Bony pelvis

The pelvic ring is formed anteriorly by the pubic symphysis and posteriorly by the sacroiliac joints. The pelvic brim (linea terminalis: sacral promontory, arcuate lines of the ilia, pecten pubis, pubic crest, upper border of the symphysis) divides the greater (false) pelvis above — part of the abdominal cavity, supporting the intestines — from the lesser (true) pelvis below, which contains the bladder, rectum and internal genital organs and forms the birth canal. The pelvic inlet is the plane of the brim; the pelvic outlet is bounded by the coccyx, sacrotuberous ligaments, ischial tuberosities, ischiopubic rami and the inferior pubic ligament. In the anatomical position the pelvis is tilted so that the ASIS and pubic tubercles lie in the same vertical plane.

Weight is transmitted from the sacrum through the sacroiliac joints along the thick posterior ilium to the acetabula when standing (or to the ischial tuberosities when sitting); the pubic rami act as tie-beams preventing the ring from spreading.

Sex differences reflect childbirth in females and greater muscularity in males: the female pelvis is wider and shallower, with a round/oval (gynaecoid) inlet (heart-shaped/android in males), a subpubic angle ≈80–90° or more (≈50–60° in males — "thumb–index angle" vs "index–middle angle"), a wide greater sciatic notch (≈90°; narrow ≈50–60° in males), everted ischial tuberosities and less projecting ischial spines (larger interspinous distance), an oval or triangular obturator foramen (round in males), a smaller acetabulum facing more anteriorly, a shorter, wider, less curved sacrum, and lighter bones with less marked muscle markings. A preauricular sulcus is more common in females.