It is the bony canal through which the fetus must pass during birth. It has a brim, a cavity or an outlet.
(A) PELVIC BRIM:-
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.
- Sacral promontory
- Sacral ala & wing
- Sacroiliac joint
- Iliopectineal line(edge formed at inner aspect of ilium)
- Iliopectineal eminence(roughened area formea where superior ramus of public bone meets the ilium.
- Superior ramus of public bone.
- Upper inner border of public bone.
- 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.
(B) PELVIC CAVITY :-
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.
(C) PELVIC OUTLET :-
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.