The evolution of the labor curve and its implications for clinical practice: the relationship between cervical dilation, station, and time during labor
2023; Elsevier BV; Volume: 228; Issue: 5 Linguagem: Inglês
10.1016/j.ajog.2022.12.005
ISSN1097-6868
AutoresEmily Hamilton, Roberto Romero, Adi L. Tarca, Philip Warrick,
Tópico(s)Pregnancy-related medical research
ResumoThe assessment of labor progress is germane to every woman in labor. Two labor disorders—arrest of dilation and arrest of descent—are the primary indications for surgery in close to 50% of all intrapartum cesarean deliveries and are often contributing indications for cesarean deliveries for fetal heart rate abnormalities.Beginning in 1954, the assessment of labor progress was transformed by Friedman. He published a series of seminal works describing the relationship between cervical dilation, station of the presenting part, and time. He proposed nomenclature for the classification of labor disorders. Generations of obstetricians used this terminology and normal labor curves to determine expected rates of dilation and fetal descent and to decide when intervention was required.The analysis of labor progress presents many mathematical challenges. Clinical measurements of dilation and station are imprecise and prone to variation, especially for inexperienced observers. Many interrelated factors influence how the cervix dilates and how the fetus descends. There is substantial variability in when data collection begins and in the frequency of examinations. Statistical methods to account for these issues have advanced considerably in recent decades. In parallel, there is growing recognition among clinicians of the limitations of using time alone to assess progress in cervical dilation in labor. There is wide variation in the patterns of dilation over time and most labors do not follow an average dilation curve.Reliable assessment of labor progression is important because uncertainty leads to both over-use and under-use of cesarean delivery and neither of these extremes are desirable. This review traces the evolution of labor curves, describes how limitations are being addressed to reduce uncertainty and to improve the assessment of labor progression using modern statistical techniques and multi-dimensional data, and discusses the implications for obstetrical practice. The assessment of labor progress is germane to every woman in labor. Two labor disorders—arrest of dilation and arrest of descent—are the primary indications for surgery in close to 50% of all intrapartum cesarean deliveries and are often contributing indications for cesarean deliveries for fetal heart rate abnormalities. Beginning in 1954, the assessment of labor progress was transformed by Friedman. He published a series of seminal works describing the relationship between cervical dilation, station of the presenting part, and time. He proposed nomenclature for the classification of labor disorders. Generations of obstetricians used this terminology and normal labor curves to determine expected rates of dilation and fetal descent and to decide when intervention was required. The analysis of labor progress presents many mathematical challenges. Clinical measurements of dilation and station are imprecise and prone to variation, especially for inexperienced observers. Many interrelated factors influence how the cervix dilates and how the fetus descends. There is substantial variability in when data collection begins and in the frequency of examinations. Statistical methods to account for these issues have advanced considerably in recent decades. In parallel, there is growing recognition among clinicians of the limitations of using time alone to assess progress in cervical dilation in labor. There is wide variation in the patterns of dilation over time and most labors do not follow an average dilation curve. Reliable assessment of labor progression is important because uncertainty leads to both over-use and under-use of cesarean delivery and neither of these extremes are desirable. This review traces the evolution of labor curves, describes how limitations are being addressed to reduce uncertainty and to improve the assessment of labor progression using modern statistical techniques and multi-dimensional data, and discusses the implications for obstetrical practice. Cesarean delivery is second only to cataract removal as the most common surgery in the United States.1HealthGradesThe 10 Most Common Surgeries in the US.healthgrades.comDate: 2022Date accessed: October 16, 2022Google Scholar In a recent review of contemporary practice in the United States, slow progress in labor was the reported indication for surgery in close to half of all intrapartum cesarean deliveries.2Zhang J. Troendle J. Reddy U.M. et al.Contemporary cesarean delivery practice in the United States.Am J Obstet Gynecol. 