Leg and ankle swelling happens when fluid leaks out of small blood vessels and gets trapped in surrounding tissues. This is called edema, and it can result from something as simple as sitting too long or as serious as a failing heart. The underlying cause determines whether the swelling is temporary and harmless or a sign that something needs medical attention.
How Fluid Ends Up Trapped in Your Tissues
Your smallest blood vessels, the capillaries, constantly balance two opposing forces. Blood pressure inside the vessel pushes fluid out into surrounding tissue, while proteins in your blood (mainly albumin, which generates about 70% of this pulling force) draw fluid back in. When either side of this equation shifts, fluid accumulates where it shouldn’t.
Swelling in the legs and ankles specifically comes down to gravity. Fluid naturally settles to the lowest point in your body, so any condition that tips the balance toward fluid leaking out of blood vessels will show up first in your feet, ankles, and lower legs. This is why swelling often worsens throughout the day and improves overnight when your legs are level with your heart.
Vein Problems in the Legs
Chronic venous insufficiency is one of the most common causes of persistent leg swelling. Your leg veins contain one-way valves that push blood upward toward your heart against gravity. When those valves become damaged, blood flows backward and pools in the lower legs, a process called venous reflux. The increased pressure in those veins forces fluid out into surrounding tissue.
Over time, the pressure buildup can become severe enough to burst capillaries. In advanced cases, the chronic swelling causes scar tissue to form in the lower leg, which traps even more fluid and makes the problem self-reinforcing. You might notice skin discoloration, a heavy or aching feeling in your legs, or slow-healing sores near the ankles alongside the swelling.
Heart, Kidney, and Liver Disease
When a major organ isn’t working properly, the effects often show up as swelling in the legs.
- Heart failure: When the heart’s lower chambers can’t pump blood effectively, blood backs up in the veins leading from the legs. This raises pressure inside those vessels and pushes fluid into the tissues of the legs, ankles, and feet. Swelling from heart failure typically affects both legs equally and may be accompanied by shortness of breath or fatigue.
- Kidney disease: Damaged kidneys lose the ability to filter excess fluid and salt from the blood efficiently. The resulting fluid buildup causes swelling in the legs and often around the eyes. In nephrotic syndrome, the kidneys also let protein leak into the urine, which lowers the blood’s ability to pull fluid back into vessels.
- Liver damage: Cirrhosis disrupts the liver’s production of albumin, the protein responsible for keeping fluid inside blood vessels. With less albumin circulating, fluid seeps into tissues more easily. Liver disease commonly causes fluid buildup in both the abdomen and the legs.
These organ-related causes tend to produce gradual, bilateral swelling (both legs equally) that worsens over weeks or months. The swelling is usually “pitting,” meaning if you press your finger into the swollen area, it leaves an indentation that takes time to fill back in.
How Doctors Measure Swelling Severity
Doctors use a simple finger-press test to grade pitting edema on a scale of 1 to 4. Grade 1 leaves a shallow 2-millimeter dent that rebounds immediately. Grade 2 creates a 3 to 4 millimeter pit that fills back in within 15 seconds. Grade 3 produces a 5 to 6 millimeter pit that takes up to a minute to rebound. Grade 4, the most severe, leaves an 8-millimeter depression that can take two to three minutes to fill back in. This grading helps track whether swelling is improving or getting worse over time.
Medications That Cause Swelling
Several common drug classes cause leg and ankle swelling as a side effect. Calcium channel blockers, widely prescribed for high blood pressure, are among the most frequent culprits. These medications relax blood vessel walls, which increases pressure in the capillaries and pushes more fluid into tissues. The swelling is dose-dependent: it occurs in 1 to 15% of people on standard doses but can affect more than 80% of those on long-term high doses.
Other medications linked to leg swelling include certain diabetes drugs, anti-inflammatory painkillers, steroids, and some hormone therapies. If your swelling started shortly after beginning a new medication or increasing a dose, that connection is worth discussing with whoever prescribed it.
Blood Clots: When One Leg Swells
A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, usually in the leg. Unlike most other causes of swelling, DVT typically affects only one leg. Warning signs include swelling accompanied by pain or cramping (often starting in the calf), warmth in the affected area, and skin that turns red or purple. Some DVTs produce no noticeable symptoms at all, which makes them particularly dangerous since the clot can break loose and travel to the lungs.
One-sided leg swelling that comes on suddenly, especially with pain or skin color changes, warrants prompt medical evaluation. Risk factors include recent surgery, prolonged immobility (like a long flight), pregnancy, and certain clotting disorders.
Swelling During Pregnancy
Some degree of leg and ankle swelling is normal during pregnancy. The growing uterus puts pressure on the veins returning blood from the legs, and the body retains more fluid overall. This type of swelling tends to build gradually and worsen in the third trimester.
The concern arises when swelling appears suddenly, particularly in the face and hands. Sudden, severe swelling combined with rapid weight gain can signal preeclampsia, a serious blood pressure condition that develops after 20 weeks of pregnancy. Preeclampsia requires medical management because it can progress to dangerous complications for both mother and baby.
Everyday and Lifestyle Causes
Not all leg swelling points to a medical condition. Prolonged sitting or standing allows gravity to pull fluid downward for hours at a time, especially when leg muscles aren’t contracting to help push blood back up. Long flights, desk jobs, and standing-heavy work shifts are classic triggers.
High sodium intake plays a direct role. Salt causes your body to retain water, increasing the volume of fluid in your blood vessels and raising the pressure that pushes fluid into tissues. For people already prone to swelling, most guidelines recommend keeping sodium intake under 2,000 milligrams per day, roughly the amount in one teaspoon of table salt. The average American consumes well over 3,000 milligrams daily, much of it from processed foods rather than the salt shaker.
Excess body weight adds pressure on the veins in the legs and pelvis, making it harder for blood to return to the heart. Heat also contributes: warm temperatures cause blood vessels to dilate, which increases capillary pressure and encourages fluid to leak into tissues. This is why swelling tends to be worse in summer months.
Patterns That Help Identify the Cause
A few key characteristics narrow down what’s behind the swelling. Swelling in both legs that gradually worsens typically points to a systemic issue: heart, kidney, or liver problems, venous insufficiency, or medication effects. Swelling in just one leg raises concern for a blood clot or a localized vein problem. Swelling that improves overnight and returns during the day suggests a gravity-dependent cause like venous insufficiency or prolonged standing. Swelling that doesn’t improve with elevation, or that comes with shortness of breath, chest pain, or reduced urination, points toward organ-related causes that need evaluation.
Pressing the swollen area gives information too. If it leaves an indentation (pitting edema), the cause is usually related to fluid pressure or protein levels. If the skin feels firm and doesn’t pit, the problem may involve the lymphatic system, which drains fluid through a separate network of vessels.