mr obstetric pelvimetry

8/3/2019 MR Obstetric Pelvimetry

http://slidepdf.com/reader/full/mr-obstetric-pelvimetry 1/5



1064

AJR:179, October 2002

Michel et al.

used to allow patients to maintain the upright position

in the scanner. With patients in the hand-to-knee posi-

tion, 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 theback 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 se-

quence 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 us-

ing the following parameters: TR/TE, 150/8.5; flip

angle, 60°; field of view, 30–34 cm; slice thick-

ness, 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 posi-

tioning, was less than 60 min in all cases.

Image Analysis

The obstetric conjugate; sagittal outlet; and in-

terspinous, intertuberous, and transverse diameters

were measured on the MR console by the same ra-

diology 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. 2 and 3). The transverse diameter (trans-

verse 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-

rous groups using the Mann-Whitney test.

A p value

of less than 0.05 was considered statistically signifi-

cant. 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 diag-

nostic quality images in every volunteer, al-

though 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 squat-

ting positions than in the supine position (3 ± 5

mm, p = 0.002 and 2 ± 5 mm, p = 0.01, respec-

tively). 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

BA

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

C



MR Obstetric Pelvimetry


1065

BA

Fig. 2.—Pelvimetric diameters. (Drawings by Roth P)A–D, Drawings show interspinous diameter (A), trans-verse diameter (B), intertuberous diameter (C), and ob-stetric conjugate and sagittal outlet (D).

DC

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.

BA

D

C

FE



1066


Michel et al.

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 posi-

tion. The obstetric conjugate was the only pa-

rameter 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 su-

pine 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 squat-

ting 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 birth-

ing positions reached statistical significance.

The Spearman’s rank correlation coefficienttest 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 squat-

ting position: the postural difference in the ob-

stetric conjugate was greater in younger

women (

p = 0.05). The data also showed a cor-

relation 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 ob-

stetric 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 andsquatting positions than in the supine position,

as was the intertuberous diameter in the squat-

ting position. The obstetric conjugate was the

only dimension to be significantly smaller in

the upright squatting position than in the su-

pine position.

Our data confirm those published by Russell

[6], who found a significant increase in inter-

spinous diameter in the last trimester of preg-

nancy 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 ra-

diographic pelvimetry; however, those authors

attributed this result to the limited size of their

study population (25 assessable views).

The transverse diameter did not change sig-

nificantly 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 sub-

ject to such influence. Clinically, a shorter ob-

stetric 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

TABLE 1Pelvic Measurements for 35 Women in Supine, Hand-to-Knee, andSquatting Positions

Parameters

Supine Hand-to-Knee Squatting

Mean ± SD

(cm)

Range

(cm)

Mean ± SD

(cm)

Range

(cm)

Mean ± SD

(cm)

Range

(cm)

Obstetric conjugate 12.4 ± 0.9 10.7–14.6 12.4 ± 0.8 10.5–14.0 12.3 ± 0.8 10.6–13.7Sagittal outlet 11.5 ± 1.3 9.5–14.3 11.8 ± 1.3 9.6–14.6 11.7 ± 1.3 9.4–14.5

Interspinous diameter 11.0 ± 0.7 9.7–12.4 11.6 ± 1.1 10.1–14.4 11.7 ± 1.0 10.0–14.7

Intertuberous diameter 12.4 ± 1.1 10.1–15.5 12.5 ± 0.8 11.2–14.5 12.7 ± 0.8 11.3–14.6

Transverse diameter 12.9 ± 0.7 11.7–14.4 12.8 ± 0.7 11.8–14.0 12.8 ± 0.8 11.3–14.3

Diameters

D i s t a n c e ( c m )

13.0

12.5

12.0

11.5

11.0

10.5

O b s t e t r i c

c o n j u g a t e

S a g i t t a l

o u t l e t

I n t e r s p i n o u s

d i a m e t e r

I n t e r t u b e r o u s

d i a m e t e r

T r a n s v e r s e

d i a m e t e r

Squatting position

Hand-to-knee position

Supine position

Fig. 4.—Graph shows mean values of obstetric conjugate; sagittal outlet; and inter-spinous, intertuberous, and transverse diameters (cm) in three positions.

