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BSTETRICS

he force required to rupture fetal membranes paradoxicallyncreases with acute in vitro repeated stretchingishal Pandey, MD; Kellie Jaremko, BS; Robert M. Moore, MS; Brian M. Mercer, MD; Bradley Stetzer, DO;eepak Kumar, MD; Jennifer M. Fox, BS; Joseph M. Mansour, PhD; John J. Moore, MD

BJECTIVE: The purpose of this study was to determine whether acuteepetitive stretching causes fetal membranes (FM) weakening.

TUDY DESIGN: Cesarean or vaginally delivered FM were repeatedlytretched and thereafter subjected to rupture testing. Rupture strengthRS), work to rupture (WR), and stiffness were determined. UnstretchedM were compared with stretched FM.

ESULTS: In the cesarean group, FM stretched to 50% or 75% of theaseline (unstretched) RS for 10-20 cycles of 10 seconds each para-

gional, biochemicallyoi: 10.1016/j.ajog.2006.10.861

ith baseline. Detailed analysis revealed that even a single stretch cy-le initiated these changes to physical properties. Vaginally deliveredM showed similar changes in physical properties, as did separatedmnion.

ONCLUSION: Acute stretch forces do not directly cause FMeakening.

ey words: Amnion, choriodecidua, fetal membrane physical
oxically showed increased RS and stiffness. WR decreased compared
ite this article as: Pandey V, Jaremko K, Moore RM, et al. The force required to rupture fetal membranes paradoxically increases with acute in vitro repeatedtretching. Am J Obstet Gynecol 2007;196:165.e1-165.e7.

Timely rupture’ of the fetal membranesFM) is an integral component of the la-or process. Preterm, premature rupture

of the membranes precedes approxi-mately one third of preterm deliveriesand is associated with increased infantmortality and morbidity.1 Until recently,rupture of membranes has been largelythought to be due to stretch-inducedweakening and tearing of the mem-branes as a result of uterine contrac-tions.2 However, in 10% of term and40% of preterm deliveries, FM rupture inadvance of contractions. This suggeststhat FM rupture is not solely the result ofthe force of contractions.3

We have recently identified a weakzone, overlying the cervix, in term, unla-bored, cesarean-delivered FM,4 and inmembranes from vaginal deliveries5 thatis characterized by biochemical modifi-cation suggestive of collagen remodelingand apoptosis.4,5 In addition, we havedemonstrated that membranes can beweakened in vitro with interleukin (IL)-1or tumor necrosis factor alpha (TNF�),6

cytokines which are frequently elevatedat end gestation.7 Such iatrogenicallyweakened FM have physical and bio-chemical characteristics similar to thosein the naturally occurring weak zone.6

These studies provide strong support forthe hypothesis that FM undergo a re-

grammed remodeling process before la-bor, which allows rupture with minimalphysical force.8

However, data also exist suggestingFM stretch is important in the weaken-ing process. In 1970, Toppozada re-ported that the contraction duringwhich the FM ruptured was not thestrongest that had been experienced tothat point, suggesting that the mem-branes weaken with the progression oflabor.9 Lavery and Miller,10,11 and muchmore recently, Oyen et al12-14 haveshown that FM are visco-elastic and,when stretched, undergo nonrecover-able deformation. However, a directdemonstration that stretching of FM re-duces the force required for subsequentrupture is lacking. The purpose of thisstudy was to test the hypothesis that re-peated stretching of term FM causesthem to weaken and fail at lower rupturestrength (force needed to rupture).

MATERIALS AND METHODSBiological samplesFM were collected from women experi-encing normal term (37-42 weeks) preg-nancies using an institutional reviewboard (IRB) approved protocol. FM of

rom the Departments of Pediatrics (Drsandey, Kumar, and Moore, Ms Jaremkond Fox, and Mr Moore) and Reproductiveiology (Drs Mercer, Stetzer, and Moore),etroHealth Medical Center, and theepartment of Mechanical and Aerospacengineering (Dr Mansour), Case Westerneserve University, Cleveland, OH.

resented at the 53rd Annual Meeting of theociety for Gynecologic Investigation,oronto, Ont, Canada, March 22-25, 2006.

eceived July 11, 2006; revised September 5,006; accepted October 6, 2006.

eprints: John J. Moore, MD, Division ofeonatology, Department of Pediatrics,etroHealth Medical Center, 2500etroHealth drive, Cleveland, OH 44109;

[email protected]

his study was supported in part by Nationalnstitutes of Health Grant HD48467 (J.J.M.).

epetitive stretch during labor hasonventionally been thought to be responsibleor fetal membrane (FM) weakening leading toupture, although recently, FM have beenemonstrated to undergo a biochemicallyediated, programmed weakening.

