The anterior triangle is formed by the pubic arch. The pelvic outlet is composed of two triangular areas that share the same base but are not in the same plane. Midpelvic contraction is more common than inlet contraction. When the diameter is less than 9 cm, the midpelvis is considered definitely contracted. Midpelvic contraction should be suspected whenever the interspinous diameter is less than 10 cm. The anteroposterior diameter of the midpelvis, which runs from the inferior aspect of the pubic symphysis to the sacral hollow at the level of the ischial spines, averages 11.5 cm or more. The distance between the ischial spines is usually the smallest diameter of the pelvis, typically measuring 10 cm or more. The lateral boundary is the pelvic sidewalls and ischial spines. The midpelvis is bounded anteriorly by the posterior aspect of the symphysis and pubis and posteriorly by the sacrum at the level of S3 or S4. When both diameters are contracted, the incidence of dystocia is much greater than when only one diameter is contracted.

The inlet usually is considered to be contracted if the obstetric conjugate is less than 10 cm or the greatest transverse diameter is less than 12 cm. The anteroposterior diameter (obstetric conjugate) is the shortest distance between the sacral promontory and the pubic symphysis. The posterior boundary is composed of the sacrum at the level of the iliopectineal lines. The pelvic inlet is bounded laterally by the iliopectineal lines, which can be traced anteriorly along the pectineal eminence and pubic crest to the symphysis. The anterior wall at the pubic symphysis measures approximately 5 cm, and the posterior wall measures approximately 10 cm. The “true” pelvis includes the inlet, the midpelvis, and the outlet. During labor, the fetus assumes positions and attitudes that are determined in part by the configuration of the mother’s pelvis.

Precipitate birth definition