Water method with water exchange has been shown to reduce medication requirement and pain experienced during colonoscopy. It increases the success rate of cecal intubation in sedated and unsedated patients undergoing screening colonoscopy. Exchange of water during scope insertion minimizes distension of the colonic lumen and decreases loop formation. Exclusion of air from the colon during insertion by omission of air insufflations and suction removal of residual air prevent elongation of the colon. These maneuvers facilitate colonoscopy insertion in average patients and may enhance the success of difficult colonoscopy.

Female gender and low-body mass index (BMI) is independently associated with incomplete colonoscopy, respectively. The investigators postulate that low-BMI female patients may benefit from using the water method for colonoscopy. In this proposal the investigators test the hypothesis that compared with conventional air insufflations the water method with water exchange significantly enhances the success rate of cecal intubation in low-BMI female patients.

The aim of the study is to compare the outcome of colonoscopy using the water method versus the conventional air method in low-BMI female patients. The primary outcome is cecal intubation success rate. The secondary outcomes include cecal intubation time, maximum pain score during colonoscopy, overall pain score after colonoscopy and adenoma detection rate.

Inclusion Criteria:

• Female patients with BMI < 20

Exclusion Criteria:

• History of colorectal surgery
• Severe colonic stricture or obstructing tumor
• Patients who cannot give informed consent and those who are hemodynamically unstable