TRUE-PELVISIt is the bony canal through which the fetus must pass during birth. It has a brim, a cavity or an outlet.


It is round except where the sacral promontory projects into it. The promontory, wing of sacrum form its posterior border, the iliac bone its lateral border & public bone its anterior border.

The fixed point on pelvic brim are known as landmark, these are.

  1. Sacral promontory
  2. Sacral ala & wing
  3. Sacroiliac joint
  4. Iliopectineal line(edge formed at inner aspect of ilium)
  5. Iliopectineal eminence(roughened area formea where superior ramus of public bone meets the ilium.
  6. Superior ramus of public bone.
  7. Upper inner border of public bone.
  8. Upper inner border of symphysis pubis.

Diameters of Brim :-

  • Anterior posterior Diameter – This diameter is a line from sacral promontory to the posterior border of symphysis pubis measures 11 cm.
  • Oblique Diameter – Diameter is line from one sacroiliac joint to the iliopectineal eminence on opposite side of pelvis is measures 12 cm.
  • Transverse Diameter – This diameter line between the point furthest apart on the iliopectineal line measures 13 cm.


The cavity extends from the brim above to the outlet below. The anterior wall is formed by public bone & symphysis pubis & its depth is 4 cm. The posterior wall is formed by the curve of sacrum, which is 12 cm in length. Because there is such a difference in measurement the cavity forms a curved canal.


It include narrow strait through which the fetus must pass. It lies between the sacrococcygeal joint , 2 ischial spine and lower border of  symphysis pubis. The outlet in diamond shaped-

  • Anterior posterior Diameter (13 cm) – This is a line from the lower border of symphysis pubis to the sacrococcygeal joint.
  • Oblique Diameter – This is said to be between obturator foramen & the sacrospinous ligament ,although there are no fixed points, measures (12 cm)
  • Transverse Diameter – This is a line between the 2 ischial spines & measures 11 cm.