Advantages of magnetic resonance (MR) pelvimetry include lack of ionizing radiation, accurate measurements, complete fetal imaging, and the potential for evaluating soft tissue dystocia (McCarthy, 1986; Stark, 1985). Zaretsky and colleagues (2005) used l\1R imaging to measure pelvic and fetal head volume to identiy those women at greatest risk of undergoing cesarean delivery for dystocia. Signiicant associations were found between some of the measures and cesarean delivery for dystocia. However, these researchers could not with accuracy predict which individual woman would require cesarean delivery. Others have reported similar indings (Sporri, 1997).
Fetal size alone is seldom a suitable explanation for failed labor. Even with current technology, a fetal size threshold to predict fetopelvic disproportion is still elusive. Most cases of disproportion arise in fetuses whose weight is well within the range of the general obstetrical population. As shown in Figure 23-5, two thirds of neonates who required cesarean delivery after failed forceps delivery weighed <3700 g. hus, other factorsfor example, malposition of the head-obstruct fetal passage through the birth canal. hese include asynclitism, occiput posterior position, and face or brow presentation.
For fetal head size estimation, clinical and radiographical methods to predict fetopelvic disproportion have proved �0 �O >� � x:5•x3 xO
FIGURE 23-5 Birthweight distribution of 362 newborns born by cesarean delivery after a failed forceps attempt at Parkland Hospital from 1989-1•999. Only 12 percent (n 44) of the newborns weighed >4000 g (dark bars).
disappointing. Mueller (1885) and Hillis (1930) described a clinical maneuver to predict disproportion. The fetal brow and the suboccipital region are grasped through the abdominal wall with the fingers, and firm pressure is directed downward in the axis of the inlet. If no disproportion exists, the head readily enters the pelvis, and vaginal delivery can be predicted. Thorp and coworkers (1993) performed a prospective evaluation of this Mueler-Hillis maneuver. They found no relationship between failed descent during the maneuver and subsequent labor dystocia.
Measurements of fetal head diameters using plain radiographical techniques are not used because of parallax distortions. he biparietal diameter and head circumference can be measured sonographically, and investigators have attempted to use this information in the management of dystocia. hurnau and colleagues (1991) used the etal-pelvic index to identiy labor complications. Unfortunately, the sensitivity of such measurements to predict cephalopelvic disproportion is poor (Ferguson, 1998; Korhonen, 2015). We believe that no current method of measurement satisfactorily predicts fetopelvic disproportion based on head size.
With this presentation, the neck is hyperextended so that the occiput is in contact with the fetal back, and the chin (mentum) is presenting (Fig. 23-6). The fetal face may present with the chin (mentum) anteriorly or posteriorly, relative to the maternal symphysis pubis (Chap. 22, p. 425). Although some mentum posterior presentations persist, most convert spontaneously to anterior even in late labor (Duf, 1981). If not, the fetal brow (bregma) is pressed against the maternal symphysis pubis. his position precludes lexion of the fetal head necessary to negotiate the birth canal. hus, a mentum posterior presentation is undeliverable except with a very preterm fetus.
FIGURE 23-6 Face presentation. The occiput is the longer end of the head lever. The chin is directly posterior. Vaginal delivery is impossible unless the chin rotates anteriorly.
450FIGURE 23-7 Mechanism of labor for right mentoposterior position with subsequent rotation of the mentum anteriorly and delivery.