The main markers of the true pelvis are the sacral promontory, the sciatic spine, and the pubic arch, as follows. 1. Sacral promontory: at the uppermost edge of the sacrum, the 1st sacral vertebrae protrudes forward to form a mark of the pelvic inlet plane, which is an important stronghold for measurement within the pelvis. 2. Sciatic Spine: It is the protruding part of the midpoint of the posterior border of the sciatic bone, located in the middle of the true pelvis. This plane is the narrowest part of the pelvic cavity. 3. Pubic arch: the anterior part of the two descending branches of the pubic bone is connected to form the pubic arch, and the angle of the pubic arch of the female pelvis is >90°, which is the sign of the exit plane of the pelvic cavity. The true pelvis, also known as the small pelvis, is located under the pelvic boundary, is the channel for the delivery of the fetus, it has two upper and lower mouth, that is, the pelvic inlet and outlet, between which is the pelvic cavity.