Pelvic Anatomy: Types, Measurements and Obstetric Relevance

The bony pelvis forms the rigid part of the birth canal. It consists of the two innominate bones, sacrum, and coccyx, joined by the pubic symphysis and sacroiliac joints. The true pelvis lies below the pelvic brim and is the passage the fetus must negotiate during birth. A fetus does not simply move straight downward through a tube. The head must engage at the inlet, flex to present a smaller diameter, descend through the midpelvis, rotate to fit the available space, and extend beneath the pubic arch at the outlet. Pelvic dimensions influence each step, but they do not determine it alone. Fetal size, head position, flexion, uterine activity, soft-tissue resistance, and previous births also affect progress. The classic pelvic types describe the shape of the inlet. They are useful for recognising possible mechanical challenges, but a labelled pelvic type is not a diagnosis of cephalopelvic disproportion (CPD). Labour progress and maternal-fetal status provide the clinically meaningful evidence.