A uterosacral ligament is a paired ligament of the female pelvis that runs between the uterus and the sacrum, a bone at the base of the vertebral column in the pelvis. Also known as a recto-uterine ligament, it is one of three paired peritoneal ligaments assisting in holding the uterus in place in the pelvic cavity. This ligament arises from the recto-uterine folds of the recto-uterine pouch, a space found just behind the cervix near the bottom of the uterus, and attaches to the front aspect of the sacrum.
A dense band of connective tissue, the uterosacral ligament is largely made up of collagen fibers. It is essentially a continuation of the recto-uterine folds bordering the lower uterus. The recto-uterine folds are made up of tissue from the peritoneum, a membrane encapsulating the abdominal cavity that extends downward into the pelvis, ending where it meets the uterus along its anterior, superior, and posterior borders.
Where the peritoneum dips behind the uterus it forms a tapered cavity known as the recto-uterine pouch or excavation. The recto-uterine folds form the walls of this pouch. From these folds along the base of the inside of this pouch arises the uterosacral ligament.
Originating immediately behind the cervix, which is the opening to the bottom of the uterus from the vagina, the paired uterosacral ligament forms to either side of the back of the uterus. Each ligament extends backward and upward along either side of the pelvis, converging posteriorly to attach to the anterior lower sacrum. The sacrum is a stacked bone — actually five fused bones in adults — that is shaped like a wedge, narrowing as it descends. It forms the posterior wall of the pelvic cavity and is bordered above by the fifth lumbar vertebra and below by the coccyx, or tailbone.
Along with the pubocervical and cardinal ligaments, the uterosacral ligament functions to hold the uterus in place within the pelvic cavity. Both of the other paired ligaments arise from either side of the cervix, with the pubocervical ligaments extending anteriorly to attach to the pubic symphysis at the front of the lower pelvis and the cardinal ligaments stretching laterally to connect to the ischial spine on either side of the pelvis. The uterosacral ligament supports the uterus from behind, of course, by affixing to the sacrum at the base of the vertebral column. This organ is further supported by the pelvic floor muscles, below; the urogenital diaphragm, in front; and the peritoneum, above and behind.