Pelvic outlet

The lower circumference of the lesser pelvis is very irregular; the space enclosed by it is named the inferior aperture or pelvic outlet. It is an important component of pelvimetry.

Boundaries
It has the following boundaries:
 * anteriorly: the pubic arch
 * laterally: the ischial tuberosities
 * posterolaterally: the inferior margin of the sacrotuberous ligament
 * posteriorly: the anterior border of the middle of the coccyx.

Notches
These eminences are separated by three notches:
 * one in front, the pubic arch, formed by the convergence of the inferior rami of the ischium and pubis on either side.
 * The other notches, one on either side, are formed by the sacrum and coccyx behind, the ischium in front, and the ilium above; they are called the sciatic notches; in the natural state they are converted into foramina by the sacrotuberous and sacrospinous ligaments.

In situ
When the ligaments are in situ, the inferior aperture of the pelvis is lozenge-shaped, bounded as follows:
 * in front, by the pubic arcuate ligament and the inferior rami of the pubes and ischia
 * laterally, by the ischial tuberosities
 * behind, by the sacrotuberous ligaments and the tip of the coccyx.