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Diagnosing onset of labor: a systematic review of definitions in the research literature Hanley, Gillian E; Munro, Sarah; Greyson, Devon; Gross, Mechthild M; Hundley, Vanora; Spiby, Helen; Janssen, Patricia A Apr 2, 2016

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RESEARCH ARTICLE Open AccessDiagnosing onset of labor: a systematicreview of definitions in the researchliteratureGillian E. Hanley1,7, Sarah Munro2,6,7, Devon Greyson2, Mechthild M. Gross3, Vanora Hundley4, Helen Spiby5and Patricia A. Janssen6,7*AbstractBackground: The diagnosis of labor onset has been described as one of the most important judgments in maternitycare. There is compelling evidence that the duration of both latent and active phase labor are clinically important andrequire consistent approaches to measurement. In order to measure the duration of labor phases systematically, weneed standard definitions of their onset. We reviewed the literature to examine definitions of labor onset and theevidentiary basis provided for these definitions.Methods: Five electronic databases were searched using predefined search terms. We included English, Frenchand German language studies published between January 1978 and March 2014 defining the onset of latentlabor and/or active labor in a population of healthy women with term births. Studies focusing exclusively oninduced labor were excluded.Results: We included 62 studies. Four ‘types’ of labor onset were defined: latent phase, active phase, first stageand unspecified. Labor onset was most commonly defined through the presence of regular painful contractions(71 % of studies) and/or some measure of cervical dilatation (68 % of studies). However, there was considerablediscrepancy about what constituted onset of labor even within ‘type’ of labor onset. The majority of studies didnot provide evidentiary support for their choice of definition of labor onset.Conclusions: There is little consensus regarding definitions of labor onset in the research literature. In order toavoid misdiagnosis of the onset of labor and identify departures from normal labor trajectories, a consistent andmeasurable definition of labor onset for each phase and stage is essential. In choosing standard definitions, theconsequences of their use on rates of maternal and fetal morbidity must also be examined.BackgroundThe diagnosis of labor onset has been described as oneof the most difficult and important judgments made byproviders of maternity care [1]. The first stage of labor,through effective uterine contractions, achieves the ob-jective of shortening or effacing the cervix, and openingor dilating it to at least 10 cm in diameter to allow thepassage of the infant from the uterus to the vagina. It iscomprised of two phases; latent and active.There is compelling evidence that the duration of bothlatent and active phases of labor are clinically relevant andthus require consistent approaches to measurement. Aprolonged latent phase of labor has been associated withan increased risk for oxytocin augmentation of labor,caesarean section, meconium staining in the amnioticfluid, 5-min Apgar score less than 7, need for newborn re-suscitation and admission to the NICU [2, 3]. Womenwho are admitted to labor wards in the latent vs. activephase of labor are at higher risk for obstetrical interven-tion including electronic fetal monitoring, epidural an-algesia, oxytocin, and caesarean section [4–7]. Theremay also be important differences in durations of latentand active phase labor and their relationship to obstet-ric outcomes according to parity.* Correspondence: patti.janssen@ubc.ca6School of Population and Public Health, University of British Columbia,Vancouver, BC, Canada7Child and Family Research Institute, University of British Columbia,Vancouver, BC, CanadaFull list of author information is available at the end of the article© 2016 Hanley et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (, which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver( applies to the data made available in this article, unless otherwise stated.Hanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 DOI 10.1186/s12884-016-0857-4Despite research pointing to the importance of theduration and transition between the latent and activephases of labor, there is considerable inconsistency in defi-nitions of labor onset, a necessary component of measur-ing duration. The onset of the latent phase of labor hasbeen defined as the time of the first clinical assessment inlabor at the hospital [3, 5], or alternatively the beginningof strong regular painful contractions [2]. Similarly, incon-sistency exists in definitions of the transition from the la-tent to the active phase. This important indicator of laborprogress has been variably characterized as coincidingwith the onset of regular contractions [8], beginning at thetime at which the woman was admitted to the labor ward[9], when she seeks professional care [10], or the time atwhich she is consented for