Station of the fetal head at complete cervical dilation impacts duration of second stage of labor

(1)

Station of the fetal head at complete cervical dilation impacts duration of second stage of labor

Elisabeth Ludvigsen

^a,b

, Finn Egil Skjeldestad

^b,

*

aDivisionofSurgery,OncologyandWomen’sHealth,UniversityHospitalofNorthNorway,Norway

bResearchGroupEpidemiologyofChronicDiseases,InstituteofCommunityMedicine,UiTtheArcticUniversityofNorway,Tromsø,Norway

ARTICLE INFO Articlehistory:

Received12June2019

Receivedinrevisedform23October2019 Accepted23October2019

Availableonlinexxx

ABSTRACT

Objective:Toexaminetheassociationbetweenstationofthefetalheadatcompletecervicaldilationand durationofsecondstageoflabor,aswellasprolongedsecondstageoflabor,withoutandwiththeuseof analgesia(EA).

Study design:Weconducteda population-basedretrospectivecohortstudyof 3311womenwitha singletonpregnancy,gestationalage37⁰weeks,andcephalicpresentation.Stationofthefetalheadat completecervicaldilationwascategorizedasatthepelvicfloor,beneaththeischialspines,butabovethe pelvicfloor,andatorabovetheischialspines.Inlogisticregressionanalysis,wedefinedprolonged secondstageoflaboras>2hwithoutand>3hwithEAinnulliparouswomen,and>1hand>2h, respectively,inparouswomen.

Results:Survivalcurvesdemonstratedlongerdurationsofsecondstageoflaborinnulliparouswomen andwomenwithEAineachcategoryofstationoffetalhead.Theadjustedoddsratioofprolongedsecond stageoflaborwas13.1(95%confidenceinterval(CI):8.5-20.1)timeshigherwhenthefetalheadwas beneaththeischialspines,butabovethepelvicfloor,and32.9(95%CI:21.5-50.2)timeshigherwhenthe fetalheadwasatorabovetheischialspinescomparedtoatthepelvicfloor.

Conclusion: Stationofthefetalheadatcompletecervicaldilationwassigniﬁcantlyassociatedwith durationofsecondstageoflabor.

Introduction

The secondstage of laborencompassesthe eventsbetween completecervicaldilationanddeliveryofthefetus.Management ofthissecondstageiscommonlybasedonitsduration.

Studies[1–3]havereportedinconsistentresultsontheimpactof epiduralanalgesia(EA)ondurationofﬁrststageoflabor.However, moststudies[1,2,4,5]agreethatEAcanincreasedurationofsecond stage. In 2003 the American College of Obstetricians and Gynecologists (ACOG)⁶deﬁnedprolongedsecondstageoflaboras>2hwithoutand

>3hwithEAinnulliparouswomen,and>1hourwithoutand>2h withEAinparouswomen.Theoptimaldurationandmanagementof thesecondstageoflaborisstillbeingdebated.[7,8]Zhangetal[9–11]

challengedexistingknowledgebystatingalongerdurationofﬁrstand secondstageoflaborthanhasbeenpreviouslyaccepted.Theyfound the95^th%ofdurationofsecondstageoflabortobe3.6and2.8hin

nulliparouswomenwithandwithoutEA,respectively,andsuggested thatthe95^th%ismoreusefulintheassessmentofnormalprogression ofsecondstageoflabor.[10]Theseandotherreports[12,13]ledthe ACOG/SocietyforMaternal-FetalMedicine(SMFM)topublishnew labormanagementguidelines [14],whichacceptedanadditionalhour ofdurationofthesecondstageoflaborinbothnulliparousandparous womenbeforediagnosingarrest.Thenew acceptedduration waseven longerwhenEAisadministered.However,stationofthefetalheadat complete cervicaldilation and the rate of fetaldescentare notincluded inthenewguidelinesinregardtonormaldurationofsecondstageof labor or the deﬁnitions of prolonged second stage of labor.The objectiveofthisstudy wastoexaminetheassociationbetweenstation ofthefetalheadatcompletecervicaldilationanddurationofsecond stageoflabor,aswellasprolongedsecondstageoflabor,withoutor withtheuseofEA.

