The bony pelvis forms the rigid, ring-shaped passageway through which the fetus must descend and rotate during a vaginal birth, and its shape and dimensions strongly influence whether that birth is likely to succeed. The pelvis is built from two innominate (hip) bones, each formed by the fused ilium, ischium, and pubis, joined to the sacrum and coccyx; these bones meet anteriorly at the pubic symphysis and posteriorly at the sacroiliac joints. Together they enclose the true pelvis, the part directly relevant to childbirth. Obstetric anatomy divides the pelvis into the inlet (where the fetus enters), the midpelvis or cavity (which contains the narrowest dimension, the interspinous diameter between the ischial spines), and the outlet (bounded by the ischial tuberosities and the pubic arch). The ischial spines also serve as the reference point for fetal station, the measure of how far the presenting part has descended. Why this matters clinically: The classic Caldwell-Moloy classification describes four pelvic types, gynecoid, android, anthropoid, and platypelloid, each with different implications for labor mechanics, fetal positioning, and the likelihood of operative birth. The gynecoid...