure your vein valves must resist, or how long they must resist it. These are where prevention has real leverage.
Prolonged Standing or Sitting
Standing still is harder on your veins than walking, because a motionless calf muscle is a pump that never fires. Occupational studies consistently find elevated varicose vein rates in roles that demand hours of static posture:
- Nurses and surgical staff
- Teachers
- Retail and hospitality workers
- Hair stylists and barbers
- Security guards and traffic officers
- Construction and factory workers
Sitting is a different problem with the same result. Hip and knee flexion partially compresses the femoral and popliteal veins while the calf pump sits idle. In New York, we see this constantly in patients who commute, then sit at a desk for nine hours, then commute again. If you have ever wondered why your veins are so visible after a long workday, the answer is usually accumulated pooling from an unused calf pump.
Excess Body Weight
Obesity increases intra-abdominal pressure, which impedes venous return from the legs at the pelvic level. Higher body mass index is one of the most consistently reported risk factors in observational studies of varicose veins, with the association particularly strong in women.
Excess weight also makes diagnosis harder. Deeper veins under a thicker subcutaneous layer may cause significant symptoms, aching, heaviness, and swelling, without ever becoming visible. Some of the most severe reflux we find on ultrasound occurs in patients whose legs look relatively normal from the outside.
Inactivity and Muscle Weakness
A weak or immobile calf loses pump capacity. Patients with ankle stiffness, prior leg injury, or a sedentary routine have measurably worse venous return even with structurally normal valves. This is also part of why varicose veins can cause knee pain and why leg symptoms and joint symptoms so often arrive together.
Smoking, Constipation, and Chronic Straining
Smoking damages the vascular endothelium and impairs the vein wall repair processes described earlier. Chronic constipation and repeated heavy lifting both spike intra-abdominal pressure, transmitting force downward into the leg veins with every strain. Neither causes varicose veins alone, but both accelerate valve failure in someone already predisposed.
What Causes Spider and Varicose Veins to Appear in the Same Leg?
Spider veins and varicose veins share the same root cause, which is why they so often show up together. Both result from elevated venous pressure. The difference is vessel size and depth.
| Feature | Spider veins | Varicose veins |
|---|---|---|
| Vessel size | Under 1 mm | Over 3 mm |
| Depth | Directly in the skin | Just beneath the skin |
| Appearance | Flat red, blue, or purple webs | Raised, rope-like, twisted bulges |
| Common symptoms | Usually none, sometimes mild burning | Aching, heaviness, swelling, itching |
| Underlying cause | Elevated venous pressure, often fed by a deeper failed vein | Direct valve failure and venous reflux |
When a larger vein below refluxes, the backward pressure is transmitted outward into the small vessels above it. Those vessels dilate and become the red or blue webs on the thigh, behind the knee, or around the ankle.
This is a clinically important detail. Treating spider veins cosmetically without addressing the failed vein feeding them is the most common reason spider veins come back within a year. A cluster of spider veins near the ankle in particular often signals underlying reflux that only ultrasound will reveal.
For a fuller breakdown, see our guides on varicose veins versus spider veins and what actually causes spider veins to form in the first place.
Primary vs Secondary Varicose Veins
Most varicose veins are primary, meaning the valves failed on their own from inherited weakness plus accumulated pressure. A minority are secondary, meaning an identifiable event destroyed or obstructed the venous system. Published estimates put secondary cases at roughly 5 to 20 percent of all varicose veins.
| Primary varicose veins | Secondary varicose veins | |
|---|---|---|
| Share of cases | Roughly 80 to 95% | Roughly 5 to 20% |
| Cause | Inherited vein wall weakness plus accumulated venous pressure | An event that damaged or blocked the venous system |
| Typical triggers | Genetics, age, pregnancy, standing work | Prior DVT, pelvic vein compression, tumors, arteriovenous malformation, leg trauma |
| Onset | Gradual, over years | Often faster, sometimes with sudden swelling |
Prior deep vein thrombosis is the most common secondary cause. A clot scars deep vein valves and forces blood to reroute through superficial veins that were never designed to carry that volume.
This distinction changes management, which is why any sudden onset of leg swelling with new varicose veins warrants prompt evaluation rather than watchful waiting. Understanding the differences between DVT and varicose veins matters here, because the two conditions can coexist and one can cause the other.
Why Did I Get Varicose Veins in Only One Leg?
Varicose veins commonly affect one leg more than the other because valve failure begins at a single point in a single vein rather than symmetrically across both legs. Unilateral varicose veins are normal and usually reflect which great saphenous vein happened to develop reflux first.
That said, veins confined entirely to one leg with associated swelling deserve extra attention, because they can indicate localized obstruction such as a prior clot or pelvic compression on that side. May-Thurner syndrome, in which the right iliac artery compresses the left iliac vein, classically produces left-sided leg symptoms. If you are dealing with varicose veins in one leg only, a duplex ultrasound is worth doing rather than assuming it is coincidence.
Prevention Tips: What Actually Works
You cannot prevent varicose veins with certainty, because you cannot change your genetics, your age, or your sex. What you can do is reduce cumulative venous pressure, which meaningfully delays onset and slows progression in people at risk. Here is what has real physiological justification.