MaterialsandMethods

Weconductedapopulationbasedretrospectivecohortstudyof womenwhogavebirthfromJanuary1,2011toDecember31,2013 in thenorth and middleparts of Troms,Norway. Ofthe 4545 women who gave birth during this period, we included 3311

* Correspondingauthorat:DepartmentofCommunityMedicine,ResearchGroup EpidemiologyofChronicDiseases,UiT,TheArcticUniversityofNorway,Tromsø,N 7037,Norway.

E-mailaddress:eskjelde@online.no(F.E.Skjeldestad).

http://dx.doi.org/10.1016/j.eurox.2019.100100

ContentslistsavailableatScienceDirect

European Journal of Obstetrics & Gynecology and Reproductive Biology: X

j o u r n al h o m e p a g e : w w w . el s e v i e r . c o m / l o c a t e / e u r o x

(2)

womenwithasingletonpregnancy,gestationalage37⁰weeks, andcephalicpresentationwhoreachedthesecondstageoflabor, andhadvalidinformationonstationofthefetalheadatcomplete cervicaldilationanddurationofﬁrstandsecondstagesoflabor.

Datawereretrospectivelytransferredfromtheelectronicmedical birth record (PARTUS) into a case-report-form, which was validatedmanuallyagainst allsourceinformationbyoneofthe authors(EL)andamedicalstudent.Sourceinformationcomprised the electronic medical birth record, medical notes during pregnancyanddelivery(providedbymidwivesandobstetricians), partographs,the“antenatalfactsheet”(helsekortforgravide),the anesthesiareportformandthepersonalhealthformprovidedby thewomen.

Theprimaryoutcomewasdurationofsecondstageoflabor, definedasminfromcompletecervicaldilationtoexpulsionof thefetus.Themainexposurewasstation ofthefetalheadat complete cervical dilation (i.e.at the start of second stage of labor),whichcategorized inthreegroups; at thepelvic floor, beneaththeischialspines,butabovethepelvicfloor,andator abovethe ischial spines. Our institutionpracticed a -3to +3 scale for assessment of station of the fetal head. Cervical dilationand stationofthe fetalhead wasassessedby digital vaginalexamination,whichwasperformedatadmissionbythe midwifeortheobstetricianincharge.Regularcontractionsand cervicaldilation>3-4cmdefinedthestartoflabor.Forwomen with a cervical dilation of >4cmat admission, the midwife estimatedthestartof activelaborbased oninformation from thewomanandvaginalexamination.Thetimevariable“onsetof labor” was validated against available source information duringdataentry.

All inductions of labor were carried out at the maternity department at the University Hospital of Northern Norway.

Specialistconsultantsassessedtheindicationsforlaborinduction.

Based upon cervical ripening (Bishop Score), the method of inductionwas eithera cervical ripening agent (misoprostol 25 micrograms or when having a previous cesarean delivery, dinoproston 3 milligrams) administered vaginally or artiﬁcial

rupture of membranes (when intact) followed by an oxytocin regimenadministeredintravenously.

It is establishedknowledgethat durationof secondstageof labor varies by parity [6,15,16]; therefore all analyses on this durationwerestratifiedbyparity.Inordertocompareourresults with internationally accepted definitions (ACOG 2003) the outcome variable“prolonged second stageof labor”was trans- formedintoabinarycategoricalvariable:duration3hwithEA and2hwithoutEAfornulliparouswomenandduration2h withEAand1hourwithoutEAforparouswomen.Further,we estimatedthe50^th,90^thand95^th%ofdurationofsecondstageof labor.Thetime variable“firststageof labor” wasdichotomized basedonthe90^th%andstratifiedbyparity.Pre-pregnancybody massindexwasdefinedasweightdividedbythesquareofthe body height (kg/m²) and categorized according to the World HealthOrganization’sclassification.

