Long labor terminating in cesarean section is associated not only with maternal pain and discomfort but also with an increase in endomyometritis, amniotic infection, prolonged hospital stay, and higher risk for the newborn, as shown by a gradual decrease in fetal pH 1.
Pelvimetry was used to identify those women at greatest risk of cephalopelvic disproportion, for whom cesarean section should be considered before undergoing labor. Using external and, later, internal calibers as well as digital examination, pelvic dimensions were estimated 2.
Magnetic resonance (MR) pelvimetry has replaced conventional radiography (X-ray) and computed tomography in many specialties 3. MRI is able to predict abnormal pelvic anatomy that would preclude vaginal delivery. The lack of ionizing radiation, accurate measurements, and high-quality imaging that does not require correction for image distortion are all additional factors that favor the use of MRI for pelvimetry 4. The aim of this work was to study whether MRI has the ability to identify those women at risk for dystocia and thus requiring cesarean delivery.
Participants and methods
Women who were pregnant and at 37-week gestation with cephalopelvic disproportion confirmed clinically in the Maady Army Forced Hospital from March 2008 to December 2009 were included in this study.
Inclusion criteria were as follows: second gravid women with a singleton pregnancy who had undergone a previous lower uterine segment cesarean section due to cephalopelvic disproportion. We included women pregnant with babies weighing between 3.5 and 4 kg as estimated by ultrasound. However, we excluded women with medical disorders associated with pregnancy such as diabetes and also excluded those with fetal weight of more than 4 kg confirmed by ultrasound.
Pelvimetry was performed using a closed Magnetom expert 1.5 T MRI system (MRI MAGNETOM EXPERT, Siemens, Netherlands, Zuid-Holland). Examination was carried out between the 38th and the 40th week of gestation with the patient in the supine position using the body coil. Pelvimetric examination for pelvic assessment: measurements were made using midsagittal, transverse, and oblique coronal sections of the pelvis. The midsagittal section included the obstetric conjugate, from the sacral promontory to the top of the symphysis pubis, and the sagittal outlet, from the end of the sacrum to the bottom of the inner cortex of the symphysis. The transverse section included the interspinous distance, or the narrowest distance between the ischial spines, and the intertuberous distance, or the widest distance between the ischial tuberosities. The oblique section, in a plane through the symphysis and the promontorium, included the transverse diameter (the largest transverse distance of the pelvis).
The protocol consisted of the following:
- Localizer images, sagittal T1-weighted image (WI; repetition time ms/echo time ms, 500/20; field of view, 300; section thickness, 10 mm; matrix, 128_256).
- Transverse and coronal T1 WI (500/25; field of view, 200; section thickness, 5 mm; matrix, 270_256).
- Oblique coronal T1 WI (550/25; field of view, 250; section thickness, 5 mm; matrix, 128_256).
- The number of sections varied with patient size to include the region of interest.
- No intravenous or vaginal contrast agents were used. There was no bowel preparation required before the examination.
- The total imaging time was approximately 25 min.
- Standardized measurements were made with electronic calipers at a workstation.
Trial of labor was carried out in all cases between 38 and 40 gestational weeks either spontaneously or by induction of labor. Patients were classified into three groups: group A, women who delivered normally; group B, women who delivered by lower uterine segment cesarean section; group C, women who delivered by assisted vaginal delivery.
Data were statistically described in terms of range, mean±SD, and median. Comparison of quantitative variables between the study groups was carried out using the Mann–Whitney U-test for independent samples when comparing two groups, and Kruskal–Wallis analysis of variance test with the Mann–Whitney U-test for independent samples as post-hoc multiple two-group comparisons when comparing more than two groups. A P-value of less than 0.05 was considered statistically significant. All statistical calculations were carried out using the computer programs Microsoft Excel 2003 (Microsoft Corporation, New York, USA) and SPSS (SPSS Inc., Chicago, Illinois, USA) version 15 for Microsoft Windows.