participation in a randomizedcontrolled trial [11]. Recently researchers have used thewoman’s self-report as the time of labor onset [8, 12–14].Friedman originally defined the onset of the activephase of labor as the point in time when the rate ofchange of cervical dilatation significantly increases [15].In practice many clinicians view 3 or 4 cm cervical dila-tion as the beginning of active phase labor [16], includingthe WHO’s partograph which is based on the principlethat active phase of labor commences at 3 cm cervicaldilatation and that during active labor the rate of cervicaldilatation should not be slower than 1 cm/h [17]. Zhanget al.’s study of 1329 women in spontaneous labor at termwith a singleton fetus in vertex presentation found con-trasting findings. They reported that the cervix dilatedsubstantially more slowly in the active phase than hadbeen reported by Friedman, taking approximately 5.5 h todilate from 4 cm to 10 cm, compared with Friedman’sreported 2.5 h and concluded that most women enteredthe active phase between 3 cm and 5 cm of cervical dila-tion [18]. A more recent retrospective study that analyzedlabor trajectories of 62,415 women who vaginally delivereda singleton fetus with vertex presentation reported thatthe 95th percentile rate of active phase dilation was sub-stantially slower than the standard rate derived fromFriedman’s work, varying from 0.5 cm/h to 0.7 cm/h fornulliparous women and from 0.5 cm/h to 1.3 cm/h formultiparous women [19].Influenced by this work, the American College of Obste-tricians and Gynecologists recently released an obstetriccare consensus statement explicitly stating that contempor-ary labor progresses at a rate substantially slower thanhistorically believed. They state that because the maximalslope in the rate of change of cervical dilatation (i.e., the ac-tive phase of labor) did not start until at least 6 cm, a cer-vical dilatation of 6 cm should be considered the thresholdfor the active phase of most women in labor [20].The controversy around definitions of labor onset prob-ably stems, at least in part, from the lack of clear under-standing of the biology of parturition. Changes in levels offetal adrenal, pituitary, and placental hormones, paracrinesignalling molecules and inflammatory mediators, occuron a continuum over a period of days to weeks and initiatefactors that act to promote uterine activity [21], butnone of these mechanisms have been completely eluci-dated [22, 23]. Consequently clinicians must rely on ob-servable characteristics of labor to define its onset.To clarify concepts surrounding the definition of onsetof the latent and active phases of labor, and to determinewhat, if any, scientific rationale these definitions arebased on, we performed a systematic review of the litera-ture. Our review asks: 1) Among healthy women labor-ing spontaneously, how is the onset of the latent phaseand the active phase of labor defined?; and 2) What, ifany, evidentiary basis is provided by authors to supporttheir definitions of labor onset?MethodsSearch methodsWe searched for English, French or German-languageoriginal research papers published from 1978 to March2014 that examined onset of the latent and active phasesof the first stage of labor. The starting date of this searchwas chosen to reflect the publication date of the secondand most recent edition of Friedman’s seminal book onthe topic entitled “Labor: Clinical Evaluation and Man-agement” [15]. We followed the PRISMA statement forreporting, although we declined to undertake risk of biasassessment as it was not pertinent to our research ques-tion, and no review protocol exists for this study.We sought original research that defined or operational-ized the onset of latent labor and/or active labor in apopulation of healthy women with term births. To focuson healthy women, we excluded studies that specificallyfocused on cohorts of women with health conditions inlabor (e.g., women with gestational diabetes, gestationalhypertension, or obesity). In order to identify appropriatestudies an information specialist (DG) searched the fol-lowing electronic databases: CINAHL, EMBASE, MED-LINE, the Web of Science, and Evidence-Based MedicineReviews (which incorporates ACP Journal Club, CochraneCentral Register of Controlled Trials, Cochrane Databaseof Systematic Reviews, Cochrane Methodology Register,Database of Abstracts of Review of Effectiveness, HealthTechnology Assessment and NHS Economic EvaluationDatabase). We also traced citations to and from relevantarticles, and searched our personal libraries for add-itional articles. As we were primarily interested in un-derstanding how studies were defining the onset oflabor, we searched databases using subject heading andkey words clustered around the concepts of latent andactive phase of labor onset or onset of the first stage oflabor overall. See Appendix 1 for the full electronicHanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 2 of 11search strategy for each database. No review protocolwas published for this study.Study