Chi-square test for independence was used for categorical variablestoexploretherelationshipbetweenmaternalandlabor characteristicsandstationofthefetalheadandprolongedsecond stageof labor.The MannWhitneyU testwas used toexamine durationofsecondstageoflaborbyparity,andtheKruskal-Wallis testwas usedtocomparemediandurationsoffirstand second stagebetweengroups.Thedistributionofdurationofthesecond stageoflaborbycategoryofstationoffetalheadwasdisplayedby survivalcurves,censoringwomenwithcesareandelivery.Further- more,binarylogisticregressionwasperformedtopredicttheodds ofhavingprolongedsecondstageoflaborbasedonstationofthe fetalhead,afteradjustingforpossibleconfoundingfactors.Parity anduseofEAwereincludedinthedefinitionofprolongedsecond stageoflabor,andthuswedidnotadjustforthem.Oxytocinwas consideredamediatingfactorandthereforewasnotincludedin theanalyses.StatisticalanalyseswereperformedusingIBMSPSS statisticsversion24.0.P-value<5%wasconsideredstatistically significant.

TheRegionalCommitteeforMedicalandHealthResearchEthics (RECNorth2013/1208)and thePatientOmbudsman, University HospitalofNorthNorway,Tromsø,approvedthestudyprotocol.

Table1

Maternalandlaborcharacteristicsbystationofthefetalheadatcompletecervicaldilation.

Variables At

pelvicﬂoor n=1553(%)

Beneath ischialspines n=985(%)

At/above ischialspines N=773(%)

P-value

Maternalage(years)

<25

25-34 35

254(16.4) 975(62.8) 324(20.9)

214(21.7) 566(57.5) 205(20.8)

140(18.1) 473(61.2) 160(20.7)

.02

Parity Nulliparous Parous

453(29.2) 1100(70.8)

536(54.4) 449(45.6)

397(51.4) 773(48.6)

<.01

Pre-pregnancyBMI(kg/m²)

<18,5

18,5-24,9 25,0-29,9

>30,0

Missing

179(11.5) 884(56.9) 304(19.6) 174(11.2) 12(0.8)

107(10.9) 560(56.9) 190(19.3) 113(11.5) 15(1.5)

60(7.8) 433(56.0) 166(21.5) 102(13.2) 12(1.6)

.07

Gestationalage(weeks) 37-38

39-40 41-42

226(14.6) 889(57.2) 438(28.2)

140(14.2) 524(53.2) 321(32.6)

85(11.0) 395(51.1) 293(37.9)

<.01

Inductionoflabor 324(20.9) 205(20.8) 163(21.1) .99

Prolonged1^ststageoflabor¹ 106(6.8) 109(11.1) 120(15.5) <.01

Oxytocinin1^ststage 180(11.6) 183(18.6) 167(21.6) <.01

EAin1^ststage 188(12.1) 225(22.8) 249(32.2) <.01

Fetalbirthweight>4000g 275(17.7) 149(15.1) 185(23.9) <.01

P-valuesfromChi-squaretests.

1Duration>90^th%.

BMI,bodymassindex;EA,epiduralanalgesia.

2 E.Ludvigsen,F.E.Skjeldestad/EuropeanJournalofObstetrics&GynecologyandReproductiveBiology:Xxxx(2019)100100

(3)

Results

Of the 3311 women included in the analysis 42 % were nulliparousand58%wereparous.Thematernalcharacteristicsage andparity;andthelaborcharacteristicsgestationalage,onsetof labor,useofEA,prolongedﬁrststageoflaborandfetalbirthweight differedsigniﬁcantlybycategoriesofstationofthefetalheadat complete cervical dilation, whereas pre-pregnancy body mass indexandonsetoflabordidnot(Table1).

