AJR 2002; 179:1063-1067
© American Roentgen Ray Society
MR Obstetric Pelvimetry: Effect of Birthing Position on Pelvic Bony Dimensions
Sven C. A. Michel1,
Annett Rake2,
Karl Treiber1,
Burkhardt Seifert3,
Rabih Chaoui2,
Renate Huch2,
Borut Marincek1 and
Rahel A. Kubik-Huch1,4
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.
Received October 11, 2001;
accepted after revision March 18, 2002.
Supported in part by a grant from the EMDO Foundation, Zürich,
Switzerland.
Address correspondence to R. A. Kubik-Huch.
Abstract
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.
Introduction
The 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
[1,2,3,4,5].
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
[7,8,9,10,11,12,13],
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
[14,15,16].
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.
1A,1B,1C,1D).
A special wooden construction was 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 mid-sagittal, 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 x 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 mid-sagittal plane. The interspinous and intertuberous diameters were
assessed in the axial plane
[17,
18] (Figs.
2A,2B,2C,2D
and
3A,3B,3C,3D,3E,3F).
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].
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Fig. 2A. —Pelvimetric diameters. (Drawings by Roth P) Drawings show
interspinous diameter (A), transverse diameter (B),
intertuberous diameter (C), and obstetric conjugate and sagittal outlet
(D).
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Fig. 2B. —Pelvimetric diameters. (Drawings by Roth P) Drawings show
interspinous diameter (A), transverse diameter (B),
intertuberous diameter (C), and obstetric conjugate and sagittal outlet
(D).
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Fig. 2C. —Pelvimetric diameters. (Drawings by Roth P) Drawings show
interspinous diameter (A), transverse diameter (B),
intertuberous diameter (C), and obstetric conjugate and sagittal outlet
(D).
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Fig. 2D. —Pelvimetric diameters. (Drawings by Roth P) Drawings show
interspinous diameter (A), transverse diameter (B),
intertuberous diameter (C), and obstetric conjugate and sagittal outlet
(D).
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Fig. 3A. —T1-weighted MR images show pelvimetric diameters in
24-year-old woman from nullipara group. MR images obtained in supine position
show interspinous (A) and intertuberous (B) diameters.
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Fig. 3B. —T1-weighted MR images show pelvimetric diameters in
24-year-old woman from nullipara group. MR images obtained in supine position
show interspinous (A) and intertuberous (B) diameters.
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Fig. 3C. —T1-weighted MR images show pelvimetric diameters in
24-year-old woman from nullipara group. MR images obtained in hand-to-knee
position show interspinous (C) and intertuberous (D)
diameters.
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Fig. 3D. —T1-weighted MR images show pelvimetric diameters in
24-year-old woman from nullipara group. MR images obtained in hand-to-knee
position show interspinous (C) and intertuberous (D)
diameters.
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Fig. 3E. —T1-weighted MR images show pelvimetric diameters in
24-year-old woman from nullipara group. MR images obtained in squatting
position show interspinous (E) and intertuberous (F)
diameters.
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Fig. 3F. —T1-weighted MR images show pelvimetric diameters in
24-year-old woman from nullipara group. MR images obtained in squatting
position show interspinous (E) and intertuberous (F)
diameters.
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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 parous 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
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.
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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.
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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 measurements 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-to-knee 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 squatting 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 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 pregnancy-related joint laxity in late gestation, for which there is ample
documentation [6,
19,20,21,22,23,24,25].
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,
11,12,13,14,15,16,17,18],
although gynecologic reference values are based on radiographic examinations
[26,27,28,29].
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 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
1A,1B,1C,1D
and
2A,2B,2C,2D,;
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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Abstract
Introduction