selectionWe included studies of healthy women in uncomplicatedlabor at term written in English, French or German. Inorder to be eligible for inclusion, studies were requiredto be original, empirical research, and a study outcomemust have involved labor onset or duration of labor. Weexcluded studies that focused exclusively on women withinduced labor (although populations that included somewomen with induced labor were included), as well ascase-studies, case-series and studies that did not presentany original data (such as commentaries and reviews).Papers were screened without blinding through a se-quence of title (by SM and GH), and abstract (by SMand GH), and any discrepancies were resolved throughdiscussion and agreement. If agreement could not bereached, a third screener (PJ) made the final decision[24]. A larger group conducted full text review (SM, GH,PJ, MG, HS, and VH). Each paper was reviewed by oneof the original screeners (SM, GH and PJ) as well as asecond screener (MG, HS and VH). Discrepancies wereresolved by one of the original screeners (SM, GH andPJ) who had not read the full text of the article.Screeners did not screen or extract articles they hadauthored or coauthored [24].Data extraction and analysisA standardized data extraction form was developed [24]to include details about the study design, setting, timeperiod, and the inclusion and exclusion criteria used todefine the study population, as well as information aboutthe sample size, the intervention(s) of interest, and theoutcome(s) of interest. Finally the reviewers independ-ently extracted the definition of labor onset used accord-ing to whether it defined the onset of the latent, oractive phase of labor or simply the onset of the firststage of labor. In addition, the reviewers extracted infor-mation about whether, and what, rationale the authorsprovided regarding their choice of definition of labor on-set, including supporting citations.Prior to beginning data extraction, all six full textreviewers independently piloted the standardized dataextraction form on a random sample of three of the in-cluded studies [24]. Responses were compared for dis-crepancies and all reviewers were involved in revisingthe data extraction form to ensure consistency and im-prove data quality. Once the form was finalized, full textreviewers (SM, GH, PJ, MG, HS, and VH) independentlyextracted data from the studies. Each study was ex-tracted by two reviewers including one of the originalscreeners (SM, GH, and PJ). We did not contact anystudy authors for data confirmation. As our primaryinterest was the definition of labor onset, rather than thevalidity of the conclusion or the study outcomes, we didnot assess risk of bias in our included studies.Synthesis of resultsWe examined key aspects of the included studies, in-cluding study design, research objective, sample size,country of origin, years of data, and publication year,and constructed tables and figures to illustrate key find-ings. We also assessed differences in labor definitions ac-cording to parity.ResultsDescription of included studiesWe identified a total of 1683 potentially relevant cita-tions (Fig. 1). Following title review, 549 were retainedand review of these abstracts eliminated all but 117studies. After full text screening, 62 studies were deemedeligible for inclusion in our review (see Table 1). Of the62 included studies, 22 (35 %) were from the UnitedStates [25–44] and six (10 %) were from Germany [12–14, 45–48]. The remaining studies included four eachfrom Italy [49–52] and Nigeria [53–56], three (5 %) eachfrom Iran [57–59] and Norway [60–62], two each fromIsrael [63, 64] and South Africa [65, 66], and one eachfrom Australia [67], Austria [68], Bahrain [69], Canada[70], France [45], India [71], Ireland [72], Jordan [73],Korea [74], Kuwait [75], New Zealand [76], Pakistan [77],Philippines [78], Saudi Arabia [79], South Korea [80], andSweden [81]. Most of the included studies (n = 39, 63 %)were published between 2005–2013 (Fig. 2) [13, 14, 28,30–34, 36, 38, 39, 42–48, 50–52, 55–59, 63, 67, 69–71,73–77, 79–81]. The majority of studies were retrospectivecohort studies (n = 29, 47 %) [2, 25, 27, 28, 30–36, 38, 40,41, 43, 44, 49–51, 55, 61, 62, 65, 66, 70, 72, 80, 82, 83],while 29 % were prospective cohort studies (n = 18) [26,29, 37, 39, 42, 46–48, 52–54, 56, 60, 67, 71, 74, 79, 81] and11 % (n = 7) were randomized controlled trials or cohort[57–59, 69, 75, 77, 78]. The remaining eight studies (13 %)employed a range of qualitative, case control, mixedmethods, or other research designs [12–14, 45, 63, 68, 73,76]. Five studies (8 %) defined definitions of labor onsetdifferently for nulliparous and multiparous women [36,40, 41, 54, 66]. Of these five studies (8 %), four were pub-lished in 1986 or earlier [40, 41, 54, 66].1) How is the onset of labor onset defined?Types of labor onset defined by the included studiesWe classified the type of labor onset according to whatthe authors of the included papers said they were defin-ing. In the 62 included studies, we