Left lateral decubitus is the starting position of colonoscopy insertion for many examiners. As the scope is advanced throughout the colon, the position can be changed as needed (see the article on How I do colonoscopy).
A recent study published in Gastrointestinal Endoscopy has shown that starting supine was associated with lower cecal intubation times, lower pain score, lower frequency of position change, less need for abdominal compression. Also, the supine starting position improved patients’ acceptance of unsedated colonoscopy.
Read the article by Shengbing Z, et al. Impact of the supine position versus left horizontal position on colonoscopy insertion: a 2-center, randomized controlled trial – here.