Leg swelling happens when excess fluid builds up in the tissues outside your blood vessels and cells. This can result from something as simple as sitting too long or eating a salty meal, or it can signal a serious problem with your heart, kidneys, or veins. Understanding the pattern of swelling, whether it’s in one leg or both, and how quickly it appeared helps narrow down the cause.
How Fluid Leaks Into Your Tissues
Your capillaries, the smallest blood vessels, constantly balance pressure inside and outside their walls. Fluid seeps out to nourish surrounding tissues, then gets pulled back in or drained away by the lymphatic system. Swelling occurs when that balance tips in one of several ways: pressure inside the blood vessels climbs too high, the vessel walls become too leaky, your blood loses the proteins that pull fluid back in, or your lymphatic drainage slows down. Most causes of leg swelling trace back to one or more of these disruptions.
Venous Insufficiency
The most common vascular cause of chronic leg swelling is venous insufficiency, which affects roughly 1 in 20 adults. Your leg veins contain one-way valves that push blood upward toward your heart against gravity. When those valves weaken or become damaged, blood flows backward and pools in the lower legs. The resulting pressure forces fluid out of the veins and into surrounding tissue.
Venous insufficiency tends to develop gradually. You might first notice swelling around your ankles that improves overnight and worsens through the day, especially after standing. Over time, the skin can darken, become leathery, or develop open sores near the ankles. Compression stockings, leg elevation, and regular movement all help counteract the gravitational pooling. In more advanced cases, procedures to close or remove damaged veins may be recommended.
Heart, Kidney, and Liver Problems
When your heart can’t pump blood efficiently, pressure builds in the veins leading back from your legs. That elevated pressure pushes fluid into your tissues the same way a backed-up garden hose leaks at its joints. Heart failure typically causes swelling in both legs, often accompanied by shortness of breath, fatigue, or a need to prop yourself up on pillows to sleep comfortably.
Your kidneys play a central role in managing how much sodium and water your body holds onto. When kidney function declines, the kidneys struggle to excrete sodium effectively, so fluid accumulates throughout the body. The legs, being the lowest point when you’re upright, collect much of that excess. Liver disease works through a different mechanism: a damaged liver produces fewer blood proteins, and those proteins are what normally pull fluid back into your bloodstream. Without enough of them, fluid drifts into the tissues and stays there.
Swelling from these organ-level problems is almost always bilateral, meaning it shows up in both legs roughly equally. It also tends to worsen over weeks or months rather than appearing overnight.
One Leg vs. Both Legs
The pattern of swelling carries important diagnostic information. Sudden swelling in a single leg is a red flag for deep vein thrombosis (DVT), a blood clot in one of the deep veins. A DVT can be dangerous because pieces of the clot can break off and travel to the lungs. If one leg swells rapidly, especially with warmth, redness, or pain in the calf, that warrants urgent medical evaluation.
Chronic swelling in both legs points toward systemic causes: vein disease, heart failure, kidney problems, or liver issues. Swelling that’s limited to one leg and develops slowly is more likely related to a localized vein problem, a prior injury, or lymphatic damage on that side.
Lymphedema
Your lymphatic system acts as a secondary drainage network, collecting fluid that capillaries don’t reabsorb and routing it back into your bloodstream. When lymph vessels or nodes are damaged or blocked, fluid accumulates in the affected area. This is lymphedema, and it differs from venous swelling in some distinctive ways. The swelling tends to feel firmer, and in advanced cases you can’t pinch the skin on top of the toes, a finding clinicians call Stemmer’s sign.
Lymphedema can be inherited or develop after surgery, radiation therapy, infection, or trauma to lymph nodes. In severe cases of venous insufficiency, the chronic fluid overload can actually overwhelm the lymphatic system too, creating a combined condition called phlebolymphedema. Treatment centers on specialized massage techniques, compression garments, and exercises that encourage lymph flow.
Medications That Cause Swelling
Several common medications can trigger leg swelling as a side effect. Calcium channel blockers, a widely prescribed class of blood pressure drugs, are among the most frequent culprits. These medications relax blood vessel walls, which can increase the pressure that pushes fluid into tissues. At standard doses, ankle swelling occurs in 1 to 15 percent of people taking them. At higher doses used long-term, that figure can exceed 80 percent.
Other medications known to cause fluid retention include anti-inflammatory painkillers (NSAIDs), certain diabetes drugs, steroids, and some hormone therapies. If your swelling started or worsened shortly after beginning a new medication, that timing is worth mentioning to your prescriber. Often a dose adjustment or switch to a different drug resolves the problem.
Salt, Inactivity, and Other Everyday Causes
Your body works hard to keep sodium concentrations stable. When you eat a lot of salt, your body holds onto extra water to dilute it. A consistently high salt intake can cause the body to retain roughly 1.5 liters of extra fluid, and gravity pulls much of that into the legs and ankles. Reducing sodium intake allows your kidneys to gradually release that stored fluid.
Prolonged sitting or standing also contributes. Without the muscle contractions of walking, your calf muscles can’t pump blood back up toward your heart efficiently. Long flights, desk jobs, and extended bed rest all create conditions for fluid to settle in the lower legs. Heat makes it worse because warm temperatures dilate blood vessels, increasing the amount of fluid that escapes into tissues.
Carrying excess weight puts additional pressure on leg veins and slows venous return. Pregnancy produces swelling through a combination of increased blood volume, hormonal changes that relax blood vessel walls, and the growing uterus compressing veins in the pelvis.
Swelling During Pregnancy: When to Worry
Mild ankle swelling in the second and third trimesters is extremely common and usually harmless. What isn’t normal is sudden swelling of the face or hands, rapid weight gain, or swelling that appears alongside headaches, vision changes, upper abdominal pain, or nausea in the second half of pregnancy. These can be signs of preeclampsia, a serious blood pressure condition that develops in some pregnancies. A blood pressure reading of 160/110 or higher with these symptoms indicates severe disease. Preeclampsia can develop quietly without obvious warning signs, which is one reason prenatal visits include regular blood pressure checks.
How Pitting Edema Is Graded
One way to assess the severity of swelling is the pitting test: pressing a finger firmly into the swollen area for several seconds, then releasing. If the pressure leaves a visible dent, that’s pitting edema. Clinicians grade it on a 1 to 4 scale based on how deep the dent is and how long it takes to bounce back.
- Grade 1: A shallow indent of 2 mm or less that disappears within a few seconds.
- Grade 2: A 2 to 4 mm indent that fills back in within 10 to 15 seconds.
- Grade 3: A noticeably deep indent of 4 to 6 mm that can last over a minute.
- Grade 4: A very deep indent of 6 to 8 mm that persists for 2 to 5 minutes.
Not all swelling pits. Lymphedema in its later stages often feels firm and doesn’t leave a dent, which can help distinguish it from fluid buildup caused by vein or organ problems. If you’re noticing persistent pitting at grade 2 or above, that typically warrants a medical workup to identify the underlying cause.