observed four distincttypes of labor onset including “active labor”, “latent orearly phase labor”, “first stage labor” or simply “labor”Hanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 3 of 11without further specification, which we call “unspecifiedlabor”. The majority of studies defined the onset of ac-tive phase labor only (n = 22, 35 %) [26, 27, 31, 35, 41,42, 45, 50, 51, 53, 56, 57, 59, 61, 63, 65, 66, 71, 74, 77,79, 83]. Three studies only defined the latent phase oflabor (5 %) [40, 55, 76], while 11 studies only defined on-set of the first stage of labor (18 %) (see Table 1) [32–34,38, 47, 62, 68–70, 73, 78]. Approximately one quarter ofstudies provided only a definition for unspecified labor(n = 15, 24 %) [12–14, 25, 30, 36, 37, 39, 43, 44, 46, 48,49, 52, 58]. With respect to studies that defined morethan one ‘type’ of labor, 10 studies (16 %) provided a def-inition for both active and latent phase of labor [2, 28,29, 54, 60, 67, 72, 75, 81, 82], while one (1.6 %) definedboth active labor and unspecified labor [80].Components of definitions of labor onsetMost studies (68 %) included measures of cervical dila-tion in their definition with only 20 studies omitting aspecific measurement of dilatation from definitions oflabor in their paper [12–14, 25, 29, 30, 32, 33, 35, 37, 40,46–48, 61, 62, 70, 73, 76, 83] (Table 1). Regular painfulcontractions were also frequently referenced in defini-tions of onset of labor (71 %), with only 18 studies omit-ting mention of contractions [26, 27, 31, 36, 39, 41–45,53, 55, 57, 59, 63, 68, 78, 79]. Studies also varied in theirdescriptions of the length and frequency of contractionsat onset of labor. Twenty-one studies (34 %) includedmention of either length or frequency of contractions intheir definition of the onset of labor [29, 30, 32, 33, 35,49–51, 54–56, 60–62, 66, 67, 69, 71, 74, 75, 83]. Onestudy stated that onset of labor in general is also charac-terized by intact, rather than ruptured, membranes [28].Below we outline how these commonly referenced com-ponents of labor definitions varied according to the typeof labor defined.Latent phase onsetAmong the 14 studies that defined latent phase labor, 11(79 %) included cervical dilatation in the definition. Onsetof the latent phase of labor was defined using various mea-sures of cervical dilation, most commonly <4 cm (n = 7,50 %) [2, 28, 54, 60, 75, 81, 82]; however, ≤2 cm, [72] and<3 cm [29, 55, 67] were also included in definitions. Onestudy provided different definitions for the end of latentphase labor according to parity, indicating that a cervicaldilation of 3 cm marked the end of the latent phase of laborfor primiparous women, while for multiparous women itwas 4 cm [54]. Cervical effacement was included in the def-inition of latent phase of labor in three of thirteen studies(23 %). While two stated that effacement should be at leastFig. 1 PRISMA/QUORUM diagramHanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 4 of 1180 % [29, 75], the third study defined latent phase labor on-set as when the cervix has “minimal or no effacement” [55].All thirteen studies (100 %) that provided definitionsfor the onset of latent phase labor included the presenceof regular painful contractions in their definition [2, 28,29, 40, 54, 55, 60, 67, 72, 75, 76, 81, 82]. Three studies(23 %) stated that during the onset of the latent phase oflabor there should be at least one painful uterinecontraction every 8–10 min [29, 54, 55], and one studystated that there should be at least two painful uterinecontractions every 10 min [75]. The duration of eachcontraction was not included in these definitions. Onlythree studies (23 %) included other physiological symp-toms in their definitions. These included bloody show[29, 72, 76] and fluid loss [72, 76], as well as gastrointes-tinal symptoms or irregular (non-repetitive) pain [72,76].Active laborOf the studies that included a definition of the onset ofactive labor (n = 33), 27 (82 %) included cervical dilata-tion in their definition [2, 26–29, 31, 42, 45, 50, 51, 53,54, 56, 57, 59, 60, 63, 65–67, 71, 74, 75, 77, 79–81]. Two(6 %) included ≥2 cm cervical dilation as the measure oflabor onset [50, 51], ten (30 %) cited 3–4 cm [29, 45, 53,54, 59, 65–67, 77, 79], while fourteen (45 %) included≥4 cm cervical dilation in their definition of active laboronset [2, 26–28, 31, 42, 56, 57, 60, 63, 71, 74, 75, 80, 81].Two studies (6 %) did not quantify the amount of dila-tion present at onset of active labor and stated ratherthat there should be contractions leading to “cervicalchange” [35, 83]. Four studies (12 %) characterized onsetof active phase labor as the point at which the cervix be-gins to dilate >1 cm per hour [2, 41, 63, 79].Cervical effacement was mentioned in six definitions(21 %) of onset of active labor [50, 51, 66, 72, 74, 81].One study mentioned the cervix being generally effaced[81], one suggested that ≥75 % effacement was indicativeof active labor [72], while three others considered thecutoff to be at least 80 % effaced [50, 51, 74], and