Yes, the sacrum is part of the pelvis. It is one of five bones that make up the bony pelvis, sitting at the back of the structure where it forms the posterior pelvic wall. The sacrum is often called the “keystone” of the pelvis because of its central position and critical role in holding the entire structure together.
Where the Sacrum Fits in the Pelvis
The bony pelvis is built from five bones: two hip bones (each formed by the fused ilium, ischium, and pubis), the sacrum, and the coccyx (tailbone). The sacrum is a spade-shaped bone at the base of the spine, formed when the last five vertebrae of the spine fuse together. That fusion process isn’t complete until puberty. The coccyx attaches to the bottom of the sacrum, and healthcare providers sometimes refer to both bones together as the “pelvic spine.”
The sacrum connects to the two hip bones through a pair of sacroiliac (SI) joints, one on each side. These joints are held together by strong ligaments and are largely immobile. That near-total rigidity is the point: the sacrum and SI joints form the back wall of the pelvic ring, and their stability is what keeps the pelvis functioning as a single, weight-bearing unit.
Axial Skeleton, but Functionally Pelvic
Anatomists classify the sacrum as part of the axial skeleton, the group that includes the skull, spine, and ribcage. The hip bones, by contrast, belong to the appendicular skeleton, the group that includes the limbs and the structures that attach them to the body’s core. So the sacrum and the hip bones come from different skeletal divisions, yet they meet at the SI joints to form one continuous pelvic structure. This is why anatomy courses describe the hip bones as being “firmly joined to the axial skeleton via attachment to the sacrum of the vertebral column.”
How the Sacrum Supports Your Body
Every pound of your upper body weight passes through the sacrum on its way to your legs. The weight travels down the spinal column, enters the sacrum, crosses the SI joints into the hip bones, and continues down through the hip sockets into the femurs. The SI joints absorb shock during this transfer, cushioning the forces generated by walking, running, and jumping.
Because of this central role, a fractured sacrum or a disrupted SI joint can destabilize the entire pelvic ring. High-energy injuries like car accidents or falls from height can crack the sacrum vertically, tear the SI joint ligaments, or produce some combination of the two. These posterior pelvic ring injuries are particularly serious because the sacrum also houses nerve roots that control sensation and movement in the legs, bladder, and bowel. Displaced sacral fractures can compress or damage those nerves.
Male and Female Sacral Differences
The sacrum looks slightly different depending on biological sex, mirroring the broader shape differences between male and female pelvises. In females, the sacrum is wider and shorter with less front-to-back curvature. In males, it is longer and more curved. These differences contribute to the wider pelvic opening in the female pelvis, which accommodates childbirth.
Why the Sacrum Evolved This Way
The sacrum exists because land animals needed a rigid connection between their spine and hind limbs. The story goes back hundreds of millions of years. Early lobe-finned fish that walked along the sea floor developed heavy ligaments connecting a primitive pelvis to enlarged rib-like structures on their pelvic vertebrae. By the time Tiktaalik, an alligator-shaped fish, began walking on land in the late Devonian period, some of those pelvic structures had already started fusing together into what researchers consider a primitive sacrum.
Amphibians and reptiles gradually increased the number of fused vertebral segments from one to two, gaining greater stability for walking. Modern mammals, including humans, have fully formed sacra made of several fused vertebrae that attach to the pelvis through the SI joints. In birds, the sacrum is actually fused directly to the pelvis for extra rigidity during flight. The human version, with its strong but slightly mobile SI joints, strikes a balance between the stability needed for upright walking and the flexibility needed to absorb the impact of each step.