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1 Department of Obstetrics and Gynecology, University of Bern, Inselspital,
Effingerstr. 3010 Bern, Switzerland.
2 Department of Obstetrics and Gynecology, Kantonsspital, 1708 Freiburg,
Switzerland.
3 Institute of Diagnostic Radiology, University of Bern, Inselspital, 3010 Bern,
Switzerland.
Received January 2, 2001;
accepted after revision January 24, 2002.
Address correspondence to S. Spörri.
Abstract
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SUBJECTS AND METHODS. Antepartum fetal sonography and maternal MR imaging pelvimetry measurements were performed at term in 38 pregnant women at risk for dystocia with a single fetus in cephalic presentation. Various methods used to diagnose cephalopelvic disproportion were evaluated in a blinded manner for their accuracy to predict both the presence of cephalopelvic disproportion and the mode of delivery (vaginal vs cesarean).
RESULTS. None of the methods tested yielded both high sensitivity (15-100%) and high specificity (24-92%) for determining the presence of cephalopelvic disproportion and high levels of accuracy for predicting labor outcome (overall predictability, 50-74%).
CONCLUSION. To achieve increased reliability of MR imaging pelvimetry in the diagnosis and treatment of dystocia and in predicting labor outcome, new methods assessing fetal-pelvic compatibility, including measurements of the pelvic outlet and the shape and configuration of the pelvis, need to be established and prospectively tested before firm recommendations for clinical use can be made.
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An increasing need exists for a method to accurately predict the presence of cephalopelvic disproportion and then select the route of delivery. Little progress has been made thus far with various attempts to recognize cephalopelvic disproportion and to predict the delivery outcome—that is, whether birth will be spontaneous or surgical, or whether high likelihood exists for a cesarean delivery.
In diagnosing cephalopelvic disproportion, radiographic pelvimetry has become less popular and has even been abandoned by many institutions because the value of the measurements of pelvic dimensions for diagnosing cephalopelvic disproportion and predicting labor outcome remains limited [6]. However, since the first description of MR imaging pelvimetry by Stark et al. [7], interest in pelvimetry has been renewed. MR imaging provides contrast resolution superior to that provided by CT and permits accurate pelvic measurements with no ionizing radiation [8, 9]. MR imaging pelvimetry is accurate in bony mensuration, with a 1% variation rate versus 10% for radiographic pelvimetry [7]. Other advantages of MR imaging include high-quality multiplanar imaging even in obese patients, direct measurement of pelvic diameters without repositioning of the subject, no requirement of correction of measured dimensions and placement of rulers, and good patient acceptance because short-bore MR imaging systems reduce patient claustrophobia [9,10,11]. The contraindications to the use of MR imaging include the presence of cardiac pacemakers, cerebral aneurysm clips, cochlear implants, and orbital foreign bodies; recent surgery; and severe claustrophobia. However, a technically superior measurement method does not have any value in clinical practice if the concept of imaging pelvimetry has no benefit.
This study investigated the use of MR imaging pelvimetry, which is an interdisciplinary topic between the radiologist and the obstetrician, in providing valuable information and objective data for the diagnosis and treatment of the pregnant woman at risk for dystocia. The objective of this prospective study was to evaluate the accuracy of current methodologies used to diagnose cephalopelvic disproportion and to predict cesarean delivery. Both antepartum MR imaging pelvimetry and sonographic fetal measurements at term were used as simple and exact techniques to evaluate fetal-pelvic compatibility. Both techniques have shown good interobserver reproducibility [12, 13].
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Patients who met the inclusion criteria were primiparas with singleton pregnancy and vertex presentation associated with at least one of the following risk factors: maternal height of 164 cm or smaller [14], early first trimester maternal weight of 90 kg or greater [15], early first trimester maternal body mass index of 29 kg/m2 or greater [15], a maternal age before pregnancy of 28 years or older [16], a history of pelvic trauma and a clinically small pelvis, and clinically suspected macrosomia, with a fetal weight shown on sonography of 90th percentile or greater at 32 weeks. In addition, multiparas who had had a previous cesarean delivery at term because of dystocia and who fulfilled at least one of the previously listed risk factors required of primiparas were invited to participate.