finallyone study referred to a “fully effaced” cervix [66].Over half of the studies defining the onset of activelabor included regular painful contractions in their def-inition (n = 20, 60 %) [2, 28, 29, 35, 50, 51, 54, 56, 60, 61,65–67, 71, 72, 74, 77, 81–83]. Among the studies thatdefined onset of active phase labor, two indicated thatcontractions should be five minutes apart [66, 67], andtwo stated that there should be at least three contrac-tions in ten minutes [71, 74], while two more suggestedcontractions should occur every 3–5 min [35, 83]. Onestudy indicated that onset of active labor is characterizedby contractions that are 20–25 s in length [71], whiletwo studies (with the same first author) stated that con-tractions be >40 s long [50, 51]. Two studies includedadditional physiological symptoms in their definition ofonset of active phase labor: fluid loss [72] and bloodyshow [29, 72].First stage labor onsetOf the 11 studies that defined onset of the first stage oflabor without referring to a particular phase [32–34, 38,47, 62, 68–70, 73, 78], five (45 %) provided a specificTable 1 Characteristics included in definitions of onset of laborCharacteristic included N (%)Type of labor definedLatent 3 (5)First stage 11 (18)Active 22 (35)Labor (unspecified) 15 (24)More than one of the above 11 (18)Cervical dilation 42 (68)In latent phase labor< 2 cm 1 (2)3–4 cm 3 (5)> 4 cm 7 (11)In active labor≥ 2 cm 2 (3)3–4 cm 10 (16)> 4 cm 14 (23)In first stage labor3–4 cm 4 (6)> 4 cm 1 (2)In unspecified labor≥ 2 cm 2 (3)3–4 cm 2 (3)> 4 cm 2 (3)Cervical effacement 12 (19)Regular painful contractions 44 (71)Frequency of contractions 12 (19)1 in 8–10 min 3 (5)2 in 10 min or five minutes apart 3 (5)3 in 10 min 5 (8)1 every 3–5 mins 2 (3)Other physiological symptoms 7 (11)Rationale for definitionReferred to women’s reports of onset or routine clinicalpractice11 (18)Cited Friedman 8 (13)Cited Gross 4 (6)Cited textbooks 3 (5)Cited another study 2 (3)Hanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 5 of 11cervical dilatation in their definition, including four thatdefined first stage labor onset when the cervix was 3–4 cm dilated [34, 38, 68, 69] and one study that used acervical dilatation of ≥4 cm [78]. Three studies did notquantify dilation but stated that at first stage labor onsetthere should be “cervical change” [32, 33, 70]. Only onestudy that defined first stage labor included effacementin its definition (9 %), and mentioned only that thereshould be demonstrable effacement and dilatation of thecervix in their definition of first stage labor [38].Most studies that defined onset of the first stage oflabor included regular painful contractions in their def-inition (n = 9, 82 %) [32–34, 38, 47, 62, 69, 70, 73]. Onlyone study referred to duration or frequency of contrac-tions at onset of first stage labor and indicated that con-tractions should be >40 s long [69].Unspecified labor onsetAmong the 16 studies that included a definition of laborthat did not specify a phase or stage [12–14, 25, 30, 36, 37,39, 43, 44, 46, 48, 49, 52, 58, 80], six (40 %) included a spe-cific cervical dilatation in their definition. These six wereevenly split between 2 cm [49, 52], 3–4 cm [43, 48], and>4 cm [36, 39]. Two studies included cervical effacementin their definition of onset of unspecified labor, statingthat the cervix should be “partially” effaced [49] or ≥50 %effaced [52].Twelve out of sixteen studies (75 %) that definedlabor onset for an unspecified stage or phase of laborincluded regular painful contractions in their defin-ition [12–14, 25, 30, 37, 46, 48, 49, 52, 58, 80]. Of thesestudies, four had the same first author [12–14, 46] andused a definition of onset of first stage labor that in-cluded multiple physiologic symptoms derived from aqualitative study on women’s experience of onset oflabor at term [12]. Three studies diagnosed the onsetof unspecified labor when one of the symptoms in-cluded contractions occurring at least three times in aten-minute interval [30, 49, 52].Definitions according to parityFive studies provided a definition of labor onset that dif-fered according to parirty [36, 40, 41, 54, 66]. One studyindicated that latent phase labor and active phase beganwhen the woman’s cervix was 3 cm or 4 cm dilatationfor primiparous and multiparous women respectively[54]. Another suggested that labor (unspecified) began at4 and 5 cm cervical dilatation for nulliparous and mul-tiparous women respectively [36]. Two studies by thesame authors reported that cervical dilatation was ex-pected to occur at different rates based on parity(1.2 cm/h for nullips vs. 1.5 cm/h for multips) [40, 41].Definition by caregiver vs. parturientMost studies did not attribute diagnosis of labor to be inthe domain of a specific type of caregiver (e.g., nurse,midwife, physician). Nineteen studies (31 %) indicatedthat the woman’s self-reported symptoms were used todiagnose onset of labor [12, 13, 25, 30, 32, 33, 35, 37, 40,46–48, 66, 67, 70, 76, 80–82]. In seven studies (11 %)clinicians included in their definition that the onset oflabor was the time at which the woman was admitted tohospital [14, 28, 38, 46, 47, 66, 73]. Three studies com-pared definitions between women and their caregivers[46, 47, 66].Temporal patternsOver the study inclusion period (1978–2013; see Fig. 2),there were no temporal patterns observed regarding theFig. 