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Downloaded from www.ajronline.org by 82.32.41.86 on 12/07/15 from IP address 82.32.41.86. Copyright ARRS. For personal use only; all rights reserved MR Obstetric Pelvimetry: Effect of Birthing Position on Pelvic Bony Dimensions Sven C. A. Michel 1 Annett Rake 2 Karl Treiber 1 Burkhardt Seifert 3 Rabih Chaoui 2 Renate Huch 2 Borut Marincek 1 Rahel A. Kubik-Huch 1,4 OBJECTIVE. The aim of our study was to measure the impact of supine and upright birthing positions on MR pelvimetric dimensions. MATERIALS AND METHODS. MR pelvimetry was performed in 35 nonpregnant female volunteers in an open 0.5-T MR imaging system with patients in the supine, hand-to-knee, and squatting positions. The obstetric conjugate; sagittal outlet; and interspinous, intertuberous, and transverse diameters were compared among positions. RESULTS. With patients in the hand-to-knee and squatting positions, the sagittal outlet (11.8 ± 1.3 cm and 11.7 ± 1.3 cm) exceeded that in the supine position (11.5 ± 1.3 cm; p = 0.002 and p = 0.01, respectively), as did the interspinous diameter (11.6 ± 1.1 cm and 11.7 ± 1.0 cm vs 11.0 ± 0.7 cm; p < 0.0001, in both cases). Intertuberous diameter was wider with patients in the squatting position than in the supine position (12.7 ± 0.8 cm vs 12.4 ± 1.1 cm; p = 0.01). Only the obstetric conjugate was smaller with patients in the upright squatting position than in the supine position (12.3 ± 0.8 cm vs 12.4 ± 0.9 cm; p = 0.01). Transverse diameter did not change significantly in any position. CONCLUSION. An upright birthing position significantly expands female pelvic bony dimensions, suggesting facilitation of labor and delivery. T Received October 11, 2001; accepted after revision March 18, 2002. Supported in part by a grant from the EMDO Foundation, Zürich, Switzerland. 1 Institute of Radiology, University Hospital, Rämistra. 100, 8091 Zürich, Switzerland. 2 Clinic of Obstetrics, University Hospital, 8091 Zürich, Switzerland. 3 Department of Biostatistics, University of Zurich, Sumatrastr. 30, 8006 Zürich, Switzerland. 4 Present address: Institute of Radiology, Cantonal Hospital Baden, CH-5404 Baden, Switzerland. Address correspondence to R. A. Kubik-Huch. AJR 2002;179:1063–1067 0361–803X/02/1794–1063 © American Roentgen Ray Society AJR:179, October 2002 he respective merits of supine versus upright (e.g., squatting, hand-to-knee, and sitting in birthing stools) birthing positions have been debated for centuries, with concerns ranging from the strictly scientific to the modish or politically correct [1–5]. An accurate characterization of the impact of posture on pelvic bony dimensions, however, has been lacking, although in 1969, using conventional outlet radiography, Russell [6] reported that a change from the supine to the sitting position significantly increased interspinous diameter both in the last trimester of pregnancy and 6 weeks after childbirth. Today, not only has MR imaging become the imaging modality of choice for assessing the maternal bony pelvis [7–13], but also vertically open configuration magnet systems no longer restrict the examination to patients in the supine position. Imaging with the patient in the sitting position has already been used for assessment of the female pelvic floor, defecography, and interventional MR imaging [14–16]. Our aim was to determine whether female pelvic outlet dimensions obtained in an open 0.5-T system differ with birthing positions. Materials and Methods Subjects The study population comprised 35 nonpregnant female volunteers 22–43 years old (mean ± SD, 28 ± 5 years), each of whom provided their informed written consent after receiving a full explanation of the examination procedure. The study protocol was approved by our institutional review board. The women were recruited into two groups: a nulliparous group (n = 25; age range, 22–35 years; mean age, 27 ± 4 years; height range, 157–181 cm; mean height, 166 ± 5 cm; weight range, 48–72 kg; mean weight, 58 ± 6 kg; mean body mass index, 21 ± 3 kg/m2) and a parous group (n = 10; age range, 27–43 years; mean age, 33 ± 4 years, p = 0.0008 vs nulliparous women; height range, 164– 175 