Patients who met exclusion criteria were primiparas who were noncandidates for a trial of labor according to generally accepted criteria and who had an increased risk of a cesarean delivery regardless of prenatal or labor management [17, 18]. These exclusion criteria included women with known congenital uterine or vaginal malformations or both; a history of uterine surgery other than a low-transverse uterotomy; previous vaginal surgery for rectovaginal fistula; a pelvic tumor obstructing the outlet; a placenta previa; a hemoglobinopathy or thrombocytopathy or both; chronic systemic disease (diabetes, hypertension, epilepsy, cardiac disease, lung disease, renal disease, positive test for human immunodeficiency virus); and women with certain complications arising and diagnosed during the pregnancy up to 32 weeks of gestation such as diabetes, hypertension, proteinuria, extensive weight gain, incompetent cervix, hydramnios or oligohydramnios, fetal malformation, and fetal growth restriction. In addition, women with a contraindication to the use of MR imaging were excluded.
After providing written consent, the 48 healthy women were scheduled for MR imaging pelvimetry, which was performed at 37 weeks of gestation. In 10 women (20.8%), the MR imaging was not performed: three women (6.3%) withdrew from the study; two women each had severe claustrophobia (4.2%), had severe difficulty lying on the back (4.2%), or had a premature delivery (4.2%); and fetal presentation as a breech was present in one woman (2.1%) at 36 weeks of gestation.
In the remaining 38 women (the study participants), standardized MR imaging pelvimetry, requiring a scanning time of approximately 15 min, was performed using a Signa 1.5-T magnet system (General Electric Medical Systems, Milwaukee, WI) with a body coil. The MR imaging protocol consisted of axial, sagittal, and oblique coronal T1-weighted fast spin-echo images (TR range/TE, 500-600/14). The section thickness was 5 mm with a 1-mm intersection gap. The field of view was 40 x 40 cm with a 512 x 256 acquisition matrix. Two to three acquisitions were performed. To prevent interobserver variability, the same senior radiology staff member performed the standardized pelvic diameter measurements on the respective sections as previously described [11, 19] and used in our previous study [10]. On the midsagittal section, the sagittal diameters of the pelvic inlet, mid pelvis, and pelvic outlet were measured (Fig. 1). On axial sections, the interspinal and the intertuberous diameter were visualized and measured (Figs. 2 and 3). On the oblique coronal section that corresponds to the inlet plane at the level of the obstetric conjugate, the transverse diameter of the pelvic inlet was determined (Fig. 4). In addition, on the midsagittal section, abnormalities of the pelvic configuration, such as a high assimilation pelvis, a flat sacrum, a sacrum with an extremely deep sacral curve (Fig. 5), or an os coccyx forming an angle of almost 90° with the sacrum (Fig. 6), were recorded. On the oblique coronal section, the shape of the pelvic inlet was determined using the classification of Caldwell and Moloy [20]. The android (Fig. 7) and platypelloid types were considered abnormal because both are associated with an increased risk of cephalopelvic disproportion [10, 21].
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Fetal sonographic measurements included the biparietal diameter, the
abdominal diameter, and the femur length. These measurements were performed
routinely according to common practice at term or on admission to the labor
room (or both) by trained or supervised obstetric residents. Fetal weight was
estimated using measurements described by Rose and McCallum
[22]. The fetal head
circumference and fetal head volume were calculated as previously described
[10]: fetal head circumference
(cm) = (0.35 x biparietal diameter [mm]) + 1, and fetal head volume
(cm3) = (21.8 x biparietal diameter [mm]) - 1388.0. The fetal
abdominal circumference was calculated as abdominal diameter x 
.
Neonatal birth weight and birth length were taken by midwives within 1 hr
postpartum. The largest neonatal head circumference measurement was recorded
during the first routine examination within 12 hr postpartum by pediatric
residents. The neonatal head volume was calculated according to the formula
[23]: neonatal head volume
[cm3] = (largest head circumference [cm])3 /
(6x2).
