Varicose veins develop when tiny one-way valves inside your leg veins stop closing properly, allowing blood to pool and stretch the vessel walls outward. This affects 10 to 30% of the adult population, and the cause is rarely a single factor. Instead, it’s usually a combination of genetics, hormones, daily habits, and the simple physics of being upright.
How Veins Fail From the Inside
Your leg veins contain small flap-like valves spaced along their length. These valves open to let blood flow upward toward the heart and snap shut to prevent it from sliding back down. When the vein wall loses its structural proteins, specifically the elastic and collagen fibers that keep it firm, the vessel widens. Once it widens enough, the valve flaps can no longer meet in the middle, and blood leaks backward.
That backward flow creates sustained high pressure inside the vein. The pressure stretches the vessel further, which makes even more valves fail downstream. This is why varicose veins tend to get worse over time rather than staying the same size. The bulging, ropy appearance is the vein elongating and twisting under that constant excess pressure.
Genetics Play a Major Role
If your parents have varicose veins, the odds are strongly stacked. When both parents are affected, the risk of developing them reaches roughly 90%. A UK study found that 85% of people with varicose veins reported a positive family history, compared to just 22% of people without them. What you inherit isn’t the varicose veins themselves but the tendency toward weaker vein walls and valves that are more prone to failure.
Family history also influences whether varicose veins stay cosmetic or progress to something more serious. The Edinburgh Vein Study, which tracked people over 13 years, found that a family history of varicose veins nearly doubled the odds of the condition worsening over time.
Hormones and Why Women Are More Affected
Varicose veins are more common in women, and hormones are a central reason. Estrogen interferes with the ability of vein walls to contract. It blocks the calcium-dependent tightening mechanism that keeps veins firm, essentially relaxing the vessel and making it more distensible. Women with varicose veins have roughly twice as many progesterone receptors embedded in their vein walls compared to women without them, and estrogen receptors are elevated throughout every layer of the affected vessel.
This receptor increase appears to drive the actual remodeling of the vein, stimulating the smooth muscle cells and lining cells to proliferate in ways that increase the diameter, thickness, and tortuosity of the vessel. It also explains why varicose veins often first appear or worsen during hormonal shifts: puberty, pregnancy, oral contraceptive use, and menopause. High estrogen levels in menopausal women have been specifically linked to greater venous distensibility and more varicose veins.
What Pregnancy Does to Your Veins
Pregnancy creates a perfect storm for varicose veins. Plasma volume expands by 40 to 50% for a single pregnancy and even more for twins or triplets. To handle that flood of extra blood, the body shifts into a pro-vasodilatory state, meaning veins actively relax and widen to accommodate the increased volume. At the same time, the growing uterus presses on the large veins in the pelvis, partially obstructing the return flow from the legs.
The combination of higher blood volume, hormonally relaxed vein walls, and physical compression from the uterus puts enormous strain on the leg valves. Varicose veins that appear during pregnancy sometimes improve in the months after delivery as blood volume drops and pelvic pressure resolves, but the valve damage may persist and worsen with subsequent pregnancies.
Standing, Sitting, and the Calf Muscle Pump
Your calf muscles act as a built-in pump for venous blood. When you walk or flex your calves, the contraction generates about 140 mmHg of pressure, forcefully pushing blood upward out of the lower leg. When you stand still or sit for long stretches, that pump goes idle. Gravity pulls blood downward, hydrostatic pressure builds in the leg veins, and the valves bear the full load alone.
Occupational research defines prolonged standing as either standing continuously for more than one hour or standing for more than four hours total during a work shift. People who stand for more than two hours a day show increased risk of venous disease, and beyond 12 hours per day in a stationary posture (standing or sitting), the risk climbs by about 22% for each additional hour. Nurses, teachers, retail workers, hairstylists, and anyone in a desk job without movement breaks are particularly vulnerable.
How Excess Weight Adds Pressure
Carrying extra abdominal weight physically compresses the veins that drain the legs. In morbidly obese patients, the pressure inside the abdomen averages about 19 cm of water, compared to roughly 8.5 cm in normal-weight individuals. That elevated pressure transmits directly into the large veins of the upper thighs and pelvis, which measured nearly 20 cm of water versus 7.5 in controls. The downstream effect is venous stasis: blood moves sluggishly through the legs, pressure builds in the smaller veins, and valves that might otherwise function adequately begin to fail under the extra load.
How Varicose Veins Progress
Most varicose veins start as a cosmetic issue: visible, twisted veins that may ache or feel heavy after a long day. But they don’t always stay that way. In the Edinburgh Vein Study, nearly one third of people who initially had only varicose veins developed chronic venous insufficiency over 13 years. That means the sustained high pressure eventually damaged the skin and tissue around the ankle, causing discoloration, eczema-like changes, hardening of the skin, or in the most advanced cases, open ulcers that are slow to heal.
Doctors categorize the severity on a scale from C0 (no visible signs) through C6 (an active ulcer). Simple spider veins are C1. Varicose veins of 3mm or larger are C2. Swelling bumps it to C3, and skin changes like brown discoloration or thickened, leathery skin around the ankle mark C4. A history of deep vein thrombosis quadruples the odds of progression, and being overweight, older, or having a family history all push the risk higher.
The factors most within your control are movement and weight. Regular calf muscle activity, whether from walking, calf raises, or simply shifting position throughout the day, is the most direct way to counteract the gravitational pressure that drives the condition forward. Compression stockings work on the same principle, externally narrowing the vein so the valves can close more effectively. For veins that are already significantly enlarged, the valves are structurally too far apart to recover on their own, and procedural treatment becomes the path to preventing further progression.