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econium, maternal diabetes, or multi-le gestation were specifically excluded.M from patients delivered by electiveesarean section, or vaginally, eitherith artificial rupture of membranes

AROM) or spontaneous rupture ofembranes (SROM) were tested sepa-

ately. After collection, FM were sec-ioned into multiple fragments utilizingur previously published methodol-gy.4-6 The FM fragments obtained wereashed briefly in Hank’s Balanced Saltolution (HBSS; pH 7.4) and kept moisthroughout mechanical testing.

FM fragments with similar physicalroperties were desirable to carry out theeasurements in this report. Because

M are heterogeneous over their sur-aces in their histologic, biochemical,nd physical characteristics and, mostarticularly, because they contain a weakone in the region overlying the cervix,4,5

e employed our previously publishedethodology to exclude a circular area

0 cm in diameter centered on the weakone of each membrane.4,5 Also, a 1 cmide rim of FM around the placental discas discarded.

hysical testingM physical properties were determinedy American Society for Testing and Ma-erials (ASTM) standards using a modi-ed industrial rupture testing machineCom-Ten Industries, Pinellas, FL) asreviously reported.4-6,15 Briefly, mem-ranes were cut into fragments of ap-roximately 6 cm by 6 cm. For testing,ach fragment was held in a specially de-igned FM assembly containing a 2.5 cmperture. Fragments were cut largenough such that multiple tests could beerformed upon 1 fragment. The testrea of the fragment was displaced by a 1m diameter, spherical plunger that wasriven perpendicular to the FM frag-ent surface at a speed of 8.4 cm/min

ntil membrane rupture occurred, or aredetermined force was reached. Dis-lacement of the FM and the resultant

orce were collected continuously. Dis-lacement was measured from the posi-ion of the spherical plunger where theorce reached 0.44 N (0.1 pounds). The
orce at rupture (rupture strength), a
65.e2 American Journal of Obstetrics & Gynecol

ork to rupture (area under a plot oforce vs displacement) and stiffnessslope of the linear portion of the forces displacement graph) were deter-ined from the measured force/dis-

lacement curves.4-6,15

During labor, the amniotic fluid pres-ure exerts a force perpendicular to theetal membranes, which results in a bi-xial stress in the plane of FM. Ball burstuncture testing, such as we havedopted, also produces a biaxial stress inhe plane of the tested FM fragments.8

etermination of changes tohysical properties due toepetitive stretchingefore repeated stretching, baselinehysical properties of each piece wereetermined. The baseline rupturetrength of each FM fragment was deter-

ined by testing the rupture strength atach corner of the fragment and averag-ng the values obtained. A location be-ween the 4 tested sites was selected forepetitive stretching (in the center of theragment). This region of the FM frag-

ent was cyclically stretched to a prede-ermined percentage of the baseline rup-ure strength over a 10 second period.he time between cycles was approxi-ately 1 minute. In initial experiments

amples were stretched to either 50% or5% of their baseline rupture strengthor 5-20 cycles. In addition, the effects ofery large stretch cycle numbers (up to50 cycles) and of only a few cycles (1-0) were investigated. As with the mono-onic test to failure, which we utilized inll our previous reports4 – 6 and whichas used to determine baseline proper-

ies in this report, displacement in eachoad cycle was referenced to the positionf the plunger where the resultant forceeached 0.44 N. After the predeterminedumber of cycles, the FM fragment wasupture tested to determine its stretch-nduced physical properties. These werehen compared with FM fragments thatere not stretched.Since AROM and SROM membranesay have different properties, these were

nitially analyzed separately. BothROM and SROM FM behaved remark-
bly similarly after 5-20 stretch cycles; o
ogy FEBRUARY 2007

oth showed strain hardening like repeatesarean section FM. Hence, their dataere combined and discussed together.o determine the effects of large numberf stretch cycles, only elective prelaboresarean section and SROM FM wereested; AROM FM were not tested.

