The sigmoid colon is the S-shaped final segment of your large intestine, sitting low in your pelvis just before the rectum. It’s roughly 14 to 16 inches (35 to 40 cm) long and serves as the last stop for stool before it moves into the rectum for elimination. Its name comes from the Greek letter sigma, which resembles an “S.” While it’s a short stretch of intestine, the sigmoid colon plays a surprisingly outsized role in digestive health problems.
Where the Sigmoid Colon Sits
The sigmoid colon begins at the front of the pelvic brim, picking up where the descending colon leaves off. It curves in its characteristic S-shape through the pelvis and ends where the rectum begins, at the level of the third sacral vertebra (a bone near the base of your spine). A fan of tissue called the sigmoid mesocolon anchors it to the pelvic wall, giving it some freedom to move but keeping it loosely in place.
This mobility matters. Unlike some parts of the colon that are fixed tightly against the back of the abdominal wall, the sigmoid colon has enough slack to shift position. That flexibility is part of normal anatomy, but in some people it becomes a liability, contributing to a condition called volvulus where the colon twists on itself.
What the Sigmoid Colon Does
By the time digested food reaches the sigmoid colon, most of the water and nutrients have already been absorbed upstream. The sigmoid’s main job is to compact and store stool, absorbing any remaining water and electrolytes while holding waste until your body is ready for a bowel movement. Muscular contractions in the wall of the sigmoid colon push stool downward into the rectum, triggering the urge to go.
These contractions are controlled by nerve networks embedded directly in the colon wall. One network coordinates the movement of the muscular layers, while another regulates fluid secretion and blood flow closer to the inner lining. Together, they allow the sigmoid colon to function semi-independently from the brain, responding to local stretch and chemical signals as stool accumulates.
Structure of the Colon Wall
The sigmoid colon wall has four distinct layers, each with a specific role. The innermost layer, the mucosa, is a single sheet of cells that lines the interior surface and dips down into tiny pits called crypts. These cells absorb water and produce mucus (through specialized goblet cells) that lubricates stool for easier passage. A thin band of muscle at the base of the mucosa helps it move independently of the deeper layers.
Beneath the mucosa sits the submucosa, a layer of connective tissue packed with blood vessels, lymph channels, and nerves. Below that is the muscularis externa, two thick layers of smooth muscle: an inner circular layer and an outer longitudinal layer. The outer layer condenses into three visible bands called taeniae coli that run the length of the colon. Between these two muscle layers lies a nerve network that coordinates the wave-like contractions moving stool along. The outermost layer, the serosa, is a thin protective coating of tissue and fat.
Why the Sigmoid Colon Is Prone to Problems
The sigmoid colon has the smallest diameter of any segment of the large intestine. Basic physics means that a narrower tube generates higher internal pressures when its walls contract. This makes the sigmoid colon the most common site for several conditions.
Diverticular Disease
Diverticula are small pouches that bulge outward through weak spots in the colon wall. They form when high internal pressure pushes the lining through gaps where blood vessels penetrate the muscle layers. Because the sigmoid colon generates the highest pressures in the large intestine, it is the most commonly affected site. Many people develop these pouches without symptoms (a condition called diverticulosis), but when a pouch becomes inflamed or infected, it becomes diverticulitis, causing pain typically felt in the lower left abdomen, fever, and changes in bowel habits.
Sigmoid Volvulus
Volvulus happens when the sigmoid colon twists around the tissue anchoring it to the pelvic wall, cutting off blood flow and trapping stool and gas. For a volvulus to occur, the bowel needs enough slack to rotate around a narrow base of attachment. This is most common in older adults who are less mobile and have a history of chronic constipation, which gradually stretches and elongates the sigmoid colon over time. Patients are often referred from nursing homes or residential care facilities. In parts of Africa, sigmoid volvulus occurs at higher rates, partly because the base of the sigmoid mesocolon tends to be anatomically narrower in some populations, and high-fiber diets increase the bulk of stool moving through the colon.
Colorectal Cancer
The sigmoid colon is one of the more common locations for colorectal cancer to develop. Polyps, which are small growths on the inner lining, can form in this region and, over years, potentially progress to cancer. This is one reason screening is important and why doctors pay close attention to the sigmoid during examinations.
How Doctors Examine the Sigmoid Colon
Two common procedures allow doctors to look directly at the sigmoid colon. A flexible sigmoidoscopy uses a thin, lighted tube inserted through the anus and rectum to examine the lower third of the colon. Despite its name suggesting it only reaches the sigmoid, the scope actually extends a bit further into the descending colon. The procedure is quicker and requires less preparation than a full colonoscopy, but it has a significant limitation: it does not visualize the upper portions of the colon.
A colonoscopy, by contrast, examines the entire large intestine from rectum to cecum. Because colorectal cancers and polyps can develop anywhere along the colon, a colonoscopy is generally considered the more thorough screening tool. Both procedures can also be used to take tissue samples for biopsy or remove polyps on the spot.
CT scans and imaging studies can also reveal problems in the sigmoid colon. Diverticulitis, for instance, is often diagnosed with a CT scan that shows inflammation or abscess formation around the sigmoid region. In emergency situations like a suspected volvulus, X-rays may reveal a characteristic pattern of a massively dilated loop of bowel.