Water retention in the legs happens when fluid leaks out of tiny blood vessels and accumulates in the surrounding tissue faster than your body can drain it away. The underlying cause ranges from something as simple as sitting too long to serious conditions involving the heart, kidneys, or veins. Whether one leg or both are swollen, and whether the swelling came on suddenly or built up over weeks, tells you a lot about what’s driving it.
How Fluid Normally Stays Balanced
Your capillaries (the smallest blood vessels) constantly filter small amounts of fluid into surrounding tissue and reabsorb most of it back. Two opposing forces keep this in check: blood pressure inside the capillary pushes fluid out, while proteins in the blood pull fluid back in. When either side of this equation shifts, fluid escapes into your tissues and stays there.
Your lymphatic system acts as a secondary drainage network, picking up whatever fluid the capillaries don’t reabsorb and returning it to your bloodstream. If this system gets damaged or overwhelmed, fluid pools in your legs because gravity pulls it to the lowest point in your body.
Vein Problems Are the Most Common Cause
Chronic venous insufficiency is one of the leading reasons legs swell. Your leg veins contain one-way valves that push blood upward toward the heart, working against gravity. These valves become more frequent closer to the ankle precisely because the pressure from gravity is greatest there. When the valves fail, blood flows backward (called reflux), and pressure builds in the lower veins. That increased pressure forces more fluid out of the capillaries and into the tissue around your ankles and calves.
Valve incompetence in the superficial veins accounts for 70% to 80% of chronic venous disease cases. You’ll often notice the swelling worsens throughout the day, especially after standing or sitting for long periods, and improves overnight when your legs are elevated. Over time, the skin around your ankles may darken or become leathery, and in severe cases, ulcers can develop.
Heart and Kidney Problems
When the heart can’t pump blood efficiently, blood backs up in the veins. This raises pressure inside the capillaries and pushes fluid into the surrounding tissue, particularly in the legs and abdomen. At the same time, reduced blood flow to the kidneys triggers a chain reaction: the kidneys sense low blood flow and respond by holding onto more sodium and water in an attempt to increase the volume available to circulate. This extra fluid has nowhere productive to go, so it accumulates as swelling.
Kidney disease on its own causes a similar problem. Damaged kidneys lose the ability to filter excess fluid and sodium from the blood. The result is the same: fluid overload that settles in the legs. Liver disease, particularly cirrhosis, also contributes by reducing the production of blood proteins that normally pull fluid back into your vessels.
Lymphedema: A Different Kind of Swelling
Standard edema involves watery fluid leaking from capillaries. Lymphedema is distinct because it involves protein-rich fluid that the lymphatic system can no longer drain properly. Anything that blocks or damages lymph vessels can cause it: surgery (especially cancer-related procedures that remove lymph nodes), radiation therapy, infection, or, in rarer cases, inherited malformations of the lymphatic system.
Lymph fluid normally moves through the vessels with the help of muscle contractions during everyday movement and small pumps built into the vessel walls. When drainage is blocked, the protein-rich fluid accumulates and triggers inflammation that, over time, causes the tissue to harden. Unlike venous edema, lymphedema doesn’t respond well to elevation alone and tends to worsen progressively without treatment. The swelling often feels firmer, and the skin may thicken in a way that’s sometimes described as resembling an orange peel.
Medications That Cause Leg Swelling
Several common medications cause fluid retention in the legs as a side effect. Calcium channel blockers, a widely prescribed class of blood pressure medication, are among the most frequent culprits. They work by relaxing blood vessel walls, which can increase the pressure inside capillaries and push fluid into the tissue. The swelling is dose-dependent: at low doses, ankle swelling occurs in 1% to 15% of people taking these medications, but at high doses over the long term, the rate can exceed 80%.
Other medications that commonly cause leg swelling include anti-inflammatory painkillers (which cause the kidneys to retain sodium), certain diabetes medications, steroids, and some hormone therapies including estrogen. If you notice new leg swelling after starting a medication, it’s worth flagging with whoever prescribed it, since switching to a different drug in the same class sometimes resolves the problem.
Pregnancy and Hormonal Shifts
Swollen legs during pregnancy, particularly in the third trimester, are extremely common and usually harmless. Hormonal changes during pregnancy cause the body to retain more sodium, which pulls water along with it. On top of that, the growing uterus can compress the large vein (inferior vena cava) that returns blood from the legs to the heart, especially when lying on your back. This obstruction slows blood flow from both legs and increases pressure in the veins below.
Swelling that comes on suddenly in one leg during pregnancy, or swelling accompanied by severe headaches, vision changes, or high blood pressure, signals a different situation that needs prompt medical evaluation.
One Leg vs. Both Legs
Whether the swelling affects one leg or both is one of the most important clues to the cause. Bilateral (both legs) swelling usually points to a systemic issue: heart failure, kidney disease, liver disease, medication side effects, or prolonged sitting. Unilateral (one leg) swelling raises concern for a localized problem, most urgently a deep vein thrombosis (DVT), which is a blood clot in the deep veins of the leg.
Acute swelling in one leg, especially if it’s painful, warm, or red, warrants prompt evaluation. Clinicians use a scoring system based on risk factors, symptoms, and physical findings to determine who needs immediate imaging with ultrasound. Other one-sided causes include a localized infection (cellulitis), a Baker’s cyst behind the knee, or lymphatic damage from prior surgery or radiation on that side.
How Severity Is Measured
Doctors assess leg swelling by pressing a finger into the swollen area for several seconds and watching what happens. If the pressure leaves an indentation (a “pit”), it’s called pitting edema, and it’s graded on a four-point scale:
- Grade 1: A shallow 2 mm pit that rebounds immediately.
- Grade 2: A 3 to 4 mm pit that fills back in within 15 seconds.
- Grade 3: A 5 to 6 mm pit that takes 15 to 60 seconds to rebound.
- Grade 4: An 8 mm pit that takes two to three minutes to fill back in.
Higher grades indicate more fluid in the tissue and typically suggest a more significant underlying cause. Lymphedema, by contrast, often produces non-pitting edema in its later stages because the accumulated protein causes the tissue to become firm rather than spongy.
Lifestyle Factors That Contribute
Not all leg swelling signals a disease. Gravity does most of the work: sitting or standing in one position for hours lets fluid pool in the legs simply because there’s no muscle contraction pushing it back up through the veins. Long flights, desk jobs, and extended car rides are common triggers. High salt intake increases the amount of sodium in your blood, which draws water into your vessels and raises the overall fluid volume your body has to manage.
Excess body weight puts additional pressure on the veins returning blood from the legs, and obesity is also linked to obstructive sleep apnea, which itself contributes to fluid retention. Clinicians now recommend screening for sleep apnea in patients with chronic bilateral leg swelling, since treating the breathing disorder can reduce the edema. Regular movement, even just flexing your calves while seated or taking short walking breaks, activates the muscle pump in your legs and helps push fluid back toward the heart.