2010; 203: 326.e1-326.e10Abstract Full Text Full Text PDF PubMed Scopus (495) Google Scholar Labor disorders are often listed as a contributing indication for cesarean deliveries for fetal heart rate abnormalities. In addition, a labor disorder leading to cesarean delivery is often considered a potentially recurring condition, which leads the decision to opt for a repeat cesarean delivery instead of attempting a trial of labor in later pregnancies. Thus, directly or indirectly, labor disorders are by far the leading indication for cesarean delivery. Disorders in labor progression impart a high burden on women, their families, and healthcare services. World Health Organization (WHO) statistics from 2014 attribute 9% of the global maternal deaths and a large portion of serious birth-related morbidities to obstructed labor.3Say L. Chou D. Gemmill A. et al.Global causes of maternal death: a WHO systematic analysis.Lancet Glob Health. 2014; 2: e323-e333Abstract Full Text Full Text PDF PubMed Scopus (3366) Google Scholar, 4Neilson J.P. Lavender T. Quenby S. Wray S. Obstructed labour.Br Med Bull. 2003; 67: 191-204Crossref PubMed Scopus (139) Google Scholar, 5McClure E.M. Garces A. Saleem S. et al.Global Network for Women's and Children's Health Research: probable causes of stillbirth in low- and middle-income countries using a prospectively defined classification system.BJOG. 2018; 125: 131-138Crossref PubMed Scopus (40) Google Scholar Although serious maternal and fetal trauma related to prolonged labor are uncommon in modern obstetrics in high income countries, they do occur. Prolonged labor is associated with adverse outcomes such as infection, low Apgar scores, shoulder dystocia, and trauma.6Levine M.G. Holroyde J. Woods Jr., J.R. Siddiqi T.A. Scott M. Miodovnik M. 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Uncertainty in the assessment of labor progress leads to both over-use and under-use of cesarean delivery and neither of these extremes is desirable. Thus, the ramifications and actions related to labor assessment touch many women, often more than once, and can have serious consequences. With the introduction of oxytocin and the development of safer cesarean delivery operations, better assessment of labor progress became germane because obstetricians could intervene when progress did not occur as expected. The seminal work of Emmanuel Friedman transformed labor assessment from qualitative and vague—like counting sunsets—to quantitative and objective. Between 1954 and 1969, Friedman described the relationship between cervical dilation, station of the presenting part, and time and proposed nomenclature for the classification of labor disorders. This body of work began when Friedman, a young obstetrical resident, spent a night on call recording dilation and station observations on a series of laboring women at the behest of the attending anesthesiologist, Dr Virginia Apgar, who wanted to know the effect of caudal anesthesia on labor. By the next morning, he observed that his cervical dilation vs time recordings showed a sigmoid (S)-shaped curve.21Romero R. A profile of Emanuel A. Friedman, MD, DMedSci.Am J Obstet Gynecol. 2016; 215: 413-414Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar These observations had a marked effect on his subsequent analytical methods and publications. A series of landmark publications followed. In 1954 in The Graphic Analysis of Labor, he described this sigmoid-shaped pattern of dilation based on data from 100 primipara.22Friedman E. The graphic analysis of labor.Am J Obstet Gynecol. 1954; 68: 1568-1575Abstract Full Text PDF PubMed Scopus (352) Google Scholar At that time, the rate of cesarean delivery was 1%, and the rate of forceps use was 68%.22Friedman E. The graphic analysis of labor.Am J Obstet Gynecol. 1954; 68: 1568-1575Abstract Full Text PDF PubMed Scopus (352) Google Scholar He characterized the first stage of labor as a process with 4 phases, namely a latent phase with little change in dilation, an acceleration phase with a brief transition period, a phase of constant maximal dilation rate, and a deceleration phase. In 1955, the dilation curve was updated slightly with data from 500 primigravida.23Friedman E.A. Primigravid labor; a graphicostatistical analysis.Obstet Gynecol. 1955; 6: 567-589Crossref PubMed Scopus (376) Google Scholar In 1965, the first curves of descent were published based on observations from 421 nulliparae and 389 multiparae.24Friedman E.A. Sachtleben M.R. Station of the fetal presenting part. I. Pattern of descent.Am J Obstet Gynecol. 1965; 93: 522-529Abstract Full Text PDF PubMed Google Scholar In scientific parlance, these curves of dilation or descent are models. In general, models are simplified ways to represent a process, an entity, or a concept. They are useful because it may be difficult to interact directly with the original subject matter. Pediatric growth charts for the height and weight of children are examples of graphic time-based models. The Friedman labor curves are graphic models that embodied the experience of hundreds of births. They were reproduced in numerous publications around the world. Generations of obstetricians used these normal labor curves and terminology to determine expected rates of dilation and fetal descent and to decide when intervention was required. In 1969, Friedman moved one step forward by providing a mathematical equation for the relationship between dilation and time.25Friedman E.A. Kroll B.H. Computer analysis of labour progression.J Obstet Gynaecol Br Commonw. 