EAwas administeredto 32.3%of nulliparous and 11.1 %of parouswomenduringtheﬁrststageoflabor.Mediandurationof theﬁrststageoflaborwas249mininnulliparousand150minin parouswomennotreceivingEA,andthisdurationnearlydoubled inbothnulliparousandparouswomenreceivingEA.Thestationof thefetalheadatcompletecervicaldilationwasdiagnosedat/above theischialspinesin 37.1% ofnulliparousand 38.8%ofparous womenwhoreceived EAversus24.6%and 17.1%,respectively, amongwomenwhodidnot(Fig.1).

Mediandurationofthesecondstageoflaborinnulliparousand parouswomenwas71minand14min,andthe90^th(95^th)%were 192(244)minand68(113)min,respectively.ReceivingEAinthe firststageoflaborwasassociatedwithalongersecondstageof labor,withamedian(95^th%)durationthatwaswas32(103)min and 17 (123) min longer in nulliparous and parous women, respectively.Pairwiseanalysisoncategoriesofstationoffetalhead foundsignificantlylongerdurationsofsecondstageoflaboramong both nulliparous and parous women who received EA when comparedtothosewhodidnot(forallpercentiles,Table 2).In addition,weobserveda consistentpatternoflongerdurationof secondstageoflaborbyincreasingdistancefromthepelvicfloorat fullcervicaldilationforbothparityclassesandsubsetsofEAuse.

This pattern is graphically illustrated by survival curves for durationofsecondstageoflabor(Fig.2a-d).

Intotal,470women(14.2%)hadaprolongedsecondstageof labor;5.3%whenthefetalheadwasatthepelvicﬂoor,36.2%when itwasbeneaththeischialspines,butabovethepelvicﬂoor,and 58.5% when the fetal head was at/above theischial spines at

completecervicaldilation.Womenwithprolongedsecondstageof labor were older, more often nulliparous and had a higher gestationalage(41-42 weeks)(Table 3).In addition,theymore oftenhad prolongedﬁrststageoflaborand fetalbirthweight>

4000g.Stationofthefetalheadatcompletecervicaldilationwas signiﬁcantly associated with prolonged second stage of labor (Table4).

Theadjusted oddsratio(aOR) forprolonged secondstageof laborwas13.1timeshigherwhenthefetalheadwasbeneaththe ischialspines,but abovethepelvicfloor, and32.9timeshigher whenthefetalheadwasat/abovetheischialspinescomparedto whentheheadwasatthepelvicfloor.Longdurationoffirststageof labor (> 90^th %), fetal birthweight >4000g,and maternalage independentlypredictedprolongedsecondstageoflabor,whereas gestationalageandonsetoflaborwerenotassociatedwiththis outcome. Thestrengthof theassociationbetweencategories of stationoffetalheadandprolongedsecondstageoflaborvariedless than 4% across all investigated confounders (Table 4) (no confounding).Inaddition,theimpactofstationofthefetalhead onprolongedsecondstageoflaborwasevenlydistributedacross categoriesofpossiblepredictors(nointeraction).

Comment

Wefoundastrongassociationbetweenstationofthefetalhead atfullcervicaldilationanddurationofsecondstageoflaborinboth nulliparousandparouswomen.Weobservedaconsistentpattern of increasingduration of second stageof laborwithincreasing distancefromthepelvicﬂoorforbothparityclassesandparity- stratiﬁedsubsetsofEAuse.Clinicallythisassociationisreasonable, however few studies have systematically assessed and documented this relationship. This information can be helpful for health care providers when presenting expectations for labor progress duringthesecond stageof labor,and for encouraging laboringwomentoendureatime-demandingdelivery.

New ACOG/SMFMrecommendations¹⁴ from2014state that pushingcancontinuefor3hwithoutprogressinfetaldescentor

Fig.1.Flowchart-selectionofstudypopulation.

(4)

rotation in nulliparous women and 2h in multiparous women priortodiagnosinglaborarrest,andthislimitisextendedforan additional hour when EA is provided as long as progress is documented.However,ourstudyfoundthatstationofthefetal headhadamajorimpactonthedurationofsecondstageoflabor.