etermination of the minimumorce required to initiate changesn FM physical propertiesach FM was sectioned into 10 frag-ents. FM baseline rupture strength of

ach fragment was determined in theanner described above. Each fragmentas then stretched a single time at a pre-etermined percentage (ie, 10%, 20%,0%,. . ., 90%) of the average baselineupture strength for 10 seconds. Ruptureesting was then carried out to determinehe changes in physical properties as aesult of the single stretch application.

tatistical analysesifferences in physical properties be-

ween baseline and stretched FM wereompared using paired and unpairedtudent t test, or analysis of varianceANOVA), as appropriate, with 2-tailedvalue � .05 considered statistically sig-ificant. All experiments were per-

ormed at least 3 times.

ESULTSeventy-four (74) term FM were ob-ained for study. Twelve (12) were elim-nated because spontaneous separationf amnion and chorion had occurred, orecause damage to the FM (torn FM)ad occurred with extraction of the pla-enta. The remaining 62 FM, 34 ob-ained following prelabor cesarean sec-ion births and 28 obtained followingaginal deliveries (AROM/SROM), wereested.

esarean section deliverieshe rupture strength of FM from prela-or cesarean section (n � 15 FM) deliv-ry increased significantly after 10tretch cycles, utilizing a stretch force ofither 50% (P � .05) or 75% (P � .001)f the baseline rupture strength. Smalldditional increases in rupture strength
ccurred with 15 and 20 stretch cycles (P

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.05; Table 1, Figure 1). Stiffness mark-dly increased with 10 stretch cycles (P �001). The trend toward further increaseith 15 and 20 cycles was not significant.

n contrast, work to rupture decreased (P .01, Table 1, Figure 1) after 10 stretch

ycles utilizing a stretch force of either0% or 75% of the baseline rupturetrength. After the initial decrease with0 cycles, the trend toward increase inork to rupture with 15 and 20 cycles didot reach significance.

aginal deliveriesembranes obtained after vaginal deliv-

ry were stretched for either 5 or 10 cy-les utilizing 75% of the baseline ruptureorce. No differences were seen betweenROM and SROM (n � 5 FM, each) so

he data were combined. The stretch in-uced changes seen in rupture strength,tiffness, and work to rupture in vagi-ally delivered FM paralleled the find-

ngs in FM obtained after cesarean sec-ion deliveries until very high cycle

TABLE 1Comparison of physical propertiesrepeated stretching. Changes in Rucycled at 50% or 75% of baselinemean � standard deviation. RS�RCESAREAN DELIVERIES...................................................................................................................

Rupture Strength (N) Baseline...................................................................................................................

cycles at 50% base RS 10.67 � .35...................................................................................................................

cycles at 75% base RS...................................................................................................................

Work to Rupture (J)...................................................................................................................

cycles at 50% base RS 0.026 � 0.00...................................................................................................................

cycles at 75% base RS...................................................................................................................

Stiffness (N/cm)...................................................................................................................

cycles at 50% base RS 18.25 � .66...................................................................................................................

cycles at 75% base RS...................................................................................................................

VAGINAL DELIVERIES...................................................................................................................

Rupture Strength (N)...................................................................................................................

cycles at 75% base RS 10.13 � 2.96...................................................................................................................

Work to Rupture (J)...................................................................................................................

cycles at 75% base RS 0.047 � 0.03...................................................................................................................

Stiffness (N/cm)...................................................................................................................

cycles at 75% base RS 17.0 � 1.4...................................................................................................................

* P � .05† P � .001, difference from baseline

umbers were utilized. The rupture s

trength of stretched, vaginally deliv-red, FM increased with respect to base-ine after 5 (P � .003) or 10 (P � .001)ycles (Table 1). Stiffness also increasedfter stretch cycling (P � .001). Also, aseen in cesarean section FM, work toupture decreased after 5 (P � .01) or 10P � .006) cycles (Table 1). With a largerumber of cycles (25-50, n � 10), rup-

ure strength ultimately decreased inaginally delivered FM (Figure 2A). Sim-lar weakening could not be demon-trated in prelabor cesarean section FMven with a very large number of cycles120-150, n � 10) at 75% of the baselineupture strength (Figure 2B).

mnion and choriodeciduamnion and choriodecidua (n � 5 FM)ere separated and tested alone to deter-ine their individual response to stretch

ycling. Amnion pieces responded totretch similarly to intact FM. Rupturetrength for amnion increased whentretched to 75% of baseline rupture

pre-labor cesarean section and vagure Strength, Work to Rupture, andpture Strength for 5, 10, 15 or 20 cture strength; N�Newtons; J�Joul

.........................................................................................................................

