Small amount of free fluid in a reproductive aged women on ultrasound is a common and normal finding. Small amount of free fluid in the pelvis is a non specific finding however. It can be from multiple causes. Since ultrasound is best for looking at the uterus and ovaries, we do not get a good look at other structures which can be a cause of small amount of free fluid in the pelvis.
While small amount of free fluid in the pelvis on ultrasound is often normal in reproductive age women, it can be caused by inflammatory and cancerous conditions as well. It is however assumed that in a reproductive age women, a small amount of free fluid is normal. Often, there is no further testing for this finding unless there is something suspicious in the clinical presentation or imaging findings.
Other conditions can cause free fluid like rupture of a cyst. Most often this resolves on its own. I have seen rare cases where a cyst bleeds and requires surgery. This will lead to moderate to large amounts of fluid and blood. Most cysts do not rupture and are likely not responsible for the fluid.
Inflammatory conditions like pelvic inflammatory disease can lead to fluid in the pelvis. There will sometimes be additional findings on ultrasound like dilated fallopian tubes, abscess (pus collection) or other inflammatory findings. Most importantly, this diagnosis will be suspected based on clinical history and findings on the physical exam.
Other inflammatory conditions which lead to small amount of free fluid in the pelvis can include inflammatory bowel disease such as Crohn’s and ulcerative colitis. Appendicitis and pancreatitis will also sometimes cause fluid to accumulate in the pelvis. Often the pelvic ultrasound will not identify the primary inflammatory conditions causing the fluid. Your doctors suspicion and a possible CT will best diagnose these conditions.
Cancers can cause free fluid in the pelvis. Cancers of the uterus, ovaries and Fallopian tubes will often be identified on the ultrasound. Cancers outside the field of view of the ultrasound will not be identified. Sometimes there will be nodules in the fluid or some other suspicious features which will prompt a recommendation for a CT scan. Your doctor may be suspicious of cancer based on your symptoms and order appropriate testing.
Small amount of free fluid in the pelvis on ultrasound is therefore most commonly a normal finding in a reproductive aged women. There are however many other causes ranging from benign self limited causes such as a ruptured cyst all the way to inflammatory conditions and cancer. Given the focus of ultrasound on the uterus and ovaries, it is important to consider any other causes of free fluid based on the clinical history and physical exam findings.