Clinicalassessmentofstationofthefetalheadwhenreachingthe second stage of labor, and documentation of EA use, can help cliniciansunderstandthelargevariationsinthedurationcurvesof thesecond stageof labor, especially when supplementalultra- soundexaminationarenotavailabletodeterminestationofthe fetal head.[17] Furthermore,continuous fetal monitoring must conﬁrmneonatalsafety[7,18].

Wherewefound91%,Grasecketal.[19]reportedthat95%ofall womenhadastationofthefetalheadatorbelowtheischialspines atfull cervicaldilation. Morethan 50years ago,Friedman [20]

reportedthat“thehigherthestationattheonsetofthedeceleration phase,themoreprotractedthelaborinthedecelerationphaseand secondstageislikelytobe”.Piperetal[21]reportedthatstationand positionofthefetalheadatcompletecervicaldilationweretwoof themanyfactorsthatinﬂuencedurationofsecondstageoflabor.

Kimmich et al [22] reported that EA may decelerate fetal descent in the active phase of labor. In our study, a higher proportionofwomenwhoreceivedEAhadarecordedstationof thefetalheadat/abovetheischialspinesatthestartofthesecond Table2

Durationofthesecondstageoflabor(min)bystationofthefetalheadatcompletecervicaldilation,stratiﬁedbyparityanduseofepiduralanalgesia

Parity Stationofthefetalheadat

completecervicaldilation

WithoutEA 50^th/90^th/95^th¹

WithEA 50^th/90^th/95^th¹

Nulliparous Atpelvicﬂoor 34/85/99 45/98/122

Beneathischialspines 75/163/202 92/223/282

At/aboveischialspines 105/237/288 143/291/344

Parous Atthepelvicﬂoor 10/24/35 14/38/54

Beneaththeischialspines 20/63/103 42/164/212

At/abovetheischialspines 35/129/188 87/216/290

1Percentilesofduration.

EA,epiduralanalgesia.

0.0

0 50 100 150

Time (minutes) Time (minutes)

Cum. proportion undelivered (%) Cum. proportion undelivered (%)

200 250 300 0.2

0.4 0.8 1.0

P0, EA (–)A P1+, EA (–)A

P0, EA (+)A P1+, EA (+)A

Fetal head at or above ischial spines Fetal head beneath ischial spines Fetal head at pelvic floor 0.6

0 50 100 150 200 250 300

0.2 0.4 0.8 1.0

0.6

0.0

0 50 100 150 200 250 300

0.2 0.4 0.8 1.0

0.6

0.0

0 50 100 150 200 250 300

0.2 0.4 0.8 1.0

0.6

Fig.2.a-dDurationcurvesofdurationofsecondstageoflaborbystationofthefetalheadstratiﬁedbyparityandEAuse.

Theverticaldottedlinesrepresenttheupperlimitforacceptabledurationofsecondstageoflabordeﬁnedby>2hwithoutand>3hwithepiduralanalgesiainnulliparous women,and>1hand>2h,respectively,inparouswomen.Cesareandeliverieswerecensored.

P0,nulliparouswomen;P1+,parouswomen;EA,epiduralanalgesia.

(5)

stageoflabor,whichcouldberelatedtoahigherstationwhenEA wasadministered.Womenwithslowlaborprogressmaybemore likelytoaskforEA,andthus,byindication,bemorelikelytohave prolongedﬁrststagelabor.Indeed,womenwithprolongedﬁrst stageof labor(>90^th%),the proportionwithprolongedsecond stagenearlydoubled,whichisinagreementwithNelsonetal[23]

who reported that the length of the second stage of labor in nulliparouswomenincreasedsigniﬁcantlywithincreasinglength oftheﬁrststageoflabor.