5 cycles 10 cycles.........................................................................................................................

12.19 � 1.6*.........................................................................................................................

14.90 � 1.9†

.........................................................................................................................

.........................................................................................................................

0.016 � 0.004†

.........................................................................................................................

0.020 � 0.004†

.........................................................................................................................

.........................................................................................................................

35.24 � 5.0†

.........................................................................................................................

43.00 � 5.1†

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

14.50 � 3.24* 12.71 � 3.72*.........................................................................................................................

.........................................................................................................................

0.027 � 0.021* 0.026 � 0.028*.........................................................................................................................

.........................................................................................................................

38.6 � 3.2† 39.2 � 2.7†

.........................................................................................................................

trength for 10 cycles (5.29 � 2.75 N to s


0.73 � 2.36 N; P � .02), while work toupture decreased (0.022 � 0.010 J to.014 � 0.005 J; P � .043). Choriode-idua, when tested alone, did not exhibithese changes in physical properties.horiodecidua remained elastic to theoint of rupture (data not shown).

haracterization of changes inM physical properties withcute stretchecause an increase, rather than an ex-ected decrease in rupture strength wasbserved with increasing cycles in theange initially tested (5-20 stretch cy-les), further examination was made toetermine the minimum number of cy-les necessary to cause changes in physi-al properties. Stiffness was the propertyost sensitive to repetitive stretching

nd, thus, the data for stiffness were pre-ented in detail. Repetitive stretching to5% of baseline rupture strength re-ulted in a progressive increase in mem-rane stiffness beginning with the first

ally delivered FM before and afterffness are shown following stretches of 10 seconds. Data representN/cm� Newtons/cm.

..................................................................................................................

15 cycles 20 cycles..................................................................................................................

13.67 � 0.9* 14.18 � 1.0†

..................................................................................................................

15.50 � 2.6† 16.24 � 1.5†

..................................................................................................................

..................................................................................................................

0.017 � 0.001† 0.020 � 0.002†

..................................................................................................................

0.019 � 0.003† 0.023 � 0.003†

..................................................................................................................

..................................................................................................................

42.14 � 2.8† 39.86 � 3.1†

..................................................................................................................

47.59 � 7.0† 48.06 � 5.7†

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hange in stiffness was also seen with therst cycle. Although present in all deliv-ry groups, this change was most pro-ounced in FM obtained following pre-

abor cesarean section delivery (FigureA).One stretch cycle caused a significantodification in FM physical properties.

t thus remained to determine the mag-itude of stretching force necessary to

nitiate stiffness changes. Fragmentsrom a single FM were each stretchednce, with forces ranging from 10-90%f baseline rupture strength, and thenupture tested (Figure 3B). A single

FIGURE 1Effect of stretch cycling onphysical properties

hanges in physical properties due to repeatedtretching of FM obtained following electivere-labor cesarean section. Changes in Rupturetrength (upper panel), Work to Rupture (middleanel), and Stiffness (lower panel) are shownollowing stretch cycled at 50% or 75% ofaseline Rupture Strength for 10, 15 or 20ycles of 10 seconds. Data represent mean �tandard deviation.

tretch cycle of only 10% of baseline rup- p


ure strength caused a large (2.5-fold, P.001) change in stiffness in FM ob-

ained following cesarean section deliv-ry (Figure 4A and B). Higher stretchorces resulted in smaller, additional in-reases in stiffness in this patient group2.5-fold at 10% baseline rupturetrength to 3.5-fold at 90% baseline rup-ure strength, P � .05). A small, but sig-ificant, increase in FM stiffness was in-uced with a single stretch cycle of 50%f baseline rupture strength in AROMatients, and with a single stretch cycle of0% of baseline rupture strength inROM patients (both P � .05). Interest-ngly, because the baseline stiffness inM of both patient groups that had ex-

