Free fluid in the pelvis seen by chance on an ultrasound

What the report words mean

Free fluid. Liquid lying loose inside the belly lining, not inside an organ or a cyst. The lining is normally kept moist by a small amount of fluid. (MedlinePlus peritoneal disorders)
Pouch of Douglas or cul-de-sac. The lowest space in the pelvis, behind the womb in women. Fluid tends to collect here because it is the lowest point. In men on CT, small fluid with no cause was found deep in the pelvis. (Hall 1979)(Tadayoni 2019)(Yu 2010)
Small, trace or minimal. A few millilitres. In men, small fluid with no cause averaged about 2.3 mL on CT and 2.5 mL on MRI, and almost all was under 10 mL. (Yu 2010)(Fox 2014)
Simple or anechoic fluid. Clear fluid with nothing floating in it. On CT, small fluid with no cause looked like simple fluid, not blood. (Yu 2010)
Isolated. Fluid on its own, with no injury, swelling or other finding on the scan that explains it. (Yu 2010)(Fox 2014)

How common it is by age and sex

In women of childbearing age, small amounts of fluid were seen in many women at ovulation, and fluid is commonly seen on scans just after ovulation. (Hall 1979)(Pupols 1984) The NHS says ovulation happens about 14 days before a period. (NHS ovulation pain) In men it is less common and falls with age. (Fox 2014)

Small free fluid in the pelvis with no cause in several studies
Group and scan	Share with fluid
Males 5 to 29, pelvic MRI for back or leg pain (Fox 2014)	44%
Men 30 to 49, same study	10%
Men over 50, same study	6%
Men after blunt injury with no cause found, CT (Yu 2010)	About 5%
Women 50 and over, or 25 and over with a family history, ovarian screening ultrasound (Gorski 2022)	4.1% at some visit

Each study used a different scan and a different group, so the numbers cannot be compared directly. MRI picks up smaller amounts than ultrasound. (Tadayoni 2019) In an ultrasound study of 10 healthy young men, 4 had small amounts of fluid. (Brown 2012) In healthy children aged 8 to 13 on MRI, fluid was seen in 91% of girls and 67% of boys. (Tadayoni 2019)

When it matters

Small fluid with no cause is often normal. Studies in healthy men conclude that small amounts do not by themselves point to a disease. (Brown 2012)(Fox 2014) In 1,000 men scanned after an injury, none with small isolated fluid had a missed bowel injury. (Yu 2010)
Fluid that goes away. In a screening programme for women with no symptoms, fluid in women with normal scans was seen on one visit only in 57%, lasted beyond three visits in 9%, and cleared in 77%. Only 17.5% of these amounts were over 10 mL. (Gorski 2022)
When another test is suggested. Fluid was more common in women later found to have a benign or cancerous ovarian tumour than in women with normal scans. The authors say there are no clear guidelines for small fluid found by chance, and suggest a repeat ultrasound until it clears or the ovary looks suspicious. (Gorski 2022) A larger amount, fluid that is not clear, fluid higher in the belly, or fluid with another finding on the scan is read differently. (Yu 2010) Your doctor decides.
Seeing a GP. The NHS says to see a GP for pelvic pain or bloating that comes and goes or does not go away, losing weight without trying, feeling full quickly or having no appetite, or constipation or diarrhoea that does not go away. It says to call 999 or go to A&E for severe pelvic pain that is getting worse or hurts when you move, feeling faint or passing out, pain in the tip of the shoulder, difficulty breathing or heavy bleeding from the vagina. (NHS pelvic pain)
Worry is real. See after an abnormal screening result.

How this differs from screening

A scan done for something else is not a screening test for any cancer. For what the evidence says about checking the ovaries with ultrasound in women without symptoms, see ovarian cancer screening.

Other things a scan may show

The same scan may mention other findings, each with its own page: an ovarian cyst, fibroids, Nabothian cysts and pelvic vein stones. Fluid in other places has its own pages: fluid around the lung and fluid around the heart.

Who it applies to

Adults told that an ultrasound, CT or MRI, done for something else, shows a small amount of free fluid in the pelvis. People who are pregnant, people scanned after an injury, people with belly pain, fever or a known liver, kidney, heart or cancer condition, and a scan done because of symptoms are outside this page.

Routine age-based screening

A finding on a scan does not replace the routine screenings for your age. See screenings by age and gender.

Where they differ:

Normal or a sign to look further. Studies in healthy men and in women around ovulation treat small fluid as often normal. (Brown 2012)(Fox 2014)(Hall 1979) An ovarian screening study says fluid should not be disregarded in women with no symptoms and suggests repeat scans. (Gorski 2022)
After an injury. In men after blunt injury, small isolated fluid on CT was not a sign of a missed injury. (Yu 2010) In women of reproductive age after blunt injury, fluid in the pelvis on ultrasound was linked to more injuries and should not be assumed normal. (Ormsby 2005)
Ovulation as the cause. Cycle studies link fluid to ovulation. (Hall 1979)(Pupols 1984) The ovarian screening study found fluid about as rarely before menopause as after it in women with normal scans, and thought ovulation was an unlikely cause in its group. (Gorski 2022)

Your doctor decides what your result means for you.

Questions people ask

What does free fluid in the pelvis mean?

It means a scan shows some liquid lying loose in the lowest part of the belly, outside the bowel, bladder and womb. A thin layer of fluid normally coats the lining of the belly, and a small amount often collects low in the pelvis. (MedlinePlus peritoneal disorders) (Yu 2010)

Is a small amount of pelvic fluid normal in women?

Often, yes. Studies of the normal menstrual cycle found small amounts of fluid in many women around ovulation, and it is commonly seen just after ovulation. (Hall 1979) (Pupols 1984)

Can men have free fluid in the pelvis?

Yes. Small amounts were seen in 4 of 10 healthy young men on ultrasound, and on MRI in 44% of males aged 5 to 29, falling to 6% over 50. (Brown 2012) (Fox 2014)

Can pelvic fluid be a sign of ovarian cancer?

In one large screening programme, pelvic fluid was more common in women later found to have an ovarian tumour, benign or cancer, than in women with normal scans. Most fluid in women with normal scans cleared. Your doctor decides what your result means for you. (Gorski 2022)

When should I see a doctor?

The NHS says to see a GP for pelvic pain or bloating that comes and goes or does not go away, weight loss without trying, feeling full quickly, or bowel changes that do not go away. (NHS pelvic pain)