cm; mean height, 170 ± 3 cm; weight range, 52–69 kg; mean weight, 60 ± 5 kg; mean body mass index, 21 ± 2 kg/m2). Nine parous women had one child; one had two children. All had delivered vaginally at least 9 months before inclusion. Imaging Technique A 0.5-T low-field vertically open configuration magnet system (Signa SP; General Electric Medical Systems, Milwaukee, WI) was used with the body flex surface coil. Imaging was performed with patients in the supine, hand-to-knee, and squatting positions (Fig. 1). A special wooden construction was 1063 Downloaded from www.ajronline.org by 82.32.41.86 on 12/07/15 from IP address 82.32.41.86. Copyright ARRS. For personal use only; all rights reserved Michel et al. used to allow patients to maintain the upright position in the scanner. With patients in the hand-to-knee position, the knees were situated in the bore of the system, with the elbows resting on a shelf to simulate a typical labor position and to maintain the position during scanning. To avoid displacement, we fixed the body flex coil to the clothing when imaging with the patient in the hand-to-knee position and to the clothes on the back or to a cushion between the legs when imaging the subject in the squatting position. During scanning pauses, the women sat on this cushion to rest. A T1-weighted fast spoiled gradient-echo sequence was performed with the patient in the midsagittal, axial, and oblique (in the plane of sacral promontory to the top of the symphysis) planes using the following parameters: TR/TE, 150/8.5; flip angle, 60°; field of view, 30–34 cm; slice thickness, 5 mm; gap, 0 mm; number of excitations, 2; matrix, 256 × 192; and bandwidth, 21 kHz. Each sequence took approximately 3 min to acquire, and the total individual study time, including positioning, was less than 60 min in all cases. Image Analysis The obstetric conjugate; sagittal outlet; and interspinous, intertuberous, and transverse diameters were measured on the MR console by the same radiology technician. The obstetric conjugate and the sagittal outlet were both assessed in the midsagittal plane. The interspinous and intertuberous diameters were assessed in the axial plane [17, 18] (Figs. 2 and 3). The transverse diameter (transverse pelvic inlet) was assessed on oblique images acquired in a plane from the sacral promontory to the top of the symphysis [10]. Statistical Analysis Continuous variables were presented as means and standard deviations. Absolute pelvic measurements in the three positions and the differences between them were compared using Wilcoxon’s signed rank test with Bonferroni’s adjustment. The data were tested for correlation with body weight, body mass index, and age using Spearman’s rank correlation coefficient and for differences between the nulliparous and pa- A rous groups using the Mann-Whitney test. A p value of less than 0.05 was considered statistically significant. Statistical analysis was performed using Stat view 5.0.1 software (SAS Institute, Cary, NC). Results MR pelvimetry in the three positions proved feasible in all subjects, yielding diagnostic quality images in every volunteer, although the hand-to-knee and squatting positions were found difficult to maintain. Dimensions in the three positions are listed in Table 1 and plotted in Figure 4. The sagittal outlet was wider in the hand-to-knee and squatting positions than in the supine position (3 ± 5 mm, p = 0.002 and 2 ± 5 mm, p = 0.01, respectively). The interspinous diameter was greater in the hand-to-knee and squatting positions than in the supine position (6 ± 7 mm and 8 ± 7 mm; p < 0.0001 in both cases). Intertuberous B C Fig. 1.—MR pelvimetry in vertical open configuration magnet system. (Drawings by Roth P) A, Photograph shows female volunteer in hand-to-knee position (rear view). B, Drawing illustrates patient in hand-to-knee position in labor. C, Photograph shows female volunteer in squatting position (lateral view). D, Drawing illustrates patient in squatting position in labor. D 1064 AJR:179, October 2002 MR Obstetric Pelvimetry Downloaded from www.ajronline.org by 82.32.41.86 on 12/07/15 from IP address 82.32.41.86. Copyright ARRS. For personal use only; all rights reserved Fig. 2.