FIGURE 2Different effects of very largenumbers of stretch cycles onSROM and C-section FM

hanges in physical properties of FM frag-ents obtained following SROM vaginal de-

ivery and elective pre-labor cesarean sectiontretched multiple times. A, Stretching SROMM fragments for 25-50 cycles (labeled mul-

iple stretches) at 75% baseline Rupturetrength resulted in a decrease in Rupturetrength (P � .001). B, Stretching pre-laborM fragments for a larger number (120-150)f cycles (labeled multiple stretches) at 75%aseline Rupture Strength did not decrease

he Rupture Strength. Data are displayed asox and whisker plots.

erienced labor (AROM, SROM) was s

ogy FEBRUARY 2007

igher than baseline stiffness in cesareanection FM, stiffness became similar inll the patient groups after a single

FIGURE 3Changes in stiffness withstretch cycle number andmagnitude of stretch

ffects on pre-labor FM Stiffness with number oftretch cycles or with the magnitude of stretch., FM fragments stretched to 75% of Rupturetrength showed a trend of increasing Stiffness

increasing slope) with increasing cycle number.tiffness is the slope of the tangent drawn at thetraight portion of the force vs. displacementurves shown for each numbered cycle. Most ofhe change occurred with the first stretch cycle.ata from a representative fragment of a FMbtained after cesarean section is shown. Threeragments each from five FM were tested. Loweranel B, Effect of the magnitude of stretch onM Stiffness: Each fragment had previously un-ergone a single, ten second stretch cycle of thendicated percentage of the baseline Rupturetrength. Data shown with B represents the

ragment with no prior stretching. Stiffness ishe slope of the tangent drawn at the straightortion of the force vs displacement curve.embranes became increasingly stiff with

igher stretch forces but most of the changeccurred at only 10% of the baseline ruptureorce. Force vs displacement curves for 10 frag-

ents from a single representative FM obtainedollowing cesarean section delivery are shown.

tretch cycle (Figure 4A and B). Rupture

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trength also increased with a singletretch cycle in all patient groups. How-ver, higher stretch forces (80% of base-ine rupture strength) were required toause a change in rupture strength thanre necessary to increase stiffness (FigureC).

OMMENThe effect of acute stretch on the physicalroperties of human FM was studied initro to determine its contribution to FMeakening. Our initial hypothesis was

hat membranes would progressivelyeaken with successive stretching. Un-

xpectedly, the membranes becametronger and stiffer. These findings werebserved in both FM obtained after a his-ory of minimal in vivo stretching, prela-or cesarean section, and in membranesollowing spontaneous vaginal deliver-es. These data suggest that acute stretchoes not reduce the strength of term FM,s has been conventionally believed.

Toppozada, et al, in 1970, showed thatn laboring mothers, the specific con-raction causing FM rupture was rarelyhe strongest contraction experienced upo that time.9 They rationalized that theM became progressively weaker due torevious contractions and, as a result,ave way at a lesser force than they hadreviously experienced. Lavery andiller, over 2 decades ago, reported that

M exhibit creep, stress relaxation, andhinning as part of their investigationsnto viscoelastic properties of theM.10,11 More recently, Oyen et al haveonfirmed and extended the findings ofavery and Miller.12–14 All of these inves-igators assumed that the FM becomeeaker after experiencing nonrecover-

ble deformation. We show here thatonrecoverable deformation likely oc-urs even after minor stretch but doesot cause weakness. Stretch relatedhanges in FM physical properties maye a protective mechanism against pre-ature FM rupture (PROM).Our data demonstrate that changes in

he properties of the FM can occur with aingle stretch cycle, and in the case of

embranes delivered by repeat cesareanection, in response to only 10% of the

FIGURE 4Stiffness and rupture strength in patient groups

tiffness and Rupture Strength changes in Patient Groups. Changes in FM Stiffness in fragments cut fromM obtained after pre-labor cesarean section, AROM vaginal delivery, and SROM vaginal delivery (N �for each delivery group). Each FM was cut into ten fragments and tested exactly in the manner of Figure

