An abnormal bony pelvic girdle is the most frequent obstruction to delivery of the fetus. The lower part of the pelvis, called the pelvis minor or true pelvis, supports the muscles of the pelvic floor, and gives shape and trend to the parturient canal. The inlet and outlet of the true pelvis are narrower than its middle portion and are called the superior and inferior straits. The inlet is somewhat cordate in outline, and normally from front to back, at its so-called conjugata vera, it averages 11 centimetres (4-5/16 inches) in depth; from side to side it measures 13 centimetres (5-1/8 inches); obliquely from the right posteriorly to the left anteriorly it is 12-1/2 centimetres (nearly 5 inches), and the other oblique conjugate is 12 centimetres (4-3/4 inches) long. The transverse diameter of the outlet, from right to left, is 11 centimetres; the diameter from front to back, because the coccyx can be pushed back in labor, is from 9-1/2 (3-3/4 inches) to 12 cm. Normal fetal head measurements average from side to side at the widest part, 9-1/2 cm. (3-3/4 inches); from the root of the nose to the occiput, 11 cm.; from the chin to the occiput, 13 cm.; from the vertex to the neck behind, 9-1/2 cm. The size of the fetal head is the most important factor in delivery, so far as the child is concerned, because, as a rule, when the head is delivered the compressible trunk follows readily. Normally the child presents in delivery with the vertex of the head first; other presentations are transitional, abnormal or pathologic. In 48,499 cases Karl Braun found vertex presentations in 95.9 per cent., and Schroeder in 250,000 cases found an average of 95 per cent. The child's head is "engaged" when its largest diameter has passed the plane of the inlet. An abnormal pelvis may be generally contracted, dwarfed, in all its diameters; it may be flat or narrow from front to back; it may be contracted from side to side; it may be generally contracted and flat at the same time; it may be obliquely contracted (Nägeli's pelvis); or it may be crowded together irregularly. Rachitis, osteomalacia, curvature of the spine, habit scoliosis, hip dislocation, and similar pathologic states cause these distortions and contractions.