1969; 76: 1075-1079Crossref PubMed Scopus (37) Google Scholar That is, the graphic model was expressed as a mathematical model or equation. Examples of mathematical models in obstetrics include estimating fetal weight based on several fetal measurements taken by ultrasound or estimating the likelihood of trisomy 21 based on biochemical and ultrasound markers. The report by Friedman and Kroll25Friedman E.A. Kroll B.H. Computer analysis of labour progression.J Obstet Gynaecol Br Commonw. 1969; 76: 1075-1079Crossref PubMed Scopus (37) Google Scholar on their modeling approach describes their choice to use the structure of an existing formula known as the Gompertz function and how they derived its coefficients from data in the National Collaborative Perinatal Project.Gompertz,26Gompertz B. On the nature of the function expressive of the law of human mortality, and on a new mode of determining the value of life contingencies.Philos Trans R Soc Lond. 1825; 115: 513-585Crossref Google Scholar an actuary, published a function in 1825 to describe mortality rates with increasing age. The Gompertz function produces a sigmoid shaped curve and is presented as follows:y(t)=ae−be−ct Later, this function was used to describe some biological systems, such as bacterial cell growth in an environment with finite nutrition. Once cell doubling had produced a substantial number of bacteria, growth of the colony became extremely rapid until limited resources caused the growth rate to flatten. The function also described tumor expansion in which case cell growth eventually slows as it outstrips its vascular supply. The National Perinatal Collaborative data set included labor information from 58,831 births between 1959 and 1965 in 19 United States hospitals. Using this data and the Gompertz function, Friedman and Kroll published the following equation linking dilation (y) to time (x):y(x)=2.30+7.70(0.016)0.34x Inspecting this equation shows that dilation can only begin its upward turn when the time variable (x) exceeds 0. Time0 is not necessarily observed but estimated from a graph of dilation vs time by extrapolating a straight line through the phase of maximal constant dilation downward to find where it crosses a horizontal line drawn at 2.5 cm. Times before this point were given negative numbers.25Friedman E.A. Kroll B.H. Computer analysis of labour progression.J Obstet Gynaecol Br Commonw. 1969; 76: 1075-1079Crossref PubMed Scopus (37) Google Scholar In short, the model was built in on several assumptions. The 4 most notable are as follows:1.Cervical dilation was related to a single variable—time.2.The course of dilation over time had a sigmoid shape. The Gompertz function will always produce a sigmoid shaped curve regardless of the data that it is based on.3.All women had a discernable point in time (time0) when dilation transitioned relatively quickly from the latent phase to a faster rate. A specific time0 was determined for all women even if one could not be observed.4.Time 0 occurred when dilation was around 2.5 cm dilation. The extrapolation method to find time0 was anchored to dilation = 2.5 cm. The formula constant of 2.3 means that, on average, the transition from the latent phase begins at 2.3 cm. The Friedman curves of dilation and descent and related terminology were widely welcomed because they brought structure and order to the analysis of labor. They appeared in textbooks and shaped labor management guidelines across continents and partograms, and Alert and Action Lines based on the dilation curves were formally recommended by the WHO from 1987 to 2020.27Philpott R.H. Castle W.M. Cervicographs in the management of labour in primigravidae. II. The action line and treatment of abnormal labour.J Obstet Gynaecol Br Commonw. 1972; 79: 599-602Crossref PubMed Scopus (115) Google Scholar, 28Philpott R.H. Castle W.M. Cervicographs in the management of labour in primigravidae. I. The alert line for detecting abnormal labour.J Obstet Gynaecol Br Commonw. 1972; 79: 592-598Crossref PubMed Scopus (39) Google Scholar, 29Studd J. Partograms and nomograms of cervical dilatation in management of primigravid labour.Br Med J. 1973; 4: 451-455Crossref PubMed Scopus (106) Google Scholar, 30Mahler H. The safe motherhood initiative: a call to action.Lancet. 1987; 1: 668-670Abstract PubMed Google Scholar, 31Lavender T. Alfirevic Z. Walkinshaw S. Effect of different partogram action lines on birth outcomes: a randomized controlled trial.Obstet Gynecol. 2006; 108: 295-302Crossref PubMed Scopus (54) Google Scholar Debates ensued about the presence or absence of a deceleration phase and what dilation values defined the onset of the acceleration phase.32Hendricks C.H. Brenner W.E. Kraus G. Normal cervical dilatation pattern in late pregnancy and labor.Am J Obstet Gynecol. 