2 Frequency of included studies by publication yearHanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 6 of 11types of labor onset defined by studies (i.e., latent vs ac-tive) or the measures of cervical dilation (i.e., 3 cm vs4 cm) that studies used to define onset of labor. Rather,studies used heterogeneous definitions throughout thetime period. However, the majority of the studies thatdefined labor onset differently for nulliparous versusmultiparous women were published in 1986 or earlier[40, 41, 54, 66].2) What, if any, evidentiary basis is provided byauthors to support their definitions of labor onset?The majority of studies did not provide any rationale fortheir definition of onset of labor (n = 37, 60 %) [12, 25,31–34, 39, 43–45, 47, 50, 51, 53–59, 61–63, 65, 68–71,73–78, 80, 81, 83]. Eleven described women’s reports orroutine clinical practice as a rationale [26–28, 30, 35–37,52, 60, 66, 67]. For instance, the authors of one studystated “we chose 4 cm as a commonly acceptedchangeover point” between the latent and activephases of labor [28].Eight studies (13 %) cited publications that were writ-ten by Friedman or used his 1954 definition of the laborcurve as their rationale [2, 29, 40–42, 72, 79, 82], how-ever not all studies used the Friedman definition cor-rectly. For example, only three of these studiesmentioned rate of dilatation [2, 41, 79], which is consid-ered an important component of Friedman’s labor curve[84]. Three studies cited obstetrical and obstetrical an-aesthesiology textbooks [38, 49, 82], including a chapterin a maternal-fetal medicine text [85], an obstetricanesthesiology textbook [86], and two chapters fromWilliams’ Obstetrics [87]. Two studies [2, 72] cited clin-ical studies of length of labor [88, 89]. Finally, four Ger-man studies sharing a common author [13, 14, 46, 48]referenced the definition of onset of labor from a quali-tative study they had previously authored [12] onwomen’s experiences of onset of labor at term.DiscussionThis systematic review provides an overview of howlabor onset for healthy women is defined in the researchliterature and summarizes the evidence being used tosupport these definitions. We found studies providingdefinitions for four different types of labor onset; latentphase, active phase, first stage and unspecified labor. Allfour definitions commonly referenced cervical dilatation,cervical effacement, and uterine contractions, with littlemention of other physiologic indications, such as bloodyshow and gastrointestinal symptoms. Cervical dilatationand regular painful contractions were the most commonindicators of labor onset, regardless of stage or phase.However, there was little consensus on the degree ofdilatation or regularity of contractions, even within defi-nitions for the same stage or phase. The majority of in-cluded studies (60 %) did not provide any evidentiarybasis for their definition of labor onset. Among studiesthat did provide evidence for their definition, the mostcommon was a citation of Friedman’s labor curve.We report that there is considerable discrepancy indefinitions of labor onset in the research literature. Evenamong studies referencing the same type of labor onset(e.g., active phase labor) and indication of labor onset,there was little consensus, with the exception that 100 %of definitions of latent phase labor referenced the pres-ence of regular painful contractions. This lack ofconsistency may be driven in part by the lack of stan-dardized documentation of labor onset in the patient’smedical record. The lack of consistent documentationmay both contribute to and result from the lack of astandardized definition. This discrepancy in definitionsis also not surprising given that the physiologic mecha-nisms that stimulate the transition of uterine musclefrom quiescence to regular contractions occur over aperiod of time, and on multiple levels, none of which areobservable, and none of which yield clear biologicmarkers which would permit a definitive diagnosis oflabor onset. The process of parturition begins days orweeks prior to the onset of observable labor. Placentalestrogens, relaxin, and prostglandins ‘soften’ the collagenfibers in the cervix and make it more distensible [90].Under the influence of estrogen, prostaglandins and dis-tension of uterine tissue, uterine tissue is prepared forlabor through cell multiplication and hypertrophy. Uter-otropins, including oxytocin, raise levels of intracellularcalcium, which stimulates contractions. Oxytocin se-creted by the fetus also is a major contributor to increas-ing oxytocin levels in uterine tissue [91]. Oxytocinreceptors increase in numbers in uterine muscle underthe influence of estradiol as term