Because levator ani (LA) muscle injuries occur in approximately 13% of all vaginal births, insights are needed to better prevent them. In Part I of this paper, we conducted an analysis of the bony and soft tissue factors contributing to the geometric “capacity” of the maternal pelvis and pelvic floor to deliver a fetal head without incurring stretch injury of the maternal soft tissue. In Part II, we quantified the range in demand, represented by the variation in fetal head size and shape, placed on the maternal pelvic floor. In Part III, we analyzed the capacity-to-demand geometric ratio, g, in order to determine whether a mother can deliver a head of given size without stretch injury. The results of a Part I sensitivity analysis showed that initial soft tissue loop length (SL) had the greatest effect on maternal capacity, followed by the length of the soft tissue loop above the inferior pubic rami at ultimate crowning, then subpubic arch angle (SPAA) and head size, and finally the levator origin separation distance. We found the more caudal origin of the puborectal portion of the levator muscle helps to protect it from the stretch injuries commonly observed in the pubovisceral portion. Part II fetal head molding index (MI) and fetal head size revealed fetal head circumference values ranging from 253 to 351 mm, which would increase up to 11 mm upon face presentation. The Part III capacity-demand analysis of g revealed that, based on geometry alone, the 10th percentile maternal capacity predicted injury for all head sizes, the 25th percentile maternal capacity could deliver half of all head sizes, while the 50th percentile maternal capacity could deliver a head of any size without injury. If ultrasound imaging could be operationalized to make measurements of ratio g, it might be used to usefully inform women on their level of risk for levator injury during vaginal birth.

References

References
1.
Lien
,
K.-C.
,
DeLancey
,
J. O. L.
, and
Ashton-Miller
,
J. A.
,
2009
, “
Biomechanical Analyses of the Efficacy of Patterns of Maternal Effort on Second-Stage Progress
,”
Obstet. Gynecol.
,
113
(
4
), pp.
873
880
.
2.
Silva
,
M. E. T.
,
Oliveira
,
D. A.
,
Roza
,
T. H.
,
Brandao
,
S.
,
Parente
,
M. P. L.
,
Mascarenhas
,
T.
, and
Natal Jorge
,
R. M.
,
2015
, “
Study on the Influence of the Fetus Head Molding on the Biomechanical Behavior of the Pelvic Floor Muscles, During Vaginal Delivery
,”
J. Biomech.
,
48
(
9
), pp.
1600
1605
.
3.
Yan
,
X.
,
Kruger
,
J. A.
,
Nielsen
,
P. M.
, and
Nash
,
M. P.
,
2015
, “
Effects of Fetal Head Shape Variation on the Second Stage of Labour
,”
J. Biomech.
,
48
(
9
), pp.
1593
1599
.
4.
Shek
,
K.
, and
Dietz
,
H.
,
2010
, “
Intrapartum Risk Factors for Levator Trauma
,”
BJOGP
,
117
(
12
), pp.
1485
1492
.
5.
Margulies
,
R.
,
Huebner
,
M.
, and
DeLancey
,
J.
,
2007
, “
Origin and Insertion Points Involved in Levator Ani Muscle Defects
,”
Am. J. Obstet. Gynecol.
,
196
(
3
), pp.
251
255
.
6.
Mant
,
J.
,
Painter
,
R.
, and
Vessey
,
M.
,
1997
, “
Epidemiology of Genital Prolapse: Observations From the Oxford Planning Association Study
,”
Br. J. Obstet. Gynaecol.
,
104
(
5
), pp.
579
585
.
7.
Ashton-Miller
,
J. A.
, and
DeLancey
,
J. O.
,
2009
, “
On the Biomechanics of Vaginal Birth and Common Sequelae
,”
Annu. Rev. Biomed. Eng.
,
11
(
1
), pp.
163
176
.
8.
Betschart
,
C.
,
Kim
,
J.
,
Miller
,
J. M.
,
Ashton-Miller
,
J. A.
, and
DeLancey
,
J. O. L.
,
2014
, “
Comparison of Muscle Fiber Directions Between Different Levator Ani Muscle Subdivisions: In Vivo MRI Measurements in Women
,”
Int. Urogynecol. J.
,
25
(
9
), pp.
1263
1268
.
9.
Kearney
,
R.
,
Sawhney
,
R.
, and
DeLancey
,
J.
,
2004
, “
Levator Ani Muscle Anatomy Evaluated by Origin-Insertion Pairs
,”
Am. Coll. Obstet. Gynecol.
,
104
(
1
), pp.
168
173
.
10.
Lien
,
K.
,
Mooney
,
B.
,
DeLancey
,
J.
, and
Ashton-Miller
,
J.
,
2004
, “
Levator Ani Muscle Stretch Induced by Simulated Vaginal Birth
,”
Obstet. Gynecol.
,
103
(
1
), pp.
31
40
.
11.
Miller
,
J. M.
,
Brandon
,
C.