—Pelvimetric diameters. (Drawings by Roth P) A–D, Drawings show interspinous diameter (A), transverse diameter (B), intertuberous diameter (C), and obstetric conjugate and sagittal outlet (D). A B C D A B C D E F Fig. 3.—T1-weighted MR images show pelvimetric diameters in 24-year-old woman from nullipara group. A and B, MR images obtained in supine position show interspinous (A) and intertuberous (B) diameters. C and D, MR images obtained in hand-to-knee position show interspinous (C) and intertuberous (D) diameters. E and F, MR images obtained in squatting position show interspinous (E) and intertuberous (F) diameters. AJR:179, October 2002 1065 Michel et al. TABLE 1 Pelvic Measurements for 35 Women in Supine, Hand-to-Knee, and Squatting Positions Hand-to-Knee Squatting Parameters Mean ± SD (cm) Range (cm) Mean ± SD (cm) Range (cm) Mean ± SD (cm) Range (cm) Obstetric conjugate Sagittal outlet Interspinous diameter Intertuberous diameter Transverse diameter 12.4 ± 0.9 11.5 ± 1.3 11.0 ± 0.7 12.4 ± 1.1 12.9 ± 0.7 10.7–14.6 9.5–14.3 9.7–12.4 10.1–15.5 11.7–14.4 12.4 ± 0.8 11.8 ± 1.3 11.6 ± 1.1 12.5 ± 0.8 12.8 ± 0.7 10.5–14.0 9.6–14.6 10.1–14.4 11.2–14.5 11.8–14.0 12.3 ± 0.8 11.7 ± 1.3 11.7 ± 1.0 12.7 ± 0.8 12.8 ± 0.8 10.6–13.7 9.4–14.5 10.0–14.7 11.3–14.6 11.3–14.3 diameter was greater in the squatting position than in the supine position (3 ± 7 mm, p = 0.01) but not greater than in the hand-to-knee position. The obstetric conjugate was the only parameter to be significantly smaller in the upright squatting position than in the supine position (2 ± 4 mm, p = 0.01) but not in the hand-to-knee position. Transverse diameter did not change significantly in any position. The differences in each parameter between the supine and the two upright positions are plotted in Figure 5. Parous women were significantly ( p = 0.0008) older than nulliparous women, with slightly larger pelvic measurements, but only the difference in sagittal outlet in the squatting position was statistically significant (12.4 ± 1.1 cm vs 11.5 ± 1.3 cm, p = 0.04). None of the differences in the effect of birthing positions reached statistical significance. The Spearman’s rank correlation coefficient test showed no influence of body weight, body mass index, or age on absolute pelvic mea- surements in the supine position. However, age minimized the effect of changing to the squatting position: the postural difference in the obstetric conjugate was greater in younger women ( p = 0.05). The data also showed a correlation with body height in that taller women had a greater increase in interspinous diameter on changing from the supine to the hand-toknee position ( p = 0.03). Changes in the obstetric conjugate were also dependent on height, with differences when changing from the supine to the hand-to-knee position being greater in taller women ( p = 0.05). Discussion Our results show 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 squat- 13.0 Distance (cm) 12.5 12.0 11.5 11.0 Squatting position Hand-to-knee position Supine position Transverse diameter Intertuberous diameter Interspinous diameter Sagittal outlet 10.5 Obstetric conjugate Downloaded from www.ajronline.org by 82.32.41.86 on 12/07/15 from IP address 82.32.41.86. Copyright ARRS. For personal use only; all rights reserved Supine ting position. The obstetric conjugate was the only dimension to be significantly smaller in the upright squatting position than in the supine position. Our data confirm those published by Russell [6], who found a significant increase in interspinous diameter in the last trimester of pregnancy and after childbirth on changing from the supine to the sitting position. On the other hand, our data contrast with those of Gupta et al. [3], who found no significant change in inlet and outlet dimensions between patients in the sitting and squatting positions using lateral radiographic pelvimetry; however, those authors attributed this result to the limited size of their study population (25 assessable views). The transverse diameter did not change significantly in any position, and the obstetric conjugate was the only parameter to be smaller with patients in the squatting position than