Fluid builds up in the abdomen when the body can no longer keep liquid inside blood vessels or drain it properly through the lymphatic system. The medical term for this is ascites, and in the United States, about 80% of cases are caused by liver disease, specifically cirrhosis. The remaining cases stem from cancer, heart failure, kidney disease, or a handful of less common conditions. Understanding the cause matters because treatment depends entirely on what’s driving the fluid accumulation.
Liver Disease Is the Leading Cause
Cirrhosis, the advanced scarring of liver tissue, is responsible for roughly 8 out of every 10 cases. When the liver becomes heavily scarred, blood can no longer flow through it easily. This creates a backup of pressure in the portal vein, the large vessel that carries blood from the intestines to the liver. That elevated pressure, called portal hypertension, forces fluid out of blood vessels and into the open space of the abdominal cavity.
At the same time, a damaged liver produces less albumin, a protein that acts like a sponge to hold fluid inside your bloodstream. With less albumin circulating, fluid leaks more readily into surrounding tissues. The kidneys compound the problem by retaining extra sodium and water, as they respond to misleading signals that the body is dehydrated, even though fluid is actually pooling in the wrong place. When ascites develops in someone with cirrhosis, it signals a shift from stable, “compensated” liver disease to a more serious, decompensated phase.
Cancer-Related Fluid Buildup
About 10% of ascites cases are caused by cancer. Ovarian, colorectal, pancreatic, breast, lung, and stomach cancers are the most common culprits. In roughly 20% of cancer-related cases, the original tumor is never identified.
Cancer causes fluid buildup through a different mechanism than liver disease. Tumors that spread to the lining of the abdomen (the peritoneum) release signaling molecules that make blood vessel walls more permeable, essentially leakier. At the same time, tumor deposits can physically block lymphatic channels, the tiny drainage vessels that normally absorb excess fluid from the abdomen and return it to the bloodstream. The combination of leakier vessels and blocked drainage creates rapid fluid accumulation that often returns quickly even after it’s removed.
Heart Failure, Kidney Disease, and Other Causes
Heart failure accounts for about 3% of ascites cases. When the right side of the heart can’t pump effectively, blood backs up into the veins leading to the liver, raising pressure in the same portal system affected by cirrhosis. The result is similar: fluid gets pushed out of congested vessels and into the abdomen.
Kidney disease, particularly nephrotic syndrome, contributes about 1% of cases. Damaged kidneys lose large amounts of albumin into the urine, dropping blood protein levels and reducing the force that keeps fluid in vessels. In adults with nephrotic syndrome who develop ascites, there’s often an underlying liver problem or heart failure contributing alongside the kidney damage.
Tuberculosis causes about 2% of cases worldwide, while pancreatic disease, blood clots in the portal vein, and severe malnutrition make up the remaining small percentage.
How Fluid Buildup Feels
Small amounts of fluid may go unnoticed. As fluid increases, most people notice their belly growing larger, clothes fitting tighter around the waist, and a feeling of fullness or bloating that doesn’t go away after eating. Some people gain weight rapidly, sometimes several pounds in just days or weeks, without any change in diet.
Larger volumes create more obvious problems. Pressure on the stomach can kill your appetite or cause nausea after small meals. Pressure on the diaphragm makes it harder to take a deep breath, especially when lying flat. Some people develop swelling in the ankles and legs as the same fluid retention affects the lower body. The abdomen may feel tight and heavy, and rolling over in bed or bending forward becomes uncomfortable.
How Doctors Confirm It
Doctors check for abdominal fluid during a physical exam using a few specific techniques. Flank dullness involves tapping across the abdomen and listening for the sound to change from hollow (over the intestines, which float) to dull (over fluid collecting at the sides). With shifting dullness, you roll onto your side and the doctor taps again to see if the boundary between hollow and dull shifts as the fluid moves with gravity. Visible bulging of the flanks while lying on your back suggests a large volume. No single test is perfectly reliable on its own, so doctors typically combine at least two of these maneuvers.
An ultrasound can detect even small amounts of fluid and is the most common imaging tool used. Once fluid is confirmed, doctors often insert a needle to withdraw a sample for analysis. One of the most useful lab values from this fluid is the serum-ascites albumin gradient, or SAAG. This compares the protein level in your blood to the protein level in the fluid. A gradient of 1.1 g/dL or higher points strongly toward portal hypertension (liver disease or heart failure) as the cause. A gradient below that threshold suggests cancer, infection, or inflammation is more likely responsible.
How It’s Managed
Treatment targets the underlying cause, but most people also need strategies to reduce the fluid itself. For liver-related ascites, the first step is restricting salt intake, generally to no more than about 5 grams of salt per day, roughly one teaspoon of table salt. Sodium pulls water into the abdomen, so cutting back slows fluid accumulation. This alone can control mild cases.
When salt restriction isn’t enough, doctors add water pills (diuretics) that help the kidneys flush sodium and water. The standard approach pairs two types: one that blocks a hormone driving sodium retention, and another that acts on a different part of the kidney. These are typically used in a specific ratio to keep potassium levels balanced, since one type tends to raise potassium while the other lowers it. Doses start low and are adjusted gradually based on how much weight you’re losing, ideally no more than about a pound a day.
For large or uncomfortable fluid collections, a procedure called paracentesis drains the fluid directly through a needle inserted into the abdomen. It provides fast relief, sometimes removing 5 to 10 liters or more in a single session. When more than 5 liters are removed, you’ll typically receive an albumin infusion through an IV to prevent a drop in blood pressure and protect kidney function. The fluid often reaccumulates, so paracentesis may need to be repeated every few weeks in some cases.
Infection Risk
One of the most dangerous complications of ascites is spontaneous bacterial peritonitis, an infection of the abdominal fluid that develops without any obvious source like a ruptured organ. Bacteria from the gut can cross into the fluid, and because ascitic fluid has low levels of infection-fighting proteins, it’s a hospitable environment for bacterial growth.
Symptoms include fever, abdominal pain or tenderness, confusion, or a general feeling of getting worse without a clear reason. The infection is diagnosed by testing a fluid sample for white blood cells. A count above 250 immune cells per cubic millimeter is the standard threshold that confirms the diagnosis, even before bacteria are identified in a culture. This infection requires prompt treatment because it can deteriorate quickly, and people who’ve had one episode are at high risk for recurrence.