,
Jacobson
,
J. A.
,
Low
,
L. K.
,
Zielinski
,
R.
,
Ashton-Miller
,
J.
, and
Delancey
,
J. O.
,
2010
, “
MRI Findings in Patients Considered High Risk for Pelvic Floor Injury Studied Serially After Vaginal Childbirth
,”
Am. J. Roentgenol.
,
195
(
3
), pp.
786
791
.
12.
Brandon
,
C.
,
Jacobson
,
J.
,
Low
,
L.
,
Park
,
L.
,
DeLancey
,
J. O.
, and
Miller
,
J.
,
2012
, “
Pubic Bone Injuries in Primiparous Women: Magnetic Resonance Imaging in Detection and Differential Diagonosis of Structural Injury
,”
Ultrasound Obstet. Gynecol.
,
39
(
4
), pp.
444
451
.
13.
Luo
,
J.
,
Ashton-Miller
,
J. A.
, and
DeLancey
,
J. O. L.
,
2011
, “
A Model Patient: Female Pelvic Anatomy can be Viewed in Diverse 3-Dimensional Images With a New Interactive Tool
,”
Am. J. Obstet. Gynecol.
,
205
(
4
), pp.
391.e1
391.e2
.
14.
Alperin
,
M.
,
Lawley
,
D. M.
,
Esparza
,
M. C.
, and
Lieber
,
R. L.
,
2015
, “
Pregnancy-Induced Adaptations in the Intrinsic Structure of Rat Pelvic Floor Muscles
,”
Am. J. Obstet. Gynecol.
,
213
(
2
), pp.
191
197
.
15.
Brooks
,
S. V.
,
Zerba
,
E.
, and
Faulkner
,
J. A.
,
1995
, “
Injury to Muscle Fibers After Single Stretches of Passive and Maximally Stimulated Muscles in Mice
,”
J. Physiol.
,
488
(
2
), pp.
459
469
.
16.
Kirilova
,
M.
,
Stoytchev
,
S.
,
Pashkouleva
,
D.
, and
Kavardzhikov
,
V.
,
2011
, “
Experimental Study of the Mechanical Properties of Human Abdominal Fascia
,”
Med. Eng. Phys.
,
33
(
1
), pp.
1
6
.
17.
Kim
,
J.
,
Ramanah
,
R.
,
DeLancey
,
J.
, and
Ashton-Miller
,
J. A.
,
2011
, “
On the Anatomy and Histology of the Pubovisceral Muscle Enthesis in Women
,”
Neurourol. Urodyn.
,
30
(
7
), pp.
1366
1370
.
18.
Kim
,
J.
,
Betschart
,
C.
,
Ramanah
,
R.
,
Ashton-Miller
,
J.
, and
DeLancey
,
J.
,
2015
, “
Anatomy of the Pubovisceral Muscle Origin: Macroscopic and Microscopic Findings Within the Injury Zone
,”
Neurourol. Urodyn.
,
34
(
8
), pp.
774
780
.
19.
Larson
,
K.
,
Luo
,
J.
,
Yousuf
,
A.
,
Ashton-Miller
,
J.
, and
DeLancey
,
J.
,
2012
, “
Measurement of the 3D Geometry of the Fascial Arches in Women With a Unilateral Levator Defect and Architectural Distortion
,”
Int. Urogynecol. J.
,
23
(
1
), pp.
57
63
.
20.
Handa
,
V. L.
,
Lockhart
,
M. E.
,
Fielding
,
J. R.
,
Bradley
,
C. S.
,
Brubaker
,
L.
,
Cundiff
,
G. W.
,
Ye
,
W.
, and
Richter
,
H. E.
,
2008
, “
Racial Differences in Pelvic Anatomy by Magnetic Resonance Imaging
,”
Obstet. Gynecol.
,
111
(
4
), pp.
914
920
.
21.
Albrich
,
S.
,
Shek
,
K.
,
Krahn
,
U.
, and
Dietz
,
H.
,
2015
, “
Measurement of the Subpubic Arch Angle by 3D Translabial Ultrasound and Its Impact on Vaginal Delivery
,”
Ultrasound Obstet. Gynecol.
,
46
(
4
), pp.
496
500
.
22.
Centers for Disease Control and Prevention
,
2010
, “
Data Table for Boys Weight-for-Length and Head Circumference-for-Age Charts
,” Last accessed June 18, 2015, http://www.cdc.gov/growthcharts/who/boys_weight_head_circumference.htm
23.
Centers for Disease Control and Prevention
,
2010
, “
Data Table for Girls Weight-for-Length and Head Circumference-for-Age Charts
,” Last accessed June 18, 2015, http://www.cdc.gov/growthcharts/who/girls_weight_head_circumference.htm
24.
Sorbe
,
B.
, and
Dahlgren
,
S.
,
1983
, “
Some Important Factors in the Molding of the Fetal Head During Vaginal Delivery—A Photographic Study
,”
Int. J. Gynaecol. Obstet.
,
21
(
3
), pp.