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 = obstetricconjugate, SO = sagittal outlet, ISD = interspinous diameter, ITD = intertuberous diam-eter, TD = transverse diameter.



MR Obstetric Pelvimetry


1067

effect of gravity. Metaanalyses of birthing po-

sition studies suggest that the benefits of up-

right posture include a shorter second stage of

labor, a small reduction in assisted deliveries,

and a decreased episiotomy rate but an in-

creased risk of severe blood loss [4, 5]. The ad-

vantages of the traditional supine and left

lateral positions include better patient ac-

cess—for example, for administering an anes-

thetic [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 ap-

proximately 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 in-

cluded no pregnant women. We made this de-

cision for two reasons: the limited space in

the scanner bore (upright scanning is techni-

cally impossible for a woman in late preg-

nancy) 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 dur-

ing 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, 19–25].

However, changes in pelvic dimensions ob-

served in nonpregnant women should becomeeven more pronounced during delivery.

Another possible limitation to our meth-

odology is that it is not always possible to re-

produce the identical plane for measuring

distances when the patient is changing posi-

tions, particularly in the axial plane. How-

ever, 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 ex-

aminations [26–29].

Our study shows that MR pelvimetry can beused not only for individual clinical decision

making—for example, in cephalopelvic dispro-

portion—but also as a new research tool in ob-

stetric physiology. Our results indicate that

differences in posture can significantly increase

female pelvic dimensions and thus provide ob-

jective confirmation for time-honored parturient

experience of the advantages of changing birth-

ing 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 posi-

tions; and Renate Huch, Department of Obstet-

rics, for critical review of the study design.

References

1. Gardosi J, Sylvester S, B-Lynch C. Alternative posi-

tions in the second stage of labour: a randomized con-

trolled trial. Br J Obstet Gynaecol 1989

;96:1290–1296

2. Gardosi J, Hutson N, B-Lynch C. Randomised,

controlled trial of squatting in the second stage of

labour.

Lancet

1989

;2:74–77

3. Gupta JK, Glanville JN, Johnson N, Lilford RJ,

Dunham RJ, Watters JK. The effect of squatting

on pelvic dimensions. Eur J Obstet Gynecol Re-

prod Biol 1991

;42:19–22

4. Kelly FW, Terry R, Naglieri R. A review of alter-

native birthing positions. J Am Osteopath Assoc

1999

;99:470–474

5. Gupta JK, Nikodem VC. Woman’s position dur-

ing second stage of labour. Cochrane Database

Syst Rev

2000

;2:CD002006

6. Russell JG. Moulding of the pelvic outlet. J Ob-

stet Gynaecol Br Commonw

1969

;76:817–820

7. Pfammatter T, Marincek B, von Schulthess GK,

Dudenhausen JW. MR-pelvimetrische Referen-

zwerte. Rofo Fortschr Geb Rontgenstr Neuen

Bildgeb Verfahr

1990

;153:706–710

8. al Ahwani S, Assem M, al Mahallawi N, Abdel Hamid

H. Magnetic resonance imaging of the female bony

pelvis: MRI pelvimetry.

