Although no precise manual method permits measure of midpelvic dimensions, a suggestion of contraction sometimes can be inferred if the spines are prominent, the pelvic sidewalls converge, or the sacrosciatic notch is narrow. Moreover, Eller and Mengert (1947) noted that the relationship between the intertuberous and interspinous diameters of the ischium is suf ficiently constant that narrowing of the interspinous diameter can be anticipated when the intertuberous diameter is narrow.
A normal intertuberous diameter, however, does not always exclude a narrow interspinous diameter.
This inding usually is defined as an interischial tuberous diam eter of 8 cm or less. The pelvic outlet may be roughly likened to two triangles, with the interischial tuberous diameter con stituting the base of both. The sides of the anterior triangle are the pubic rami, and its apex is the inferoposterior surface of the symphysis pubis. The posterior triangle has no bony sides but is limited at its apex by the tip of the last sacral vertebra-not the tip of the coccyx. Diminution of the intertuberous diam eter with consequent narrowing of the anterior triangle must inevitably force the fetal head posteriorly. Floberg and asso almost 1 percent of more than 1400 unselected nulliparas with term pregnancies. A contracted outlet may cause dystocia not tion. Oulet contraction without concomitant midplane contrac tion is rare.
Although the disproportion between the fetal head and the pelvic outlet is not suiciently great to give rise to severe dystocia, it may play an important part in perineal tears. With increased narrowing of the pubic arch, the occiput cannot emerge directly beneath the symphysis pubis but is forced farther down upon the ischiopubic rami. The perineum, consequently, becomes increasingly distended and thus exposed to risk of laceration.
Vallier (2012) reviewed experiences with pelvic fractures and pregnancy. Trauma from automobile collisions was the most common cause. Moreover, they note that fracture pattern, minor malalignment, and retained hardware are not absolute indications for cesarean delivery. In determining suitability for vaginal delivery, fracture healing requires 8 to 12 weeks and thus recent fracture merits cesarean delivery (Amorosa, 2013). A history of pelvic fracture warrants careful review of previous radiographs and possible imaging pelvimetry later in pregnancy.
The techniques for clinical evaluation using digital examination of the bony pelvis during labor are described in detail in Chapter 2 (p. 30). The value of radiological imaging to assess pelvic capacity has also been examined. First, with x-ray pelvimetry alone, the prognosis for successful vaginal delivery in any given pregnancy with cephalic presentation cannot be established (Mengert, 1948). Similarly, one systematic review found insufficient evidence to support the use of x-ray pelvimetry with cephalic presentations (Pattinson, 2017).
Advantages of pelvimetry with computed tomography (CT) compared with those of conventional x-ray pelvimetry include greater accuracy and easier performance. With either method, costs are comparable, and x-ray exposure is small (Chap. 46, p. 906). Depending on the machine and technique employed, fetal doses with CT pelvimetry may range from 250 to 1500 mrad (Moore, 1989).