in the supine position. The abducted femora act as levers on flexion, opening the outlet. These changes are purely postural [6]. One reason that neither the obstetric conjugate nor the transverse diameter increased with patients in either upright position could be that these are both pelvic inlet parameters and thus less subject to such influence. Clinically, a shorter obstetric conjugate during squatting may delay the first stage of labor, during which the fetal head enters the pelvis and rotates. Although, to our knowledge, previous anatomic evidence of the increase in pelvic dimensions was limited, clinical trials had hinted at the benefits of the upright position in the second stage of labor— that is, from full dilatation of the cervix. In part, however, these were also attributed to the Diameters Fig. 4.—Graph shows mean values of obstetric conjugate; sagittal outlet; and interspinous, intertuberous, and transverse diameters (cm) in three positions. 1066 Fig. 5.—Box plot of pelvimetric differences in changing from supine to hand-to-knee (first bar in each set ) to squatting (second bar in each set) positions. OC = obstetric conjugate, SO = sagittal outlet, ISD = interspinous diameter, ITD = intertuberous diameter, TD = transverse diameter. AJR:179, October 2002 Downloaded from www.ajronline.org by 82.32.41.86 on 12/07/15 from IP address 82.32.41.86. Copyright ARRS. For personal use only; all rights reserved MR Obstetric Pelvimetry effect of gravity. Metaanalyses of birthing position studies suggest that the benefits of upright posture include a shorter second stage of labor, a small reduction in assisted deliveries, and a decreased episiotomy rate but an increased risk of severe blood loss [4, 5]. The advantages of the traditional supine and left lateral positions include better patient access—for example, for administering an anesthetic [4]. It can also be physically stressful for the patient to maintain the squatting position for a long time [4]. Indeed, all the participants in our study, despite being young and fit, found it exhausting to hold the same position for approximately 10 min during image acquisition. In some cases, image quality was impaired by motion artifacts because of trembling. A limitation of our study is that we included no pregnant women. We made this decision for two reasons: the 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 by having to remain immobile during the 10 min of image acquisition). On these ethical grounds, we even extended our noninclusion criteria to recent parturients. We are aware that this limitation prevented us from measuring the influence of pregnancyrelated joint laxity in late gestation, for which there is ample documentation [6, 19–25]. However, changes in pelvic dimensions observed in nonpregnant women should become even more pronounced during delivery. Another possible limitation to our methodology 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. MR imaging has become widely accepted as the imaging modality of choice for obstetric pelvimetry [7, 11–18], although gynecologic reference values are based on radiographic examinations [26–29]. Our study shows that MR pelvimetry can be used not only for individual clinical decision making—for example, in cephalopelvic disproportion—but also as a new research tool in obstetric physiology. Our results indicate that AJR:179, October 2002 differences in posture can significantly increase female pelvic dimensions and thus provide objective confirmation for time-honored parturient experience of the advantages of changing birthing position to facilitate vaginal birth. Acknowledgments We thank the following colleagues at Zurich University Hospital: Peter Roth, Department of Neurosurgery, for the drawings in Figures 1 and 2; Anni Meier and Nino Teodorovic, Institute of Diagnostic Radiology, for technical assistance; Regina Grimm for instruction in birthing positions; and Renate Huch, Department of Obstetrics, for critical review of the study design. References 1. Gardosi J, Sylvester S, B-Lynch C. Alternative positions in the second stage of labour: a randomized controlled trial. 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