T HE prediction of the probability of the safe passage of the fetus through the birth canal is the primary function of the attendant at the start of labor. The means of determining the relative size of the fetal head and the internal diameters of the pelvis have therefore been the objects of deep concern down through the centuries. Manual techniques of clinical evaluation of cephalo-pelvic relations reached their peak a generation or two ago. A massive nomenclature existed with respect to pelvic planes and diam- eters. To these were related various positions, attitudes, stations, and synclitisms of the fetal head. Measurements depended on digital efforts to explore the interior and on dubious implications drawn from external pelvimetry. The mechanisms of labor, as it might occur under the innumerable possible pelvic measure- ments and fetal orientations, were the subject of hours of stu- dent drilling and remained a lifelong preoccupation of the most seasoned specialist. The increasing safety of cesarean section somewhat miti- gated the consequences of error. When a borderline internal conjugate was digitally determined, a trial labor might be per- mitted with the assurance that an ultimate solution was in reserve. Mistakes of two kinds persisted, however. On the one hand, trial labor might be permitted to continue too long and, with penicillin not yet discovered, a delayed cesarean section v vi Foreword could be perilous. Alternately, to be on the safe side, many un- necessary elective sections might be carried out.