Several retrospective studies [4–6,8,10], have reported an associationbetweenEAuseandlongerdurationofsecondstage oflabor.Ourestimatesofthe95^th%durationofsecondstageof laborinnulliparouswomenwhodidnotreceiveEA(3.4h)and thosewhoreceivedEA (4.5h)are verysimilartowhat Cheng et al reported in a US study [4] comprising 42 000 women (womenwithoutEA:3.3h;womenwithEA:5.6h).LaterZhang etal[10]reportedshorter95^th%fordurationofsecondstageof

laborforbothnulliparouswomenwithoutEA(2.8h)andwith EA(3.6h).Inthelatterstudyonly1/3of thestudypopulation was eligible for duration analysis and only deliveries with normalneonataloutcomeswereincluded.Consistentresultsin linewithourresultshavebeenreported[1,5]forparouswomen anduseofEA.

Durationofﬁrststageoflabor,maternalage,fetalbirthweight and gestational age were independent predictors of prolonged second stage of labor without any confounding effects on the association between station of the fetal head and duration of second stage of labor. Despite adjustment analysis, residual confounding might be present, for example by rotation of the occiputposteriorpositionofthefetalheadwhenpassingthrough thepelvis.Wehaddataforpositionattimeofdelivery,andocciput posteriorpositionatdelivery(6.9%)wasassociatedwithprolonged secondstage,butthisvariablehadminimalconfoundingeffecton thestationestimates.Wedidnotincludepositioninthemodelas Table3

Characteristicsofthesamplebyprolongedsecondstageoflabor.

Variables Notprolonged

n=2841(%)

Prolonged n=470(%)

P-value Maternalage(years)

<25years 25-34 35

542(19.1) 1715(60.4) 584(20.6)

66(14.0) 299(63.6) 105(22.3)

.03

Parity Nulliparous Parous

1100(38.7) 1741(61.3)

286(60.9) 184(39.1)

<.01

BMI(kg/m²)

<18,5

18,5-24,9 25,0-29,9

>30,0

Missing

307(10.8) 1588(55.9) 563(19.8) 350(12.3) 33(1.2)

39(8.3) 289(61.5) 97(20.6) 39(8.3) 6(1.3)

.03

Gestationalage(weeks) 37-38

39-40 41-42

397(14.0) 1572(55.3) 872(30.7)

54(11.5) 236(50.2) 180(38.3)

<.01

Inductionoflabor 591(20.8) 101(21.5) .73

Prolonged1^ststageoflabor 225(9.0) 80(17.0) <.01

Oxytocinin1^st stage

440(15.5) 90(19.1) .05

EAinﬁrststage 543(19.1) 119(25.3) <.01

Fetalbirthweight>4000g 488(17.2) 121(25.7) <.01

Stationoffetalheadatcompletecervicaldilation Atpelvicﬂoor

Beneathischialspines At/aboveischialspines

1528(53.8) 815(28.7) 498(17.5)

25(5.3) 170(36.2) 275(58.5)

<.01

P-valuesfromChi-squaretests.

BMI,bodymassindex;EA,epiduralanalgesia;BW,fetalbirthweight.

1Duration>90^th%.

Table4

Determinantsofprolongedsecondstageoflabor.

Variablesinmodel Women

N

Prolonged secondstage n(%)

aOR 95%CI

Stationoffetalheadatcervicaldilation Atpelvicﬂoor

1553 25(1.6) 1.0 -

Beneathischialspines 985 170(17.3) 13.1 8.5-20.1

At/aboveischialspines 773 275(35.6) 32.9 21.5-50.2

Prolonged1^ststageoflabor 335 80(23.9) 1.4 1.1-1.9

Fetalbirhtweight>4000g 609 121(19.9) 1.5 1.2-1.9

Maternalage(years)

<25

608 66(10.9) 1.0 -

25-34 2014 299(14.9) 1.6 1.2-2.2

35 689 105(15.2) 1.6 1.1-2.3

Maternalpre-pregnancybodymassindex,onsetoflaborandgestationalagehadnoconfoundingeffect,thusnotincludedinthemodel.

