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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
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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
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AJR:179, October 2002
MR Obstetric Pelvimetry
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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
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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
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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.
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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
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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.
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