The pelvic brim is an approximately butterfly-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis.

The pelvic brim is obtusely pointed in front, diverging on either side, and encroached upon behind by the projection forward of the promontory of the sacrum.

In this case, the pelvic cavity coincides with the true pelvis, which is below the above-mentioned plane.

The urinary bladder lies just above the anterior pelvic brim.

[3] This article incorporates text in the public domain from page 238 of the 20th edition of Gray's Anatomy (1918)