1970; 106: 1065-1082Abstract Full Text PDF PubMed Scopus (130) Google Scholar, 33Peisner D.B. Rosen M.G. Transition from latent to active labor.Obstet Gynecol. 1986; 68: 448-451PubMed Google Scholar, 34Cohen W.R. Friedman E.A. Perils of the new labor management guidelines.Am J Obstet Gynecol. 2015; 212: 420-427Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar, 35Cohen W.R. Friedman E.A. Misguided guidelines for managing labor.Am J Obstet Gynecol. 2015; 212: 753.e1-753.e3Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar, 36Zhang J. Troendle J. Grantz K.L. Reddy U.M. Statistical aspects of modeling the labor curve.Am J Obstet Gynecol. 2015; 212: 750.e1-750.e4Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar The sections that follow are intended to expand on key aspects of mathematical modeling that are pertinent to the problem of labor assessment. Statistical terms are linked to a glossary. The Friedman curves strongly influenced researchers studying labor progression more than 5 decades later.37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar, 38Zhang J. Troendle J. Mikolajczyk R. Sundaram R. Beaver J. Fraser W. The natural history of the normal first stage of labor.Obstet Gynecol. 2010; 115: 705-710Crossref PubMed Scopus (180) Google Scholar, 39Zhang J. Landy H.J. Ware Branch D. et al.Consortium on Safe Labor. Contemporary patterns of spontaneous labor with normal neonatal outcomes.Obstet Gynecol. 2010; 116: 1281-1287Crossref PubMed Scopus (548) Google Scholar, 40Suzuki R. Horiuchi S. Ohtsu H. Evaluation of the labor curve in nulliparous Japanese women.Am J Obstet Gynecol. 2010; 203: 226.e1-226.e6Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar, 41Shi Q. Tan X.Q. Liu X.R. Tian X.B. Qi H.B. Labour patterns in Chinese women in Chongqing.BJOG. 2016; 123: 57-63Crossref PubMed Scopus (19) Google Scholar, 42Oladapo O.T. Souza J.P. Fawole B. et al.Progression of the first stage of spontaneous labour: A prospective cohort study in two sub-Saharan African countries.PLoS Med. 2018; 15e1002492Crossref PubMed Scopus (54) Google Scholar, 43Inde Y. Nakai A. Sekiguchi A. Hayashi M. Takeshita T. Cervical dilatation curves of spontaneous deliveries in pregnant Japanese females.Int J Med Sci. 2018; 15: 549-556Crossref PubMed Scopus (7) Google Scholar, 44Lundborg L. Åberg K. Sandström A. et al.First stage progression in women with spontaneous onset of labor: a large population-based cohort study.PLoS One. 2020; 15e0239724Crossref Scopus (14) Google Scholar, 45Shindo R. Aoki S. Misumi T. et al.Spontaneous labor curve based on a retrospective multi-center study in Japan.J Obstet Gynaecol Res. 2021; 47: 4263-4269Crossref PubMed Scopus (3) Google Scholar They asked the same basic question—how does cervical dilation change over time? Later, however, scientists could approach this problem with the benefit of the considerable advances in mathematical modeling and computerization. Labor data presents challenges to developing an accurate model. Women have repeated measurements of dilation and descent over time. There is variation in when they enter hospital and in the number and frequency of examinations. Dilation assessment is imprecise and sometimes erroneous. Dilation at a previous examination is likely to affect the value at the next examination. Each of these challenges can be addressed, in part, by longitudinal statistical methods to produce curves that are more accurate. Contemporary mathematical methods allow the data to determine the optimal shape of the curve instead of forcing the data to fit a presumed shape. Resulting models are evaluated with statistical tests to measure how well they represent the data. Instead of debating what kind of curve is the best representation of labor, it is possible to measure their goodness of fit and assess with statistical confidence which curve has the best fit, is the most parsimonious, or the most robust. The first of several publications using longitudinal statistical methods was published by Zhang et al37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar in 2002. It was based on an examination of 1162 term nullipara with a spontaneous onset of labor and vaginal birth in the United States. The cesarean delivery rate in the parent data set was 12.5%. Forceps were used in 9.8% of the births. A repeated-measures, mixed-effects model was used to take into account serial measurements. Representing the response variable (dilation) as a polynomial function of time in their approach allowed the average population curve to have bends (inflection points). The general shape of curves of both dilation and descent vs time were different from the classic Friedman curves. These differences are illustrated in Figure 1. Reasonable speculation arose that differences in the 2 study groups could account for the differences in the shapes of the labor curves. Cesarean delivery rates were much higher in later years, and hence the number of labors removed from consideration were very different. In addition, there were substantial changes in the rates of epidural and caudal anesthesia, oxytocin administration, forcep delivery, and differences in maternal age and body mass index (BMI). When Zhang et al37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar applied the same repeated-measures, mixed-effect modeling techniques based on a polynomial function to the National Perinatal Collaborative data from 1959 to 1965 (the same data source used in the 1969 report by Friedman and Kroll25Friedman E.A. Kroll B.H. Computer analysis of labour progression.J Obstet Gynaecol Br Commonw. 