J Belge Radiol1991

;74:15–18

9. Wright AR, English PT, Cameron HM, Wilsdon

JB. MR pelvimetry: a practical alternative. Acta

Radiol 1992

;33:582–587

10. Wentz KU, Lehmann KJ, Wischnik A, et al. Pel-

vimetry using various magnetic resonance tomog-

raphy techniques vs. digital image enhancement

radiography: accuracy, time requirement and en-

ergy exposure [in German]. Geburtshilfe Frauen-

heilkd 1994

;54:204–212

11. van Loon AJ, Mantingh A, Serlier EK, Kroon G,

Mooyaart EL, Huisjes HJ. Randomised controlled

trial of magnetic-resonance pelvimetry in breech

presentation at term. Lancet 1997

;350:1799–1804

12. Tukeva TA, Aronen HJ, Karjalainen PT, Makela

PJ. Low-field MRI pelvimetry. Eur Radiol

1997

;7:230–234

13. Levine D, Barnes PD, Edelman RR. Obstetric

MR imaging. Radiology

1999

;211:609–617

14. Fielding JR, Griffiths DJ, Versi E, Mulkern RV,

Lee ML, Jolesz FA. MR imaging of pelvic floor

continence mechanisms in the supine and sitting

positions. AJR

1998

;171:1607–1610

15. Hilfiker PR, Debatin JF, Schwizer W, Schoenen-

berger AW, Fried M, Marincek B. MR defecogra-

phy: depiction of anorectal anatomy and pathology.

J Comput Assist Tomogr

1998

;22:749–755

16. Wong TZ, Silverman SG, Fielding JR, Tempany

CM, Hynynen K, Jolesz FA. Open-configuration

MR imaging, intervention, and surgery of the uri-

nary tract. Urol Clin North Am 1998

;25:113–122

17. Aronson D, Kier R. CT pelvimetry: the foveae are

not an accurate landmark for the level of the is-

chial spines. AJR

1991

;156:527–530

18. Stark DD, McCarthy SM, Filly RA, Parer JT, Hri-

cak H, Callen PW. Pelvimetry by magnetic reso-

nance imaging. AJR 1985

;144:947–950

19. Lehmann KJ, Wischnik A, Zahn K, Georgi M. Do

the obstetrically relevant bony pelvic measure-

ments change? a retrospective analysis of com-

puted tomographic pelvic x-rays [in German].

Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Ver-

fahr

1992

;156:425–428

20. Bryant Greenwood G, Ali S, Mandel M, Green-

wood F. Ovarian and decidual relaxins in human

pregnancy. Adv Exp Med Biol 1987

;219:709–713

21. Charlton WP, Coslett-Charlton LM, Ciccotti MG.

Correlation of estradiol in pregnancy and anterior cru-

ciate ligament laxity. Clin Orthop

2001

;387:165–170

22. Schauberger CW, Rooney BL, Goldsmith L,

Shenton D, Silva PD, Schaper A. Peripheral joint

laxity increases in pregnancy but does not corre-

late with serum relaxin levels. Am J Obstet Gy-

necol 1996

;174:667–671

23. MacLennan AH, Nicolson R, Green RC. Serum

relaxin in pregnancy. Lancet 1986;2:241–243

24. Bjorklund K, Lindgren PG, Bergstrom S, Ulmsten

U. Sonographic assessment of symphyseal joint

distention intra partum. Acta Obstet Gynecol

Scand 1997;76:227–232

25. Bjorklund K, Nordstrom ML, Bergstrom S. Sono-

graphic assessment of symphyseal joint distention

during pregnancy and post partum with special refer-

ence to pelvic pain. Acta Obstet Gynecol Scand

1999;78:125–130

26. Varner MW, Cruikshank DP, Laube DW. X-ray

pelvimetry in clinical obstetrics. Obstet Gynecol

1980;56:296–300

27. Lao TT, Chin RK, Leung BF. Is X-ray pelvimetry

useful in a trial of labour after caesarean section? Eur

J Obstet Gynecol Reprod Biol 1987;24:277–283

28. Federle MP, Cohen HA, Rosenwein MF, Brant

Zawadzki MN, Cann CE. Pelvimetry by digital

radiography: a low-dose examination. Radiology

1982;143:733–735

29. Raman S, Samuel D, Suresh K. A comparative

study of X-ray pelvimetry and CT pelvimetry.

Aust N Z J Obstet Gynaecol 1991;31:217–220

mr obstetric pelvimetry

Documents