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How are the sacrum and coccyx formed?

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Front and back views of the human skeleton are shown side by side, with labeled bones including the skull, spine, ribs, pelvis, arms, legs, hands, and feet. Insets detail the bones of the foot and hand, highlighting major bone groups and anatomical terms. — human skeleton Front and back views of the human skeleton.

The sacrum and coccyx are formed through the fusion of individual vertebrae during human development.

Sacrum: This wedge-shaped bone is derived from the 25th through the 29th vertebrae of the vertebral column. In the early stages of human fetal development, the hip bones align with these segments. As development progresses, the sacroiliac joints form, and these five vertebrae fuse into a single bone, a process typically completed by early adulthood.12
Coccyx: Often referred to as the tailbone, the coccyx represents a vestigial tail and is composed of three to five smaller caudal vertebrae. These vertebrae fuse with each other during early adulthood. In later life, the coccyx may also fuse with the base of the sacrum.3

Both structures undergo ossification—the biological process by which cartilage is transformed into hard bone—as part of the maturation of the human skeleton.24