The cervix of the uterus is the only part that is retroperitoneal, meaning it lies behind the peritoneum and is fixed in the pelvis. This anatomical relationship is crucial during hysterectomy and other pelvic surgeries to avoid injury to surrounding structures such as the ureters which run close to the cervix. In contrast, the fundus, body, and isthmus are intraperitoneal and covered by peritoneum. Understanding the retroperitoneal position of the cervix helps surgeons anticipate and manage potential complications.

Reference: Gray's Anatomy, Standring S, 42nd Edition.