Fig. 31-3. Median section of the pelvis, showing the planes of the inlet and outlet. The true (T), obstetrical (O), and diagonal (D) conjugate diameters are indicated. The axis of the birth canal, that is, the path taken by the fetal head in its passage through the pelvic cavity, can be seen to turn at the uterovaginal angle. 5, fifth lumbar vertebra. Click for high resolution image.