. FM fragments were subjected to one stretch cycle of the indicated percentage of the baseline ruptureorce (10-90%) and then rupture tested. A, shows Stiffness in N/cm. B, shows the same data normalizedy dividing by the baseline Stiffness to highlight differences between the labor groups. The change intiffness of the pre-labor cesarean section FM is significantly larger than the other groups (P � .01). The

argest component of the change in these membranes occurs with a single cycle using only 10% of theaseline Rupture Strength. All data are the mean � SEM. C, Rupture Strength changes in Patient Groups:hanges in Rupture Strength in fragments cut from FM obtained after pre-labor cesarean section, AROMaginal delivery, and SROM vaginal delivery (N � 4 for each delivery group). Each FM was cut into tenragments. FM fragments were subjected to one stretch cycle of the indicated percentage of the baselineupture Strength (10 -90%) and then rupture tested. All data are the mean �SEM.

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aseline physical properties as a result ofn vitro stretch differ with delivery groupFigure 4). The largest poststretch cy-ling changes in physical properties areeen in membranes that have not beenxposed to clinical labor (cesarean �ROM and SROM groups). Thus, theapacity for change in FM physical prop-rties may be related to the “stretchingistory” during labor. This argument isupported by our finding that after all

embranes are maximally stretched, initro, patient groups trend toward simi-ar physical properties (Figure 4).

Even after extensive in vitro stretching,owever, the FM rupture potential of theatient groups is different. Vaginally de-

ivered FM ultimately weaken (rupturetrength decreases) after undergoing aarge numbers of stretch cycles while FMbtained after elective prelabor cesareanection deliveries do not. Thus, one canot duplicate all of “the additional inivo experience” of the vaginally deliv-red membranes merely by acutelytretching them. Programmed biochem-cal processes which have been postu-ated to induce the development of aeak zone in the FM, overlying the cer-ix, may have acted further on vaginallyelivered membranes that remain intero for an average of 2-3 weeks longerhan those obtained following electiverelabor cesarean section.8 Chronictretch activated gene induction with re-ultant release of biochemical mediatorsapable of weakening the FM16,17 maylso play more of a role in the vaginallyelivered FM.Although an increase in Rupture

trength following repeated stretchingight seem counter intuitive, our re-

ults may be explained, in part, bytretch-induced alterations in the tis-ue’s load bearing fibrous (collage-ous) network. When soft tissues areeformed, collagen fibers are reori-nted in the direction of tensiletrain.13,14,18-20 Permanent reorienta-ion of the fibrous network is suggestedy the non-recoverable deformationbserved in this and other studies. Re-etitive stretching may result in cumu-

ative reorientation of fibers. However,he first cycle of deformation causes
he greatest change in material behav- b

or, suggesting that most of the fibereorientation occurs after one cycleFigures 3 and 4).13,14,18-20 If deforma-ion reorients the collagen networklong the direction of the appliedtress, it is likely that the membranetrength will increase, rather than de-rease; the fibers have become aligneduch that they are best able to resistensile deformation. Although we ob-erved fiber reorientation in samplestained with picrosirius red (unpub-ished research) this finding was notonsistently quantifiable.

In our previous studies, changes inupture Strength paralleled changes inork to Rupture, that is, both increased

r both decreased together.4-6 In this in-estigation, however, the only workeasured was Work to Rupture the FM

n the final rupture test, after repeatedtretching had been completed. It is rea-onable to expect that in each stretch cy-le prior to the rupture testing, some ad-itional, non-recoverable work wasone on the sample.14 If this additionalork is added to the Work to Ruptureeasured in the final rupture test, it is

ikely that the total would be an increasever baseline unstretched FM. However,he work done during non-rupturing cy-les was not measured in our investiga-ion.

