Leg and ankle swelling happens when fluid leaks out of small blood vessels and accumulates in surrounding tissue. This is called edema, and it can result from something as simple as sitting too long or as serious as heart or kidney disease. The cause is almost always related to one of four things: increased pressure inside blood vessels, weakened vein valves, low protein levels in the blood, or inflammation that makes vessel walls more permeable.
How Fluid Builds Up in Your Legs
Your body constantly moves fluid between your bloodstream and surrounding tissues. Under normal conditions, pressure inside your blood vessels pushes some fluid out, while proteins in your blood (especially albumin) pull fluid back in. These two forces stay roughly balanced.
Swelling develops when something tips that balance. Higher pressure inside the veins forces more fluid out. Lower protein levels in the blood mean less pulling force to draw fluid back. Damage or inflammation in the vessel walls lets fluid escape more easily. Gravity makes your legs and ankles the first place this excess fluid collects, especially if you’ve been standing or sitting for hours.
Weak Vein Valves
Chronic venous insufficiency is one of the most common reasons for persistent leg swelling. Your leg veins contain one-way valves that push blood upward toward your heart against gravity. When those valves become damaged or weakened, blood flows backward and pools in the lower legs. The resulting pressure buildup forces fluid into surrounding tissue, causing swelling that typically worsens throughout the day and improves overnight when you lie flat.
Risk factors include a history of blood clots, varicose veins, obesity, pregnancy, and prolonged standing. The swelling tends to develop gradually over months or years, often accompanied by skin changes like darkening or thickening around the ankles.
Heart Failure
When the heart can’t pump blood efficiently, pressure builds in the veins leading back to it. That pressure backs up into the legs, forcing fluid into surrounding tissue. This chain reaction is a hallmark of congestive heart failure and usually causes swelling in both legs equally.
The key difference from vein-related swelling is what accompanies it. Heart failure typically causes shortness of breath (particularly when lying down or during exertion), fatigue, and sometimes a rapid or irregular heartbeat. Swelling from heart failure often leaves a visible dent when you press on the skin, a finding called pitting edema. Doctors grade this on a scale from 1 to 4: grade 1 leaves a shallow 2-millimeter pit that rebounds immediately, while grade 4 creates an 8-millimeter pit that takes two to three minutes to fill back in.
Kidney and Liver Disease
Albumin, a protein made by the liver, is responsible for about 75 to 80 percent of the force that pulls fluid back into your bloodstream. When albumin levels drop, fluid has nowhere to go but into tissue.
Two major conditions cause this drop. In kidney disease, particularly nephrotic syndrome, the kidneys leak large amounts of protein into the urine, sometimes more than 3.5 grams in 24 hours. The resulting protein loss depletes albumin faster than the body can replace it, leading to widespread swelling that often shows up first in the legs and around the eyes.
In liver cirrhosis, the liver loses its ability to produce enough albumin because functional liver cells are replaced by scar tissue. Cirrhosis also causes fluid to accumulate in the abdomen (ascites), which complicates the picture further. When both abdominal fluid and leg swelling are present, liver disease is a strong possibility.
Medications That Cause Swelling
Several common medications cause leg and ankle swelling as a side effect. Calcium channel blockers, a class of blood pressure drugs, are among the most frequent culprits. Swelling occurs in 1 to 15 percent of people taking standard doses, but at higher doses taken long-term, the rate can exceed 80 percent. One study found that combining amlodipine (a calcium channel blocker) with another blood pressure medication cut the swelling rate from about 19 percent to under 8 percent.
Other medications known to cause fluid retention include corticosteroids, certain diabetes drugs, hormone therapies containing estrogen, and some antidepressants. If you notice new swelling after starting a medication, your prescriber can often adjust the dose or switch to an alternative.
Too Much Sodium
Sodium tells your kidneys to hold onto water. A high-salt diet can push your body to retain enough extra fluid to cause visible swelling, especially if you already have a condition that makes you prone to it. For people actively dealing with edema, guidelines from Georgetown University’s nephrology division recommend keeping daily sodium intake between roughly 1,375 and 1,800 milligrams, which is well below the average American intake of over 3,400 milligrams. Processed foods, restaurant meals, and canned soups are the biggest sources for most people.
Blood Clots in the Leg
Deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, usually in one leg. Unlike most other causes of leg swelling, DVT almost always affects just one side. The swollen leg often feels warm, painful, or tender, especially in the calf. The skin may turn red or purple.
DVT is dangerous not because of the swelling itself, but because the clot can break free and travel to the lungs, a life-threatening condition called pulmonary embolism. DVT can also occur without noticeable symptoms, which is why sudden one-sided leg swelling warrants prompt medical evaluation. Risk factors include recent surgery, long flights or bed rest, cancer, and use of hormonal birth control.
Swelling During Pregnancy
Some degree of ankle swelling is normal during pregnancy, particularly in the third trimester. The growing uterus puts pressure on pelvic veins, slowing blood return from the legs, and hormonal changes encourage fluid retention.
The concern is when swelling signals preeclampsia, a potentially dangerous pregnancy complication. Normal pregnancy swelling tends to stay in the feet and ankles. Preeclampsia is more likely when swelling appears suddenly in the hands, arms, or face, accompanied by greater-than-expected weight gain from fluid retention. A blood pressure reading of 140/90 or higher, combined with protein in the urine, is the diagnostic marker. Severe preeclampsia brings additional warning signs: persistent headaches, vision changes, upper abdominal pain, and reduced urine output.
When Leg Swelling Is an Emergency
Most causes of leg swelling develop gradually and can be evaluated at a regular appointment. But certain combinations of symptoms require immediate attention. If leg swelling is accompanied by sudden shortness of breath, chest pain, coughing up pink or bloody phlegm, a rapid irregular heartbeat, or a feeling of suffocating that worsens when lying down, these can indicate fluid backing up into the lungs. Cold, clammy skin and wheezing or gasping are additional red flags. This scenario, acute pulmonary edema, is life-threatening and requires emergency medical care.
One-sided leg swelling that comes on quickly with pain, warmth, or skin color changes also warrants urgent evaluation to rule out a blood clot.