True Pelvis

True Pelvis

It 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 & pubic bone form the anterior border.

The fixed point on the pelvic brim is known as a landmark, these from posterior to anterior are-

  1. Sacral promontory
  2. Sacral ala & wing
  3. Sacroiliac joint
  4. Iliopectineal line (edge formed at the inner aspect of ilium)
  5. Iliopectineal eminence (roughened area from where superior ramus of pubic bone meets the ilium.
  6. Pectineal line
  7. Pubic tubercle
  8. Pubic crest
  9. Symphysis pubis

Diameters of Brim –

  • Anteroposterior Diameter – This diameter is a line from the sacral promontory to the posterior border of the symphysis pubis measures 11 cm.
  • Oblique Diameter – Diameter is a line from one sacroiliac joint to the iliopectineal eminence on the opposite side of the pelvis 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 obstetrical outlet below. The anterior wall is formed by pubic bone & symphysis pubis & its depth is 4 cm. The posterior wall is formed by the curve of the sacrum, which is 12 cm in length. Because there is such a difference in measurement the cavity forms a curved canal. The overall diameter is about 12 c.m.


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

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


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