In summary, our hypothesis that acuteepetitive stretch causes a decrease inupture Strength was rejected; repetitive

tretch causes an increase in FM Rupturetrength. FM that have not experiencedabor (elective cesarean section) undergoear maximal non-elastic deformationven with as little as 10% of the baselinetrength and with a single stretch cycle.tretch forces before and during laborikely have a complex effect upon FMtrength but acute stretching is clearlyot the cause of membrane weakening

eading to rupture. f

EFERENCES. Mercer B. Preterm premature rupture ofhe membranes. Obstet Gynecol 2003;101:78-193.. Polzin WJ, Brady K. Mechanical factors in thetiology of premature rupture of the mem-
ranes. Clin Obstet Gynecol 1991;34:702-714. s
ogy FEBRUARY 2007

. Parry S, Strauss JF. Premature rupture of fetalembranes. New Eng J Med 1998;338:63-670.. El Khwad M, Stetzer B, Moore RM, Kumar D,ercer B, Arikat S, Redline R, Mansour JM,oore JJ. Term human fetal membranes have aeak zone overlying the lower uterine pole andervix before the onset of labor. Biol Reprod005;72:720-726.. El Khwad M, Pandey V, Stetzer B, MercerM, Kumar D, Moore RM, Fox JM, Redline RW,ansour JM, Moore JJ. Fetal membranes from

erm vaginal deliveries have a zone of weaknessxhibiting characteristics of apoptosis and re-odeling. J Soc Gynecol Invest 2006;13:91-195.. Kumar D, Fung W, Moore RM, Pandey V, Fox, Stetzer B, Mansour JM, Mercer BM, RedlineW, Moore JJ. Pro-inflammatory cytokines

ound in amniotic fluid induce collagen remod-ling, apoptosis and biophysical weakening ofultured human fetal membranes. Biol Reprod006;74:29-34.. Bowen JM, Chamley L, Keelan JA, Mitchell.ytokines of the placenta and extra-placentalembranes: Roles and regulation during hu-an pregnancy and parturition. Placenta002;23:257-273.. Moore RM, Mansour JM, Redline RW, MercerM, Moore JJ. The physiology of fetal membrane

upture: Insight gained from the determination ofhysical properties. Placenta 2006;27:1037-051.. Toppozada MK, Sallam NA, Gaafar AA, El-ashlan KM. Role of repeated stretching in

he mechanism of timely rupture of the mem-ranes. Am. J. Obstet. Gynecol. 1970;108:43-249.0. Lavery JP, Miller CE. The viscoelastic naturef chorioamniotic membranes. Obstet. Gy-ecol. 1977;50:467-472.1. Lavery JP, Miller CE, Knight RD. The effect of

abor on the rheologic response of chorioamnioticembranes. Obstet Gynecol 1982;60:87-92.2. Oyen ML, Calvin SE, Cook RF. Uniaxialtress-relaxation and stress-strain responses ofuman amnion. J Mater Sci Mater Med004;15:619-624.3. Oyen ML, Cook RF, Calvin SE, Landers DV.echanical failure of human membrane tissues.Mater Sci Med 2004;15:651-658.4. Oyen ML, Cook RF, Stylianopoulos T, Ba-ocas VH, Calvin SE, Landers DV. Uniaxial andiaxial mechanical behavior of human amnion. Jater Res 2005;20:2902-2909.5. Arikat S, Novince RW, Mercer BM, Kumar, Fox J, Mansour JM, Moore JJ Separation ofmnion from choriodecidua is an integral evento the rupture of normal term fetal membranesnd constitutes a significant component of theork required. Am J OB Gynecol 2006;94:211-217.6. Maehara K, Kanayama N, Maradny EE,ezato T, Fujita M and Terao T. Mechanicaltretching induces interleukin-8 gene expres-
ion in fetal membranes: a possible role for the

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nitiation of human parturition. Eur J Obstetynecol Reprod Biol 1996; 70:191-196.7. Nemeth E, Millar LK, Bryant-Greenwood G.etal membrane distention II: differentially ex-ressed genes regulated by acute distention in
itro. Am J Obstet Gynecol 2000;182:60-67. fi
8. Spatz H, Kohler L, Niklas K. Mechanical be-avior of plant tissues: composite materials ortructures? J Exp Biol 1999;202:3269-3272.9. Melis P, Noorlander ML, van der Horst CM,an Noorden CJ. Rapid alignment of collagen
bers in the dermis of undermined and not un- 2

ermined skin stretched with a skin-stretchingevice. Plast Reconstr Surg. 2002;109:674-80.0. Tower TT. Neidert MR, Tranquillo R.T. Fi-er Alignment Imaging During Mechanicalesting of Soft Tissues. Ann Biomed Eng
002;30:1221-1233.

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The force required to rupture fetal membranes paradoxically increases with acute in vitro repeated stretching