After an adequate trial of labor either spontaneously or by induction, nine women [group B (36%)] underwent a cesarean delivery, two women [group C (8%)] underwent an operative vaginal delivery after an arrest of labor for more than 30 min, and 14 women [group A (56%)] experienced an uncomplicated spontaneous vaginal delivery.
The mean values and the SD for each pelvic parameter obtained by MRI in the three delivery groups are given in Table 1 (Figs 1–6).
So far, there is no consensus recognition of MRI in the obstetric decision-making setting 5. The problem is that MR pelvimetry is not part of the routine antenatal work-up. It is indicated only in patients suspected of having fetal-pelvic disproportion, breech presentation, and a history of pelvic trauma.
In the present study, pregnant women who delivered by lower uterine segment cesarean section had significantly smaller measurements in all parameters than women who delivered normally (P≤0.05). In addition, pregnant women who delivered by lower uterine segment cesarean section had significantly smaller measurements only in the obstetric conjugate, oblique diameter, interspinous diameter, and anteroposterior diameter than pregnant women who delivered by assisted vaginal delivery (P<0.05), whereas the posterior sagittal, diagonal conjugate, intertuberous diameter, and sagittal outlet did not show any significant differences (P>0.05). In contrast, there were insignificant differences in all parameters between pregnant women who delivered normally and women who delivered by assisted vaginal delivery (P>0.05).
Opinions on the effect of pelvimetry on clinical decision making are divided: MR pelvimetry can only add to the obstetrician’s arsenal by providing information about a crucial factor in the mechanism of labor. Other factors are also clinically relevant, such as fetal biometric data, the position and deformability of the fetal head, and the strength of uterine contractions. When brought together with the sum of other information at the clinician’s disposal, MR pelvimetry has allowed better selection of the delivery route, with a significantly lower rate for emergency cesarean section 6.
The results of the present study agree well with the results of Thomas and colleagues, who confirmed that pelvimetric dimensions are significantly smaller in women undergoing cesarean section and assisted delivery than in those delivering vaginally, and they have shown that the pelvimetric parameters associated with the largest intraobserver and interobserver error and intraindividual variability are the intertuberous distance and the sagittal outlet; therefore, decision makers should treat them with caution 5.
Other authors showed no pelvimetric differences between pregnant and nonpregnant women, nor any correlation between the pelvimetric profile and parity. In addition, the virtual absence of significant differences in pelvimetric data, except for transverse diameter, between spin-echo and fast spin gradient echo sequences was already reported in smaller populations 7.
Sven and colleagues showed that changes in birthing position augment pelvic dimensions and might therefore be obstetrically advantageous. The sagittal outlet and interspinous diameter were significantly greater in the hand-to-knee and squatting positions than in the supine position, as was the intertuberous diameter in the squatting position. The obstetric conjugate was the only dimension that was significantly smaller in the upright squatting position than in the supine position 8.
Analysis of birthing position studies using MRI suggests that though the benefits of upright posture include a shorter second stage of labor, a small reduction in assisted deliveries, and a decreased episiotomy rate, there is an increased risk of severe blood loss 9.
A limitation of this study is that it did not include some pregnant women because of limited space in the scanner bore (upright scanning is technically impossible for a woman in late pregnancy) and the ethics of scanning stress; particularly in the hand-to-knee and squatting positions, even nonpregnant volunteers were exhausted, having to remain immobile during the 10 min of image acquisition. Another limitation of this study is that it is not always possible to reproduce the identical plane for measuring distances when the patient is changing positions, particularly in the axial plane. However, measurement of a diameter remains the same irrespective of the exact plane.
The present study can be considered as preliminary study that can provide us with a view about the advantage of MR images in predicting risk of dystocia in pregnant women. Furthermore, to achieve an increased reliability of MR 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.
Conflicts of interest
There are no conflicts of interest.
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Keywords:© 2012 Lippincott Williams & Wilkins, Inc.
MRI; pelvimetry; pregnancy