Eight methods were used to evaluate pelvic adequancy and to diagnose
cephalopelvic disproportion. The method described by Colcher and Sussman
[24] is based on the
transverse and sagittal diameters of the pelvic inlet and the mid pelvis, and
a contracted pelvis is defined as a sum of the two dimensions of less than
22.0 cm for the inlet or less than 20.0 cm for the mid pelvis. The method
described by Mengert [25] is
based on the products of the transverse and sagittal diameters for both the
inlet and the mid pelvis, and pelvic contraction is defined as an area of less
than 123 cm2 for the inlet or less than 106 cm2 for the
mid pelvis. The method described by Borell and Fernström
[26] is based on the sum of
the transverse diameter of the mid pelvis and the transverse and sagittal
diameters of the pelvic outlet, and pelvic contraction is defined as a sum of
less than 29.5 cm. A fetal weight estimate of greater than 3600 g based on
sonography is taken as an indicator of cephalopelvic disproportion
[10]. The cephalopelvic
disproportion index [27]
compares the smallest pelvic diameter (either the sagittal diameter of the
inlet or the transverse diameter of the mid pelvis) with the fetal biparietal
diameter and indicates how much wider the smallest pelvic diameter is than the
biparietal diameter. A positive cephalopelvic disproportion index is present
if the pelvic diameter is less than 9 mm wider than the biparietal diameter.
The fetal—pelvic index
[28] compares the fetal head
and abdominal circumferences with the respective inlet and mid pelvis
circumferences. The pelvic inlet and mid pelvis circumferences were calculated
according to the formula [28]:
(sagittal diameter + transverse diameter) x / 2. A positive
fetal—pelvic index is present if the circumference of the fetal head or
abdomen is larger than that of the maternal pelvic inlet or mid pelvis, which
indicates cephalopelvic disproportion or abdominal—pelvic disproportion.
The method described by Friedman and Taylor
[23] evaluates cephalopelvic
disproportion by comparing fetal head volume with maternal pelvic inlet and
mid pelvis capacity. The pelvic capacities were calculated using the formula
[23]:
(xd3) / 6, where d equals the
appropriate sagittal or transverse diameter of the respective plane, always
taking the shorter of the two. Cephalopelvic disproportion is present if the
fetal head volume is more than 50 cm3 larger than the smallest
inlet capacity or the fetal head volume is more than 200 cm3 larger
than the interspinal capacity or both. According to the method described by
Spörri et al. [10],
cephalopelvic disproportion is present if the fetal head volume is greater
than the smallest pelvic inlet or mid pelvis capacity or both.
The obstetricians managing labor of the individual cases had no knowledge of the results of MR imaging measurements. On the other hand, investigators were not involved in the labor management and evaluated the eight methods tested without knowledge of the delivery outcome. The methods were assessed by comparing the mode of delivery with the prediction of cephalopelvic disproportion.
All data were presented as mean (±SD) or as median (range). The three delivery groups (cesarean, operative vaginal, and spontaneous vaginal) were compared using one-way analysis of variance, the Kruskal-Wallis test, or the chi-square test, as appropriate. A post hoc comparison of the groups was performed with the Dunnett or Dunn test using the spontaneous vaginal delivery group as the reference group. We used Statview version 4.5 software (Abacus Concepts, Berkeley, CA) for statistical analysis. Sensitivity, specificity, and predictive values were calculated in the standard fashion. All statistical tests were two-tailed, and statistical significance was assumed at a p value of less than 0.05.
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Maternal data and characteristics of labor for the three delivery groups are given in Table 1. Women with cesarean deliveries for cephalopelvic disproportion had significantly smaller skeletal structures, experienced longer duration of labor, and required an epidural anesthesia, which was freely available for pain relief, more often.
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Fetal and neonatal data, including neonatal conditions that were evaluated by obstetric or pediatric residents, did not differ among the groups. None of the newborns had an Apgar score of less than 7 at 5 min. Three newborns each of the operative vaginal and the spontaneous vaginal delivery groups had an umbilical artery pH of less than 7.15. The sonographic estimate of fetal weight (3296 ± 618 g) using measurements described by Rose and McCallum [22] was found to be a good predictor of birth weight (3307 ± 391 g) (r = 0.75). Fetal and neonatal head circumference (34.6 ± 1.3 cm and 34.5 ± 1.0 cm, respectively) (r = 0.50) as well as fetal and neonatal head volume (710 ± 87 cm3 vs 697 ± 63 cm3, respectively) (r = 0.51) correlated moderately with the equations recently described by Spörri et al. [10]. Fetal sonographic measurements were performed at a median of 10 hr antepartum (range, 1-72 hr).