205
212
.
25.
Baxter
,
J.
,
1946
, “
Moulding of the Foetal Head: A Compensatory Mechanism
,”
J. Obstet. Gynaecol. Br. Emp.
,
53
(
3
), pp.
212
218
.
26.
Kriewall
,
T. J.
,
Stys
,
S. J.
, and
McPherson
,
G. K.
,
1977
, “
Neonatal Head Shape After Delivery: An Index of Molding
,”
J. Perinat. Med.
,
5
(
6
), pp.
260
267
.
27.
Moloy
,
H. C.
,
1942
, “
Studies on Head Molding During Labor
,”
Am. J. Obstet. Gynecol.
,
76
, pp.
762
782
.
28.
Gardberg
,
M.
, and
Tuppurainen
,
M.
,
1994
, “
Persistent Occiput Posterior Presentation—A Clinical Problem
,”
Acta Obstet. Gynecol. Scand.
,
73
(
1
), pp.
45
47
.
29.
Borell
,
U.
, and
Fernstrom
,
I.
,
1958
, “
Die Umformung des Kindlichen Kopfes Wahrend Normaler Entbingen in Regelrechter Hinterhauptslag
,”
Geburtshilfe Frauenheilkd.
,
18
(
9
), pp.
1156
1166
.
30.
Lowder
,
J.
,
Burrows
,
L.
,
Krohn
,
M.
, and
Weber
,
A.
,
2007
, “
Risk Factors for Primary and Subsequent Anal Sphincter Lacerations: A Comparison of Cohorts by Parity and Prior Mode of Delivery
,”
Am. J. Obstet. Gynecol.
,
196
(
4
), pp.
344
348
.
31.
Jing
,
D.
,
2010
, “
Experimental and Theoretical Biomechanical Analyses of the Second Stage of Labor
,” Last accessed Aug. 21, 2015, http://deepblue.lib.umich.edu/handle/2027.42/76013
32.
Lowder
,
J. L.
,
Debes
,
K. M.
,
Moon
,
D. K.
,
Howden
,
N.
,
Abramowitch
,
S. D.
, and
Moalli
,
P. A.
,
2007
, “
Biomechanical Adaptations of the Rat Vagina and Supportive Tissues in Pregnancy to Accommodate Delivery
,”
Obstet. Gynecol.
,
109
(
1
), pp.
136
143
.
33.
Kriewall
,
T. J.
,
1982
, “
Structural, Mechanical, and Material Properties of Fetal Cranial Bone
,”
Am. J. Obstet. Gynecol.
,
143
(
6
), pp.
707
714
.
34.
Margulies
,
S. S.
, and
Thibault
,
K. L.
,
2000
, “
Infant Skull and Suture Properties: Measurements and Implications for Mechanisms of Pediatric Brian Injury
,”
ASME J. Biomech. Eng.
,
122
(
4
), pp.
364
371
.
35.
Ergaz
,
U.
,
Goldstein
,
I.
,
Divon
,
M.
, and
Weiner
,
Z.
,
2015
, “
A Preliminary Study of Three-Dimensional Sonographic Measurements of the Fetus
,”
Rambam Maimonides Med. J.
,
6
(
2
), p.
e0019
.
36.
Maharaj
,
D.
,
2010
, “
Assessing Cephalopelvic Disproportion: Back to the Basics
,”
Obstet. Gynecol. Surv.
,
65
(
6
), pp.
387
395
.
37.
Beckmann
,
M. M.
, and
Stock
,
O. M.
,
2013
, “
Antenatal Perineal Massage for Reducing Perineal Trauma
,”
Cochrane Database Syst. Rev.
,
4
, p.
CD005123
.
38.
Grobman
,
W.
,
Lai
,
Y.
,
Landon
,
M.
,
Spong
,
C.
,
Leveno
,
K.
,
Rouse
,
D.
,
Varner
,
M.
,
Moawad
,
A.
,
Caritis
,
S.
,
Harper
,
M.
,
Wapner
,
R.
,
Sorokin
,
Y.
,
Miodovnik
,
M.
,
Carpenter
,
M.
,
O'Sullivan
,
M.
,
Sibai
,
B.
,
Langer
,
O.
,
Thorp
,
J.
,
Ramin
,
S.
,
Mercer
,
B.
, and
NICHD
,
2007
, “
Development of a Nomogram for Prediction of Vaginal Birth After Cesarean Delivery
,”
Obstet. Gynecol.
,
109
(
4
), pp.
806
812
.
39.
Siafarikas
,
F.
,
Staer-Jensen
,
J.
,
Hilde
,
G.
,
,
K.
, and
Ellstrom
,
E. M.
,
2014
, “
Levator Hiatus Dimensions in Late Pregnancy and the Process of Labor: A 3- and 4-Dimensional Transperinael Ultrasound Study
,”
Am. J. Obstet. Gynecol.
,
210
(
5
), pp.
484
491
.
You do not currently have access to this content.