The term rectum refers to the lowest 12-15 centimeters of the large intestine. The rectum is located just above the anal canal. Normally, the rectum is securely attached to the pelvis with the help of ligaments and muscles. This attachment firmly holds the rectum in place. Various factors, such as age, long-term constipation, and the stress of childbirth, may cause these ligaments and muscles to weaken, which means that the rectum's attachment to the body also weakens. This causes the rectum to prolapse, meaning it slips or falls out of place. Occasionally, large hemorrhoids (large, swollen veins inside the rectum) may predispose the rectum to prolapse.

Rectal prolapse is similar to, but not the same as, rectocele, which is a common condition in women. A rectocele is a prolapsed rectum that can result when the backside (or posterior) wall of the vagina prolapses.

In the early stages of rectal prolapse, the rectum becomes poorly attached but stays within the body most of the time. This stage of rectal prolapse is called mucosal prolapse, or partial prolapse, meaning that only the inner lining of the rectum (rectal mucosa) protrudes from the anus. This occurs when the connective tissues within the rectal mucosa loosen and stretch, allowing the tissue to protrude through the anus. When long-term hemorrhoidal disease is the cause, the condition usually does not progress to complete prolapse. Determining if the problem is hemorrhoids or rectal prolapse is important. Hemorrhoids occur commonly, but they rarely cause rectal prolapse.

As the rectum becomes more prolapsed, the ligaments and muscles may weaken to the point that a large portion of the rectum protrudes from the body through the anus. This stage is called complete prolapse, or full-thickness rectal prolapse, and is the most commonly recognized stage of the condition. Initially, the rectum may protrude and retract depending on the person's movements and activities. However, if the disease goes untreated, the rectum may protrude more frequently or even permanently.