The mean values for pelvic dimensions obtained by MR imaging in the three study groups are shown in Table 2. Two measured pelvic dimensions, the obstetric conjugate and the interspinal diameter, and eight of nine calculated pelvic dimensions were smaller in women who had cesarean delivery than in those with spontaneous vaginal deliveries. Pelvic dimensions did not differ between the two vaginal delivery groups.
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On the basis of the MR imaging and the descriptive criteria mentioned in the Subjects and Methods section, abnormality of the pelvis was diagnosed in 15 of the 38 cases. Among the 13 women in the cesarean and the nine women in the operative vaginal delivery group, eight and four, respectively, showed pelvic abnormalities. Thus, a diagnosis of some degree of pelvic abnormality occurred in 55% of the women who had an operative delivery. In the spontaneous vaginal delivery group, only three (19%) of the 16 women had pelvic abnormalities.
Results of the outcome data are given in Tables 3,4,5,6. The evaluation of the diagnostic value of the eight methods, except for the cephalopelvic disproportion index [27] and the method described by Spörri et al. [10], gave low sensitivity values (15-77%). This result indicates that the presence of cephalopelvic disproportion was often not correctly diagnosed, and cesarean deliveries were not accurately predicted by the methods (positive predictive value, 44-64%). High sensitivities (85-100%) were achieved with the cephalopelvic disproportion index [27] and the method described by Spörri et al. [10] that relates the fetal head volume to the smallest pelvic capacity of either the inlet or the mid pelvis. However, these two methods yielded very low specificity values (56% and 24%, respectively) and did not accurately predict when vaginal delivery was possible. Overall, none of the methods showed high levels of accurate prediction of labor outcome (overall predictive value, 50-74%).
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Our data (Tables 3 and 4) confirm previous reports [11, 29, 30] that showed that measurements of pelvic dimensions and estimated fetal weight, when used alone, have low sensitivity (15-62%) for the detection of cephalopelvic disproportion and thus do not accurately predict when vaginal delivery is unlikely and when a cesarean delivery is needed (positive predictive value, 44-64%). The poor performance of fetal weight and bony measurements alone is not unexpected because these measurements take into account only the "passenger" and "passage," respectively.
Although our results (Tables 5 and 6), in accordance with the results of others [10, 27, 29, 31], show that the individual methods comparing the relative fit of the passenger with the passage were more accurate in identifying the presence of cephalopelvic disproportion (sensitivity, 59-100%), these methods revealed low specificity (24-72%) and thus did not accurately predict when vaginal delivery was possible. In general, any method for the prospective detection of cephalopelvic disproportion must give a high sensitivity and a very good specificity. Although false-positive cases will lead to unnecessary cesarean deliveries, in the false-negative cases the course of labor will eventually lead to the correct diagnosis, but the delay in diagnosis presents unnecessary suffering for the mother, and the postponement of an operative delivery is associated with an increased risk for the mother and the baby. Overall, none of the methods tested was accurate in identifying the presence of cephalopelvic disproportion and in predicting delivery outcome (overall predictive value, 50-74%). On the basis of these data, we conclude that MR imaging pelvimetry as used today has little value for the diagnosis and treatment of women at risk for dystocia.