1. Move Your Calf Every 30 Minutes
Twenty calf raises every half hour beats a single gym session for venous health, because the calf pump works on frequency, not intensity. Set a recurring reminder if your job keeps you seated or standing in place. Walking remains the single best exercise for venous return.
2. Wear Graduated Compression If You Stand for Work
Graduated compression stockings apply the most pressure at the ankle and less as they move upward, mechanically supporting the vein walls and improving valve closure. For nurses, teachers, retail staff, and anyone on their feet more than four hours, 15 to 20 mmHg compression worn during the workday is the highest-value preventive step available.
3. Elevate Your Legs Above Heart Level
Fifteen minutes of elevation, twice daily, drains pooled blood using gravity instead of fighting it. This is especially effective in the evening, and it is often what finally answers the question of why your legs hurt at night.
4. Maintain a Healthy Weight
Reducing abdominal pressure improves venous outflow from the pelvis. Even modest weight loss lowers the baseline pressure your leg valves are working against every hour you are upright.
5. Manage Constipation and Avoid Chronic Straining
Adequate fiber and hydration reduce repeated intra-abdominal pressure spikes. This is an overlooked prevention step that costs nothing.
6. Get an Ultrasound Early If Your Family History Is Strong
If a parent or sibling has varicose veins and you have started noticing leg heaviness, aching, or evening swelling, a baseline duplex ultrasound tells you whether reflux has already started. Reflux is measurable before veins become visible. Early detection is the one prevention tool that requires a specialist, and it is the most informative one.
When to See a Vein Doctor in NYC
Cosmetic concern alone is a legitimate reason to seek treatment, but certain signs indicate the underlying venous disease is progressing and should be evaluated promptly:
- Brown, red, or darkened skin staining above the ankle
- Persistent swelling that does not resolve overnight
- Itching, flaking, or eczema over a vein
- A hard, tender, cord-like segment along a vein
- Bleeding from a varicose vein, even minor
- Any sore near the ankle that will not heal
- New varicose veins accompanied by sudden one-sided swelling
Knowing when to worry about varicose veins matters because untreated venous reflux can progress to complications, including blood clots and venous ulcers that are far harder to treat than the original vein.
At Vein and Knee Pain Vascular Group, evaluation begins with a full history, a physical exam, and duplex ultrasound mapping of your venous anatomy. That map is what determines which vein is refluxing and where, and it is the basis for every treatment decision. Board-certified interventional radiologist Dr. Amir Salem, MD reviews each study personally.
When treatment is indicated, modern options are in-office and minimally invasive:
- Radiofrequency ablation, using heat energy to close the failed vein
- Endovenous laser ablation, sealing the vein with laser energy
- Varithena microfoam, collapsing the vein with an injected foam
- VenaSeal, closing the vein with a medical adhesive
- Sclerotherapy, best suited to smaller veins and spider veins
- Ambulatory phlebectomy, removing bulging surface veins through tiny incisions
Each closes the failed vein so blood reroutes through healthy ones. You can review the full range of varicose vein treatment options and see real patient before and after results from our New York practice.
Frequently Asked Questions About What Causes Varicose Veins
Can varicose veins go away on their own?
No. Varicose veins do not resolve on their own, because the valve damage and vein wall remodeling that caused them are permanent. Pregnancy-related veins sometimes improve within six months of delivery as blood volume and hormones normalize, but veins persisting beyond that reflect lasting valve failure and typically worsen over time.
Does crossing your legs cause varicose veins?
No. Crossing your legs does not cause varicose veins. This is a persistent myth. The pressure generated by leg crossing is far too low and too brief to damage vein valves. What matters is the total number of hours spent immobile, whether seated or standing, not the position of your legs while sitting.
At what age do varicose veins usually start?
Varicose veins most often become visible between the ages of 30 and 50, though people with strong family history can develop them in their teens or twenties. The underlying reflux typically begins years before the vein becomes visible, which is why symptoms like leg heaviness often precede any cosmetic change.
Can exercise cause varicose veins?
No. Regular exercise protects against varicose veins by strengthening the calf pump and improving venous return. Very heavy weightlifting with repeated Valsalva straining can transiently spike intra-abdominal pressure, so predisposed lifters should breathe through lifts rather than hold their breath, but exercise itself is protective.
Are varicose veins dangerous or just cosmetic?
Most varicose veins are not dangerous, but they are not purely cosmetic either. They are a visible sign of chronic venous insufficiency, which can progress to swelling, skin damage, superficial thrombophlebitis, and venous ulceration. Evaluation determines which category you fall into.
The Bottom Line
What causes varicose veins is failed vein valves allowing blood to pool in the leg, driving up venous pressure until the vein wall stretches and bulges. Genetics determines how vulnerable your valves are. Age, hormones, pregnancy, excess weight, and hours spent standing or sitting determine how quickly those valves give way.
The most useful thing to take from this is that varicose veins are progressive. The vein you can see today reflects reflux that started years ago, and the underlying chronic venous disease continues whether or not you address it. Compression, movement, and elevation slow that progression. Only treatment closes the failed vein.
If you have a family history, visible leg veins, or persistent aching and swelling, a duplex ultrasound will tell you exactly what your veins are doing. Schedule a consultation with Vein and Knee Pain Vascular Group in New York and find out where you stand before your legs make the decision for you.
