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. The oblique plane passing approximately through the pelvic brim divides the internal part of the
pelvis (
pelvic cavity) into the false or
greater pelvis and the true or
lesser pelvis. The false pelvis, which is above that plane, is sometimes considered to be a part of the
abdominal cavity, rather than a part of the pelvic cavity. 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. The
sigmoid colon also passes close to the pelvic brim. == Clinical significance ==