aOR,adjustedoddsratio;CI,conﬁdenceintervals

(6)

welackedinformationonpositionofthefetalheadthroughthe passageofthepelvis.Onestudyshowedthattwo-thirdsofocciput positionswhenreachingtheischialspineswillrotatetoananterior positionatthetimeofdelivery.[24]

Thestrengthsofourstudyincludethelong-termutilizationof anestablishedelectronicmedicalbirthrecordsystem(PARTUS),a steady study population, committed employees and validated outcome data. The extensive literature review we performed confirmsthatweincludedthemostcommonconfounderswhen analyzing station of the fetal head as a primary exposure for durationofsecondstageoflabor.Limitationsofourstudyarethe retrospectivestudydesignandthatwedidnotconsideredrateof descent.Further,thedefinitionsofonsetoffirstandsecondstages oflaborwillinfluencedurationcurves.Theassessmentofstation of the fetal head and cervical dilation was done by vaginal explorationwhenindicated,notatdeterminedintervals,thusthe definitionof onsetof thesecond stage, can in some cases, be somewhatarbitrary.Thissubjectiveexaminationmethodisnot veryreliableandmayleadtointra-andinter-observerbiases[25]

asdemonstratedin abirthsimulatorstudy[26].Tutscheketal [17]foundthatintrapartumultrasoundexaminationsweremore reproducibleintheassessmentoflaborprogressionthandigital vaginalpalpation. Furthermore, selectionof thestudy sample, clinicalpracticeandlackofstandardizedprotocolsonprolonged secondstageoflabormayalsocontributetovariationsinduration estimates. These issues may affect the generalizability of our study results, but they mimic the real-world scenario in the deliveryroom.

We foundthat stationof thefetalheadat completecervical dilationhadasigniﬁcantimpactondurationofsecondstageof labor and on the risk of prolonged second stage of labor.

Assessment of station and position of the fetal head must be considered important factors in the clinical examination of laboring women to anticipate remaining time to delivery and thelikelihoodofachievingvaginaldelivery.Changingtheguide- linesforthemanagementofthesecondstageoflaborexclusively basedonthedurationofsecondstage,maybeanoversimpliﬁca- tionofthecomplexprocessoflabor.

Authorcontributions

FES designed the study. EL did data collection. EL/FES ran consistency analyses, cleaned data, and analyzed data.

EL was lead author. EL/FES interpreted the results, evaluated literature, and agreed upon the ﬁnal manuscript forsubmission.

Funding

The study has received funding from theNorthern Norway RegionalHealthAuthority.

DeclarationofCompetingInterest None.

Acknowledgements

The authors would like to thank Caroline Henriksen and MartineLerbuktforparticipationindatacollection.

References

[1]LeightonBL,HalpernSH.Epiduralanalgesia:effectsonlaborprogressand maternalandneonataloutcome.SeminPerinatol.2002;26(2):122–35.

[2]CambicCR,WongCA.Labouranalgesiaandobstetricoutcomes.Br.J.Anaesth.

2010;105(Suppl1):i50–60.

[3]HasegawaJ,FarinaA,TurchiG,HasegawaY,ZanelloM,BaronciniS.Effectsof epiduralanalgesiaonlaborlength,instrumentaldelivery,andneonatalshort- termoutcome.J.Anesth.2013;27(1):43–7.

[4]ChengYW,ShafferBL,NicholsonJM,CaugheyAB.Secondstageoflaborand epiduraluse:alargereffectthanpreviouslysuggested.Obstet.Gynecol.

2014;123(3):527–35.

[5]KilpatrickSJ,LarosJrRK.Characteristicsofnormallabor.Obstet.Gynecol.

1989;74(1):85–7.

[6]ACOG Practice Bulletin Number 49, December 2003: Dystocia and augmentationoflabor.Obstet.Gynecol.2003;102(6):1445–54.

[7]CohenWR,FriedmanEA.Perilsofthenewlabormanagementguidelines.Am.

J.Obstet.Gynecol.2015;212(4):420–7.

[8]ChengYW,CaugheyAB.Secondstageoflabor.ClinObstetGynecol.2015;58 (2):227–40.

