Sigmoid colon

What it means

The sigmoid colon is the last curving section of the large bowel before it reaches the rectum. It sits low in the left side of the belly and forms an S-shaped loop, which is where its name comes from: sigma is the Greek letter shaped like an S. By the time stool reaches this point, most of the water has been absorbed, so the sigmoid colon mainly holds the now-formed waste until your body is ready to pass it.

Why it appears on a CT, MRI or X-ray report

Radiologists frequently name the sigmoid colon because it is a common site for several everyday findings. Reports may describe its wall thickness, how much stool or gas it holds, and the presence of small pouches in the wall called diverticula. They may note whether the surrounding fat looks clean or inflamed. Because this segment is mobile and S-shaped, it is also a region radiologists describe carefully when assessing the lower left belly.

What it usually means

In most reports the sigmoid colon is mentioned simply to say where a finding sits in the lower left belly. A very common observation here is diverticulosis, small out-pouchings of the bowel wall that are extremely frequent with age and usually cause no symptoms at all. Reports may also note stool loading or gas, which are normal. If the wall is described as thickened or the surrounding fat as inflamed, the radiologist may raise diverticulitis (inflammation of those pouches), which your doctor would interpret alongside your symptoms. Naming this segment is descriptive on its own. What it means for you depends on the full description, how you feel, and the reason the scan was done.

When to follow up

The name alone needs no action. Act on what is described. If your report notes wall thickening, inflammation, a narrowing, or recommends a colonoscopy or follow-up scan, discuss this with your doctor. Symptoms that deserve prompt attention include persistent lower left belly pain, fever with belly pain, a marked change in bowel habit, or blood in the stool. These should be reviewed by a clinician rather than left to wait.

A plain-language way to picture it

Imagine the large bowel as a winding road that ends at a holding bay before the exit. The sigmoid colon is the final S-bend in that road, tucked into the lower left corner of your belly. Like a layby just before a junction, it is where formed waste waits briefly before moving on to the rectum and out. The S-shape is exactly what gives it its name.