1969; 76: 1075-1079Crossref PubMed Scopus (37) Google Scholar), the resulting curve was another nearly exponential curve that was similar to the earlier work.38Zhang J. Troendle J. Mikolajczyk R. Sundaram R. Beaver J. Fraser W. The natural history of the normal first stage of labor.Obstet Gynecol. 2010; 115: 705-710Crossref PubMed Scopus (180) Google Scholar In short, the mathematical techniques seem to be a major source of the differences in the shapes of the Friedman and Zhang curves. Zhang et al37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar applied the same modeling method to another large data set of women who delivered between 2002 and 2008 in 19 United States hospitals and again obtained a similar curve.38Zhang J. Troendle J. Mikolajczyk R. Sundaram R. Beaver J. Fraser W. The natural history of the normal first stage of labor.Obstet Gynecol. 2010; 115: 705-710Crossref PubMed Scopus (180) Google Scholar The 3 average dilation curves for nullipara by Zhang et al37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar from these 3 different sources are similar, as is shown in Figure 2.37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar,38Zhang J. Troendle J. Mikolajczyk R. Sundaram R. Beaver J. Fraser W. The natural history of the normal first stage of labor.Obstet Gynecol. 2010; 115: 705-710Crossref PubMed Scopus (180) Google Scholar,39Zhang J. Landy H.J. Ware Branch D. et al.Consortium on Safe Labor. Contemporary patterns of spontaneous labor with normal neonatal outcomes.Obstet Gynecol. 2010; 116: 1281-1287Crossref PubMed Scopus (548) Google Scholar Reports of labor curves of dilation over time soon followed from other regions in the United States and from different countries including Japan, Nigeria and Uganda, Sweden, Israel, and China.40Suzuki R. Horiuchi S. Ohtsu H. Evaluation of the labor curve in nulliparous Japanese women.Am J Obstet Gynecol. 2010; 203: 226.e1-226.e6Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar, 41Shi Q. Tan X.Q. Liu X.R. Tian X.B. Qi H.B. Labour patterns in Chinese women in Chongqing.BJOG. 2016; 123: 57-63Crossref PubMed Scopus (19) Google Scholar, 42Oladapo O.T. Souza J.P. Fawole B. et al.Progression of the first stage of spontaneous labour: A prospective cohort study in two sub-Saharan African countries.PLoS Med. 2018; 15e1002492Crossref PubMed Scopus (54) Google Scholar, 43Inde Y. Nakai A. Sekiguchi A. Hayashi M. Takeshita T. Cervical dilatation curves of spontaneous deliveries in pregnant Japanese females.Int J Med Sci. 2018; 15: 549-556Crossref PubMed Scopus (7) Google Scholar, 44Lundborg L. Åberg K. Sandström A. et al.First stage progression in women with spontaneous onset of labor: a large population-based cohort study.PLoS One. 2020; 15e0239724Crossref Scopus (14) Google Scholar, 45Shindo R. Aoki S. Misumi T. et al.Spontaneous labor curve based on a retrospective multi-center study in Japan.J Obstet Gynaecol Res. 2021; 47: 4263-4269Crossref PubMed Scopus (3) Google Scholar, 46Cahill A.G. Roehl K.A. Odibo A.O. Zhao Q. Macones G.A. Impact of fetal gender on the labor curve.Am J Obstet Gynecol. 2012; 206: 335.e1-335.e5Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar, 47McPherson J.A. Tuuli M. Odibo A.O. Roehl K.A. Zhao Q. Cahill A.G. Labor progression in teenage women.Am J Perinatol. 2014; 31: 753-758PubMed Google Scholar, 48Ashwal E. Livne M.Y. Benichou J.I.C. et al.Contemporary patterns of labor in nulliparous and multiparous women.Am J Obstet Gynecol. 2020; 222: 267.e1-267.e9Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar Despite anthropometric differences in these populations and some differences in the inclusion or exclusion criteria and the background cesarean delivery rates, the nulliparous curves based on repeated measures and the polynomial modeling techniques were similar to those shown in Figure 2.37Zhang J. Troendle J.F. Yancey M.K. Reassessing the labor curve in nulliparous women.Am J Obstet Gynecol. 2002; 187: 824-828Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar,38Zhang J. Troendle J. Mikolajczyk R. Sundaram R. Beaver J. Fraser W. The natural history of the normal first stage of labor.Obstet Gynecol. 2010; 115: 705-710Crossref PubMed Scopus (180) Google Scholar,39Zhang J. Landy H.J. Ware Branch D. et al.Consortium on Safe Labor. Contemporary patterns of spontaneous labor with normal neonatal outcomes
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