approaches. Alsounder the influence of estrogens, the number of gapjunctions in muscles increase. Gap junctions are trans-cellular membrane channels, which allow ion exchangebetween cells to propagate an electrical signal and subse-quent muscle contraction [90].A definition of labor onset that uses both endocrinelevels and observable signs and symptoms might providea reliable and valid measure at some point in the future.In practical terms, what is needed is a point in time afterwhich labor should not only be expected to continueamong healthy women, but beyond which, failure to pro-gress would require intervention on the part of the care-giver to prevent subsequent maternal and neonatalmorbidity.Studies in our review were more likely to focus on activephase of labor than latent phase labor, which is of concerngiven the adverse outcomes associated with early hospitalHanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 7 of 11admission in latent phase labor [2, 3, 30]. A strong con-sensus around the definition of onset of latent phase laboris needed to ensure comparability of research findings,and subsequently to guide clinical diagnosis and interven-tion. Understanding when the transition between the la-tent and active phases of labor takes places is essential fordesigning initiatives to assist women to remain out of hos-pital during latent phase labor [92].Our review supports the notion that measurement ofcervical dilatation is dominant in the discussion of deter-mining labor onset and the transition from latent to ac-tive phases [76]. Thus it is perhaps not surprising thatwomen present to hospital when not in labor, as they aregenerally unable to assess their own cervical dilatation.Previous research has illustrated that descriptions oflabor onset and progression that rely on cervical dilata-tion do not provide women with the means to under-stand how far they have progressed in their labor [76].While healthcare providers may feel relatively certainabout their diagnosis when women arrive at hospitalprior to active labor, they are then faced with making amanagement decision that incorporates not only theirdiagnostic judgment but also cues regarding how wellthe woman is coping, family expectations, and institu-tional requirements. These factors may contribute to ad-mission in latent phase labor [93].A consistent and measurable definition of labor onsetfor each phase and stage is essential in order to identifydepartures from normal labor trajectories and avoid mis-diagnosis of the onset of labor with subsequent sequelae,including increased risk for oxytocin augmentation oflabor, caesarean section, meconium staining in the amni-otic fluid, 5-min Apgar score less than 7, need for new-born resuscitation and admission to the NICU [2, 3].Definitions tend to be static, for example a measure of thecervical dilatation at which a phase or stage of labor isconsidered to have begun (e.g., active labor begins at4 cm), or a degree of effacement. These static definitionsmay result from the widely held, and erroneous [84] con-clusion that Friedman defined the transition from latentto active phase labor as occurring at 3–4 cm cervical dila-tation [94, 95]. Friedman asserts instead that slow laborprogression is identified by change in dilatation over timewith active-phase cervical dilatation progressing linearly ata minimum of 1.0 cm/h in nulliparas [84]. Recent recom-mendations have changed the cervical dilatation uponwhich the transition is believed to take place to 6 cm [20].Our systematic review has revealed that there appears tobe little consensus in the amount of cervical dilatation ne-cessary to indicate that active phase labor has begun.Strengths of our systematic review include explicit, anddetailed eligibility criteria and a comprehensive search con-structed and conducted by an information specialist. Wewere also able to review studies published in English,French and German due to the multi-lingual capacity ofour international team. A limitation of our review is thatwe cannot recommend a specific definition of labor. Giventhat our review sought simply to answer what definitionswere in common use in the literature and what evidentiarybasis was provided for their use, we were unable to assesswhether specific definitions were associated with better ob-stetric outcomes than others. This is the type of researchthat will be needed to recommend a definition of labor on-set. Further research seeking practitioners’ views on themost useful definition of onset of early labor would also beuseful.ConclusionIn summary, we report very little consensus regarding def-initions of labor onset in the research literature. In par-ticular we note that latent phase onset is an understudiedphenomenon whose definition merits further investigationby clinical scientists. Most definitions referred to the pres-ence of regular uterine contractions and cervical dilatationas static concepts. Despite the fact that the current focuson static definitions of labor onset has failed to lead toconsensus, recent