Pelvic inlet

The superior circumference forms the brim of the pelvis, the included space being called the pelvic inlet (superior aperture).

It is formed laterally by the pectineal and arcuate lines, in front by the crests of the pubes, and behind by the anterior margin of the base of the sacrum and sacrovertebral angle. The superior aperture is somewhat heart-shaped, obtusely pointed in front, diverging on either side, and encroached upon behind by the projection forward of the promontory of the sacrum.

It has three principal diameters: antero-posterior, transverse, and oblique.

• The anteroposterior or conjugate diameter extends from the sacrovertebral angle to the symphysis pubis; its average measurement is about 110 mm. in the female.
• Thetransverse diameter extends across the greatest width of the superior aperture, from the middle of the brim on one side to the same point on the opposite; its average measurement is about 135 mm. in the female.
• The oblique diameter extends from the iliopectineal eminence of one side to the sacroiliac articulation of the opposite side; its average measurement is about 125 mm. in the female.