A sigmoidoscopy is a way to view the lower 20 inches of a patient’s sigmoid colon and rectum. The sigmoid colon and rectum are part of the large intestine. The large intestine helps the body process waste. The first 5 to 6 feet of the large intestine is the colon. The last 6 inches of the large intestine is the rectum, ending at the anus.
How a sigmoidoscopy works
A sigmoidoscopy is done with a device called a sigmoidoscope. A sigmoidoscope is a pencil thin, flexible tube with a light and a camera on the end. During a sigmoidoscopy, the doctor inserts the sigmoidoscope into the anus, through the rectum, and into the lower part of the colon. The doctor views the inside of the colon through the scope or on a video monitor.
Doctors often use this exam to screen for colorectal cancer  or intestinal polyps. Polyps are small abnormal growths that may become cancerous. They also use it as a diagnostic test for patients who have rectal bleeding, a change in bowel habits, or other symptoms.
Who does my sigmoidoscopy?
A gastroenterologist performs most sigmoidoscopies with the help of a nurse in a doctor's office or at a hospital. This type of doctor specializes in the gastrointestinal tract, including the stomach and intestines. A surgeon, primary care or family practice doctor, nurse practitioner, or physician assistant may also perform the procedure.
Getting ready for a sigmoidoscopy
When you schedule the exam, your doctor will give you detailed instructions on how to prepare. Tell him or her about all medications you are taking. Also, be sure to ask whether you should take them on the day of the test. You may need to stop taking medications that increase your risk of bleeding for a few days before the sigmoidoscopy. These medications include aspirin or blood thinners. Also discuss any drug allergies or other health conditions you may have.
Your lower colon must be empty for the doctor to examine it. So, you will need to use an enema or very strong laxatives the night before or the day of the sigmoidoscopy. You may also need to follow a liquid diet for up to 24 hours before the exam. This means eating only foods such as fat-free bouillon or broth, black coffee, strained fruit juice, or gelatin.
In addition, you will be asked to sign a consent form stating you understand the benefits and risks of the exam and agree to the test. Talk with your doctor about any concerns you have about the sigmoidoscopy.
Finally, check your insurance provider plan to see if it covers the sigmoidoscopy or if you will have additional out-of-pocket costs.
During the procedure
When you arrive for your sigmoidoscopy, you will need to remove your clothing and change into a hospital gown. The doctor will ask you to lie on your side on an exam table and will drape a sheet over your body. Then the doctor will gently insert the lubricated scope into your rectum. He or she will use the scope to blow air into your colon. This inflates it and makes it easier to examine. You may feel bloated or like you need to go to the bathroom.
Generally, a sigmoidoscopy is uncomfortable but not painful. The doctor will probably perform it without anesthesia. However, the doctor may give you sedatives if you are anxious about the procedure. If you start to feel pain at any time, tell the doctor so he or she can reposition the scope.
A sigmoidoscopy usually takes about 15 to 20 minutes to complete.
If the doctor finds a polyp in your colon during the exam, he or she will remove a piece of tissue or the entire polyp. The doctor does this using a tool attached to the end of the sigmoidoscope. He or she will send a sample of the tissue to a laboratory where a pathologist will examine it under a microscope to determine whether it is cancerous. Finding polyps in the lower part of the colon means there is a higher chance of having polyps elsewhere in the colon. So your doctor will likely recommend a colonoscopy to evaluate the remaining 4 to 5 feet of the colon that is not examined during a sigmoidoscopy. A colonoscopy  allows the doctor to look at the entire colon.
After the procedure
You can expect to return to your normal activities immediately after the sigmoidoscopy, including driving. However, if your doctor plans to give you a sedative, you will need to arrange for a friend or family member to drive you home. You will be groggy from the medication after the procedure.
Initially, you may feel cramps or bloating, but that discomfort usually passes quickly. People often expel the gas that was put into the colon during the procedure. And they may have some diarrhea while releasing the gas. You also may have a small amount of bleeding from your rectum, which is normal. If you have abdominal pain, major bleeding, dizziness, weakness, or a fever, you should call your doctor.
Questions to ask your doctor
Before a sigmoidoscopy, consider asking your doctor the following questions:
Who will perform the sigmoidoscopy? Will anyone else be in the room during the procedure?
How do I need to prepare for the procedure? Are there any restrictions on what I may eat or drink the day before?
What will happen during the sigmoidoscopy?
How long will the procedure take?
Will it be painful?
What are the benefits and risks of having a sigmoidoscopy?
Will I be given sedatives or anesthesia?
Will I need to avoid any activities after the sigmoidoscopy?
Will I need to have someone drive me home from the procedure?
When will I learn the results?
How will the results be communicated to me?
Who will explain the results to me?
Will I need any additional tests?
Cancer Screening 
National Institute of Diabetes and Digestive and Kidney Diseases: Flexible Sigmoidoscopy 
National Cancer Institute: Colorectal Cancer Screening