recommendations continue to use thisapproach [20]. Future research could include testing defi-nitions of labor onset that include other physiologic pa-rameters such as station of the baby and measures ofchange in parameters over time. Given that Friedmans’work seemed to be the most foundational in this body ofliterature, initial studies could compare definitions to thetraditional Friedman model. Furthermore, emerging defi-nitions need to be evaluated with respect to impact oftheir use on maternal and fetal outcomes, for examplematernal pelvic floor injury, chorioamnionitis, hypoxic is-chemic encephalopathy, and birth injury. While conduct-ing this critical research, investigators would be welladvised to keep in mind the balance between an objectiveand useful definition that will accurately indicate when in-terventions are warranted, and measures that can be usedto help women self-diagnose labor onset and assist themin remaining out of the hospital during latent phase labor.Appendix 1Search strategy by databaseMedline (Ovid MEDLINE(R) 1946 to Present with DailyUpdate)Searched on 18 January 2013; updated on March 14th20141. Labor Onset/2. Labor Stage, First/3. 1 or 24. limit 3 to yr = “1978 -Current”5. limit 4 to (english or french or german)CINAHL with Full Text (Ebsco Host)Searched on 23 January 2013Hanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 8 of 11(MH “Labor Stage, First”)Limiters: Exclude MEDLINE records; Language: Eng-lish, French, German; Source Types: Academic Journals,Books, Dissertations, CEUsEmbase 1974 to 2013 January 22 (OvidSP)Searched on 23 January 2013; updated on March 14th20141. labor onset/2. labor stage 1/3. 1 or 24. limit 3 to yr = “1978 -Current”5. limit 4 to human6. limit 5 to to (english or french or german)7. limit 6 to exclude medline journalsWeb of Knowledge (Thompson Reuters) Databases =SCI-EXPANDED, SSCI, A&HCI, CPCI-S, CPCI-SSHTimespan = All YearsSearched on 23 January 2013; updated on March 14th20141. Topic = (“labor onset” or “labor onset”)2. Topic = (“Labor stage, first” or “Labor stage, first” or“labor stage I”)3. #2 OR #14. Exclude PortugueseEBM Reviews - Cochrane Central Register of ControlledTrials December 2012 (OvidSP)Searched on 23 January 2013; updated on March 14th20141. Labor Onset/2. Labor Stage, First/3. 1 OR 24. limit 3 to yr = “1978 -Current”(n.b. no language limits available; no MEDLINE orEMBASE records to eliminate)Competing interestsThe authors declare they have no competing interests.Authors’ contributionsGEH and PAJ drafted the review protocol and designed the review. DGcompleted all literature searching. GEH, SM, and PAJ competed the title andabstract reviews. GEH, SM, MG, VH, HS and PAJ completed the full textreview. GEH drafted the data extraction form and SM, MG, VH, HS and PAJreviewed and refined the data extraction form. SM completed the analysis.GEH drafted the article. All authors edited and revised the article. All authorsread and approved the final manuscript.AcknowledgmentsThis study was supported by the Canadian Institutes of Health Research [Grant# RN146572 - 264633]. The funders were not involved in any part of the study.Author details1Department of Obstetrics & Gynaecology, University of British Columbia,Vancouver, BC, Canada. 2Interdisciplinary Studies Department, University ofBritish Columbia, Vancouver, BC, Canada. 3Midwifery Research and EducationUnit, Hannover Medical School, Hanover, Germany. 4Faculty of Health &Social Sciences, Bournemouth University, Bournemouth, UK. 5School ofHealth Sciences, University of Nottingham, Nottingham, UK. 6School ofPopulation and Public Health, University of British Columbia, Vancouver, BC,Canada. 7Child and Family Research Institute, University of British Columbia,Vancouver, BC, Canada.Received: 20 February 2015 Accepted: 23 March 2016References1. Lauzon L, Hodnett E. Antenatal education for self-diagnosis of the onset ofactive labour at term. Cochrane Database Syst Rev. 2000;2:CD000935.2. Chelmow D, Kilpatrick SJ, Laros Jr RK. Maternal and neonatal outcomes afterprolonged latent phase. 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Cheyne H, Dowding DW, Hundley V. Making the diagnosis of labour:midwives’ diagnostic judgement and management decisions. J Adv Nurs.2006;53(6):625–35.94. Rouse DJ, Owen J, Hauth JC. Active-phase labor arrest: oxytocinaugmentation for at least 4 hours. Obstet Gynecol. 1999;93(3):323–8.95. Rouse DJ, Owen J, Savage KG, Hauth JC. Active phase labor arrest: revisitingthe 2-hour minimum. Obstet Gynecol. 2001;98(4):550–4.•  We accept pre-submission inquiries •  Our selector tool helps you to find the most relevant journal•  We provide round the clock customer support •  Convenient online submission•  Thorough peer review•  Inclusion in PubMed and all major indexing services •  Maximum visibility for your researchSubmit your manuscript your next manuscript to BioMed Central and we will help you at every step:Hanley et al. BMC Pregnancy and Childbirth  (2016) 16:71 Page 11 of 11


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