OG13.1 | Maternal Pelvis — Summary & Reflection

KEY TAKEAWAYS

The maternal pelvis forms the bony birth canal through which the fetus must pass. The true pelvis is divided into three planes: the inlet, mid-pelvis, and outlet. Key diameters: at the inlet, the obstetric conjugate (~11 cm, estimated as diagonal conjugate minus 1.5–2 cm) is the critical AP measure; the transverse diameter (~13.5 cm) is the widest. At the mid-pelvis, the interspinous diameter (~10–10.5 cm) is the smallest diameter of the entire pelvis. At the outlet, the intertuberous diameter (~11 cm) is the critical transverse measure. The Caldwell-Moloy classification identifies four pelvic types: gynecoid (50%, best prognosis), android (23%, prone to mid-pelvic arrest), anthropoid (24%, favours OP delivery), and platypelloid (3%, inlet engagement difficult). Contracted pelvis is defined by thresholds: OC <10 cm, interspinous <9 cm, intertuberous <8 cm. Clinical assessment uses the diagonal conjugate, ischial spines, sacrosciatic notch, sacral curvature, and sub-pubic angle. These dimensions directly determine the mechanism and progress of labour.

REFLECT

Think about a real or simulated situation where you assessed a pregnant woman's pelvis on examination. What did you feel? Were the ischial spines prominent? Could you reach the sacral promontory? What was your impression of the pelvic type? Now that you have studied the classification and dimensions in detail, how would you revise your clinical impression? How would you counsel the woman about her chances of vaginal delivery, and at what point in labour would you be concerned about progress? Kolb's experiential learning model asks us to move from concrete experience to reflective observation, then to abstract conceptualisation and active experimentation — this question is your invitation to make that transition from anatomy recall to clinical reasoning.