During a posterior colpotomy, the incision is made in the posterior vaginal fornix, which lies directly anterior to the rectum. This anatomical proximity places the rectum at the highest risk of injury. Unlike the bladder and ureters, which reside anteriorly, the rectum is immediately posterior, making it vulnerable during dissection and entry into the pouch of Douglas. Awareness of this relationship is critical to avoid rectal perforation and associated complications.

Reference: Williams Gynecology, Hoffman et al., 3rd Edition.