Overview
Introduction
The bony pelvis is the rigid passage through which the fetus enters, rotates, and descends during vaginal birth.
The bony pelvis is the rigid passage through which the fetus enters, rotates, and descends during vaginal birth. Its inlet, midpelvis, and outlet create the available space, but pelvic anatomy is only one part of labor mechanics. Fetal size, head flexion, fetal position, uterine activity, and soft tissues also determine whether descent occurs. The ischial spines are especially useful landmarks. They mark the narrow midpelvis and provide the reference point for fetal station: a presenting part at the spines is at 0 station. Serial changes in station, cervical dilation, contraction quality, and fetal status show whether labor is progressing. Pelvic shape or a single measurement cannot determine whether vaginal birth will succeed. Cephalopelvic disproportion (CPD) is a clinical diagnosis made when labor arrests despite adequate uterine activity and no reversible obstruction. Current U.S. guidance supports a trial of labor rather than selecting a delivery route from pelvimetry alone.