The occipitoposterior position

Abstract 1. 1. The occipitoposterior position in the main is caused by the adaptation of the head to a pelvis having a narrow fore pelvis and an ample anteroposterior diameter and therefore may be considered “physiologic.” 2. 2. Degrees of the posterior position can be differentiated. A true posterior position is one in which the occiput is at or posterior to the sacroiliac synchondrosis. 3. 3. Occipitoposterior positions, although physiologic and in spite of the fact that 70 per cent deliver spontaneously, cause more dystocia than the other vertex positions. 4. 4. Dystocia in posterior positions originates with the abnormality of the pelvis in which they occur. 5. 5. Extension is a common feature of the posterior mechanism and is caused by the posterior axis of fetal drive. Extension not only results in larger presenting diameters but also causes a part of the uterine drive to be dissipated. 6. 6. Flexion greatly facilitates the mechanism and should be encouraged by posture. A forward excursion of the fetal axis must take place before flexion can occur. 7. 7. Internal rotation although receiving its impulse from the inclined planes of the pelvic floor receives its directional force from the variable resistances offered by the bony pelvis in all the pelvic planes through which the head is descending. 8. 8. Since the pelvic architecture shows marked variations in shape, it follows that the patterns of rotation show corresponding variations. 9. 9. Emphasis is placed on the importance of recognizing two fundamentally different pelvic forms frequently associated with posterior positions, viz., those forms in which the transverse diameters are contracted at the brim or midpelvis or at both levels with compensating ampleness in the anteroposterior diameters and the converse of this form in which the anteroposterior diameters are shortened by virtue of a flat posterior segment at the brim or a forward sacrum at the midpelvis level while the transverse diameters remain ample. It follows that operative techniques should be individualized for each type. 10. 10. The occipitoposterior mechanisms which have been reviewed may be translated into operative principles as follows: 10.1. a. When the posterior arrest is due to a narrow anthropoid pelvic brim preventing rotation, the head is pushed up and rotated above the brim. Version is also correct in principle. 10.2. b. When arrest is due to narrow spines in definite anthropoid types, the head is pushed up and rotated above the spines or above the brim. Version is also an alternative procedure. 10.3. c. When the posterior aspect of the inlet is flat or the sacrum is forward, rotation is carried to the transverse only and in this position the head is brought into the lower fore pelvis or the retropubic space thereby converting it into a deep transverse. Rotation is carried out at this low level. Upward displacement of head with rotation and version are incorrect procedures. 10.4. d. When the posterior aspect of the inlet is flat or the sacrum is forward and labor has progressed sufficiently to allow a deep transverse arrest to occur rotation is again carried out at a low level just behind the pubis. 10.5. e. When the head is arrested in a direct occipitoposterior, deliver without rotation only when the lower sacrum is not encroaching. In most cases the head should be rotated to the anterior. Such rotation is carried out at the level which is most suitable for the pelvis in which it occurs. 10.6. f. When the head is low, well-molded, and well-flexed manual rotation is the most desirable procedure. 10.7. g. Rotation at level of arrest is rarely a procedure of choice because in most instances there is some pelvic obstruction which is preventing rotation at that level. 10.8. h. The unmodified Scanzoni maneuver with simultaneous traction and rotation initiates the act of rotation at the level of arrest where there is obstruction. It is, therefore, an undesirable procedure in those cases which have a well-molded and flexed head. Even in this group of cases manual rotation is the operation of choice.