Our findings regarding the limited success of the fetal—pelvic index, which is the most widely used method in prospectively identifying cephalopelvic disproportion and predicting cesarean delivery, are in agreement with results recently reported by Ferguson et al. [32]. Those authors were the first to evaluate the fetal—pelvic index using digital radiography after the original investigators [28]. In the initial corroborative trial of Ferguson et al. [32], the performance of the fetal—pelvic index as a predictor of cesarean versus vaginal (operative and spontaneous) delivery yielded sensitivity, specificity, and overall predictive values of 27%, 84%, and 65%, respectively. In contrast, Thurnau et al. [29], using conventional radiographic pelvimetry, showed the fetal—pelvic index to be highly sensitive (range, 71-94%) and specific (range, 94-100%) and to have positive and negative predictive values for cesarean and vaginal delivery of 94-100% and 75-95%, respectively. The reason for this discrepancy is not known with certainty. There are probably small but significant sources of error and bias in the resultant measurements in comparison with conventional pelvimetry. Moreover, any users of radiologic techniques that rely on measurement must be aware of inherent methodologic errors. Even if a particular pelvimetric technique is shown to work well at one particular center, it may not be effective in general practice if measurement errors are large and interobserver reliability is low [8].
Regarding methodologic errors, our actual pelvimetric findings with respect to the measurement of the interspinal diameter that marks the narrowest point of the pelvis confirm an observation reported by Aronson and Kier [33] and Spörri et al. [10]. Because the spines are difficult to identify on the coronal view, the fovea of the femoral head has been used as a landmark to determine the level of the appropriate axial view. Those authors [10, 33], however, questioned the accuracy of this landmark. Using MR imaging and CT pelvimetry, respectively, 33% [10] and 35% [33] of nonpregnant women and 65% [33] of pregnant women had ischial spines below the level of the foveae. In no case were the spines above the foveae. Analysis of our axial images confirmed this observation. In no case was the smallest interspinal diameter measured above the level of the fovea, and in 68% of our women the ischial spines were below the foveae, with a maximum distance of 1.0 cm. Thus, an exact analysis of the axial sections below the foveae is crucial for identification of the smallest distance of the true pelvis in pregnant women.
In our study population, pelvic abnormalities such as abnormal shape of the pelvic inlet and abnormal pelvic configuration as a descriptive diagnosis after enrollment in the study were distinctly more often observed in women who had an operative delivery than in those with a spontaneous vaginal delivery (55% vs 19%). Although the exact impact of pelvic abnormality on the mode of delivery has yet to be evaluated in a larger population, our findings suggest that MR imaging pelvimetry provides important information about pelvic abnormalities that appear to be associated with operative delivery due to cephalopelvic disproportion. Thus, a complete MR imaging evaluation of pelvic measurements should include a statement about the pelvic configuration and the shape of the pelvic inlet.
The current methods used to evaluate the fetal—pelvic compatibility consider the pelvic inlet and mid pelvis [10, 23, 27, 28]. However, in light of the studies by Suonio et al. [34] and Floberg et al. [35], this evaluation should also include a comparison with the pelvic outlet. Those authors have shown that as pelvic outlet size decreases, a significant increase is seen in frequency of both abdominal and vaginal operative delivery.
In summary, the efficacy of imaging pelvimetry for the detection of cephalopelvic disproportion and the prediction of cesarean and other operative deliveries remains to be shown. Future research should focus on refinement of procedures and the establishment of reproducible measurement landmarks, interpretation of protocols, and collection of normative criteria for MR imaging pelvimetry. This goal can be achieved only if radiologists and obstetricians work together to obtain the knowledge of each specialty's needs and possibilities. Moreover, to achieve an increased reliability of MR imaging pelvimetry in the diagnosis and treatment of dystocia and in predicting obstetric outcome, new methods of assessing fetal—pelvic compatibility, including measuring the pelvic outlet and considering the shape and configuration of the pelvis, need to be established and prospectively tested by different centers before firm recommendations for clinical use can be made.
Finally, prospective studies should address the question of indications for antepartum MR imaging pelvimetry. The diagnostic evaluation, including MR imaging pelvimetry, must remain restricted to women at risk for dystocia and cephalopelvic disproportion. In our study, risk factors used to select women for antepartum MR imaging pelvimetry were taken from the literature. Any extension of the clinical application of MR imaging pelvimetry to patients with a lower or no risk for dystocia will have to be validated, because in any diagnostic test, sensitivity and specificity depend on the prevalence of the abnormalities to be detected.
Acknowledgments
We thank Nancy Bell for her assistance in editing the manuscript.
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