The short answer
Ascites is a buildup of fluid between the lining of the abdomen and the organs inside it. Liver disease is the most common cause. Several cancers can cause it too. A scan can measure the fluid but cannot say what is in it, which is why a needle sample often follows.
Ascites describes fluid, not a diagnosis, and a scan cannot say what the fluid contains.
MedlinePlus names liver damage as the usual cause, through high pressure in the liver veins and low blood albumin.
Heart failure, pancreatitis, kidney dialysis, clots in the liver veins and several abdominal cancers are also on the list.
A paracentesis draws off fluid to test it, and can also relieve pressure.
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The full explanation.
Fluid in a space that should be nearly dry
Ascites is a buildup of fluid between the lining of the abdomen and the organs inside it. That is MedlinePlus's definition, and it is the whole of the word.
A scan can see the fluid and estimate how much there is. It cannot see what the fluid is made of. That gap is why the word sits in a report without an explanation attached.
The liver explanation that leads the list
MedlinePlus names the usual mechanism first. High pressure in certain veins of the liver, called portal hypertension, plus low blood levels of a protein called albumin.
Anything that badly damages the liver can set this off. MedlinePlus lists long-term hepatitis B or C, years of heavy alcohol use, fatty liver disease, and cirrhosis from inherited conditions.
The other causes that are not cancer
The list is longer than most people expect. MedlinePlus also names:
- clots in the veins of the liver.
- congestive heart failure.
- pancreatitis.
- thickening and scarring of the sac around the heart.
- kidney dialysis.
None of these is a small diagnosis. But none of them is cancer, and together they account for a great deal of the ascites that gets reported.
When cancer is the question being asked
MedlinePlus says people with certain cancers in the abdomen may develop ascites. It names cancer of the appendix, colon, ovaries, uterus, pancreas, bile ducts and liver.
NCI's own definition adds the detail that matters here. In late-stage cancer, tumor cells may be found in the fluid in the abdomen.
May be found. The fluid has to be looked at before anyone knows.
Why a needle answers what the scan cannot
MedlinePlus describes the test plainly. A thin needle draws off some fluid from the belly. This is an abdominal tap, or paracentesis. NCI defines it the same way.
The fluid is then tested for two things: what caused it, and whether it is infected. Sometimes a large amount is removed at the same time to make you more comfortable.
Blood and urine tests usually run alongside it, to check how the liver and kidneys are working.
How the amount connects to symptoms
MedlinePlus sets this out clearly. A small amount of fluid may cause no symptoms at all. As more collects, pain and bloating follow. Large amounts can cause shortness of breath, because the fluid pushes up on the diaphragm and compresses the lower lungs.
So the volume on your report is not a measure of danger. It is a rough measure of how much you are likely to feel.
Signs that mean contacting your provider right away
Call 911 or get emergency care now if you are vomiting blood, passing black tarry stools, cannot be woken properly, or are struggling to breathe.
For the rest, MedlinePlus gives a specific list for people who already have ascites. Contact your provider right away for:
- a fever above 100.5 degrees Fahrenheit — 38 °C is 100.4 °F; act at 100.4 °F —, or one that will not go away.
- belly pain.
- blood in your stool, or black tarry stools.
- blood in your vomit, or easy bruising and bleeding.
- breathing problems, or new swelling in the legs or ankles.
- confusion, or trouble staying awake.
- yellowing of the skin or the whites of the eyes.
Infection of the fluid is the reason this list exists. MedlinePlus calls it a life-threatening complication, which is why fever and belly pain sit at the top.
Imaging Tests for Cancer explains what each scan type can show, and what to ask when the report is discussed.
Sources
Words to know
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Common questions
Does ascites mean cancer has spread?
Not on its own. MedlinePlus lists severe liver damage as the usual cause, along with heart failure, pancreatitis, clots in the liver veins and dialysis. Cancers of the appendix, colon, ovaries, uterus, pancreas, bile ducts and liver can also cause it. Testing the fluid is how they are told apart.
What is a paracentesis?
NCI describes it as putting a thin needle or tube into the abdomen to remove fluid from the space around the intestines, stomach and liver. MedlinePlus says the fluid is then tested to find the cause and to check for infection. A larger volume may be drawn off to make you more comfortable.
Why does a small amount of fluid cause no symptoms?
MedlinePlus says you may have none if there is only a little fluid. As more collects you may feel pain and bloating. Large amounts can cause breathlessness, because the fluid pushes up on the diaphragm and squeezes the lower lungs.
What do trace, small, moderate and large mean?
They are the radiologist's estimate of volume, not a risk score. The more useful question is whether the amount has changed since your last scan, and what the fluid looks like next to the rest of the report.
Questions to ask your doctor
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Sources last checked: 2026-08-09 what this meansLast updated: 2026-08-20Next planned review: 2027-08-09
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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