[9]ZhangJ,TroendleJ,MikolajczykR,SundaramR,BeaverJ,FraserW.Thenatural historyofthenormalﬁrststageoflabor.Obstet.Gynecol.2010;115(4):705–10.

[10]ZhangJ,LandyHJ,BranchDW,etal.Contemporarypatternsofspontaneous laborwithnormalneonataloutcomes.Obstet.Gynecol.2010;116(6):1281–7.

[11]ZhangJ,TroendleJF,YanceyMK.Reassessingthelaborcurveinnulliparous women.Am.J.Obstet.Gynecol.2002;187(4):824–8.

[12]GuiseJM,EdenK,EmeisC,etal.Vaginalbirthaftercesarean:newinsights.

Evid.Rep.Technol.Assess.(FullRep.).2010;(191):1–397.

[13]SpongCY,BerghellaV,WenstromKD,MercerBM,SaadeGR.Preventingthe ﬁrstcesareandelivery:summaryofajointEuniceKennedyShriverNational InstituteofChildHealthandHumanDevelopment,SocietyforMaternal-Fetal Medicine,andAmericanCollegeofObstetriciansandGynecologists Workshop.Obstet.Gynecol.2012;120(5):1181–93.

[14]Obstetriccareconsensus no.1: safeprevention ofthe primarycesarean delivery.Obstet.Gynecol.2014;123(3):693–711.

[15]FriedmanEA.Primigravidlabor;agraphicostatisticalanalysis.Obstet.Gynecol.

1955;6(6):567–89.

[16]Friedman EA. Laborin multiparas; a graphicostatistical analysis.Obstet.

Gynecol.1956;8(6):691–703.

[17]Tutschek B, Torkildsen EA,EggeboTM. Comparison between ultrasound parametersandclinicalexaminationtoassessfetalheadstationinlabor.

UltrasoundObstet.Gynecol.2013;41(4):425–9.

[18]LevenoKJ,NelsonDB,McIntireDD.Second-stagelabor:howlongistoolong?

Am.J.Obstet.Gynecol.2016;214(4):484–9.

[19]GraseckA,TuuliM,RoehlK,OdiboA,MaconesG,CahillA.Fetaldescentin labor.Obstet.Gynecol.2014;123(3):521–6.

[20]FriedmanEA,SachtlebenM.Stationofthefetalpresentingpart;II.Effectonthe courseoflabor.Am.J.Obstet.Gynecol.1965;93(4):530–6.

[21]PiperJM,BollingDR,NewtonER.Thesecondstageoflabor:factorsinﬂuencing duration.Am.J.Obstet.Gynecol.1991;165(4Pt1):976–9.

[22]KimmichN,JuhasovaJ,HaslingerC,Ochsenbein-KolbleN,ZimmermannR.

Impactfactorsonfetaldescentratesintheactivephaseoflabor:a retrospectivecohortstudy.JPerinatMed.2017;46(6):579–85.

[23]NelsonDB,McIntireDD,LevenoKJ.Relationshipofthelengthoftheﬁrst stageoflabortothelengthofthesecondstage.Obstet.Gynecol.2013;122 (1):27–32.

[24]VinterD, PaltieliY,HabermanS,GonenR,VilleY,NizardsJ. Prospective multicenterstudyofultrasound-basedmeasurementsfofetalheadstation andpostionthroughoutlabor.UltrasoundObstet.Gynecol.2014;46:611–5.

[25]BuchmannE,LibhaberE.Interobserveragreementinintrapartumestimation offetalheadstation.Int.J.Gynaecol.Obstet.2008;101(3):285–9.

[26]DupuisO,SilveiraR,ZentnerA,etal.Birthsimulator:reliabilityoftransvaginal assessmentoffetalheadstationasdeﬁnedbytheAmericanCollegeof ObstetriciansandGynecologistsclassiﬁcation.Am.J.Obstet.Gynecol.

2005;192(3):868–74.