Labor: Overview of Normal and Abnormal Progression PDF

Official reprint from UpToDate® www.uptodate.com © 2024 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Labor: Overview of normal and abnormal progression AUTHORS: Robert M Ehsanipoor, MD, Andrew J Satin, MD, FACOG SECTION EDITOR: Vincenzo Berghella, MD DEPUTY EDITOR: Vanessa A Barss, MD,...

Official reprint from UpToDate® www.uptodate.com © 2024 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Labor: Overview of normal and abnormal progression AUTHORS: Robert M Ehsanipoor, MD, Andrew J Satin, MD, FACOG SECTION EDITOR: Vincenzo Berghella, MD DEPUTY EDITOR: Vanessa A Barss, MD, FACOG All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jan 2024. This topic last updated: Nov 28, 2023. INTRODUCTION Labor is defined as regular and painful uterine contractions that cause progressive dilation and effacement of the cervix. The rate of cervical dilation becomes faster after the cervix is completely effaced. Normal labor results in descent and eventual expulsion of the fetus. Parity affects this process: Parous patients who have had a previous vaginal birth have faster labors than nulliparous patients. Although determining whether labor is progressing normally is a key component of intrapartum care, determining the time of labor onset, measuring its progress, and evaluating the uterine, fetal, and pelvic factors that affect its course are an inexact science. "Abnormal labor," "dystocia," and "failure to progress" are traditional but imprecise terms that have been used to describe a labor pattern deviating from that observed in most patients who have a spontaneous vaginal birth. These labor abnormalities are best described as protraction disorders (ie, slower than normal progress) or arrest disorders (ie, complete cessation of progress). By convention, an abnormally long active phase is usually described as protracted, whereas an abnormally long latent phase or second stage is usually described as prolonged. Recognizing abnormal labor progression and initiating appropriate interventions are important because it is associated with increased risks for operative delivery and maternal and neonatal morbidity. This topic will provide an overview of labor progress and discuss risk factors for abnormal progression. Management of normal labor and delivery; diagnosis and management of abnormalities of the latent phase, first stage, and second stage; and management of the third stage are reviewed separately: (See "Labor and delivery: Management of the normal first stage".) (See "Labor: Diagnosis and management of the latent phase".) (See "Labor: Diagnosis and management of an abnormal first stage".) (See "Labor: Diagnosis and management of a prolonged second stage".) (See "Management of the third stage of labor: Prophylactic pharmacotherapy to minimize hemorrhage" and "Retained placenta after vaginal birth".) DEFINITIONS FOR THE STAGES AND PHASES OF LABOR Interpretation of labor progress is based on the stage and phase: First stage: The time from onset of labor to complete cervical dilation. To document the onset of labor, patients are simply asked the time when they believe labor began (ie, when contractions started to occur regularly every three to five minutes for more than an hour). The time of complete dilation is when this finding is first identified on physical examination. It is impossible to determine the precise times of both the start of labor and complete dilation since the normal uterus contracts intermittently and irregularly throughout gestation, the initial regular contractions at the onset of labor are mild and infrequent, initial cervical changes are subtle, and physical examination to document cervical change is performed intermittently. Phases: The first stage consists of a latent phase and an active phase. The latent phase is characterized by gradual cervical change; the active phase is characterized by more rapid cervical change. The labor curve of parous patients may show an inflection point between the latent and active phases; this point occurs at approximately 5 cm dilation. In nulliparous patients, the inflection point is often unclear and, if present, occurs at a more advanced cervical dilation, typically at approximately 6 cm or more. In any case, the inflection point is a retrospective finding. Second stage: The time from complete cervical dilation to fetal expulsion. Phases: Traditionally, the second stage does not have phases; however, when pushing is delayed, some clinicians divide the second stage into a passive phase (from complete cervical dilation to onset of active maternal expulsive efforts) and an active phase (from beginning of active maternal expulsive efforts to expulsion of the fetus). Third stage: The time between fetal expulsion and placental expulsion. Fourth stage: Some clinicians identify a fourth stage of labor, which is commonly defined as the first hour or two after placental expulsion when the uterus begins the process of involution. WHAT IS NORMAL LABOR PROGRESSION? Background — In the 1950s, Emanuel Friedman described criteria for the normal progress of labor (mean, 5th and 95th percentiles of cervical dilation over time) [4,5]; these criteria were used for assessment and management of labor for decades. (See 'Friedman (historic) observations' below.) Since 2010, several studies have evaluated the normal progress of labor in thousands of patients to establish contemporary criteria [6-8]. Most notably, Zhang et al studied data from the Consortium of Safe Labor, which included over 62,000 laboring patients at 19 hospitals in the United States and provided robust contemporary data (see 'Contemporary observations' below). Importantly, these data describe time spent in labor resulting in a vaginal birth in contemporary United States hospitals, which is not necessarily a natural or unaided process. Although patients included in the contemporary dataset began labor spontaneously, over 45 percent received oxytocin for labor augmentation and nearly 75 percent received epidural analgesia. Contemporary criteria are different from those described by Friedman. Specifics of these observations are detailed below (see 'Contemporary observations' below). The active phase can start at a more advanced cervical dilation, and dilation can be slower than originally described and can still be normal (ie, associated with a high chance of vaginal birth and normal newborn outcome) [9,10]. This change in the labor curve can be attributed to changes in patient characteristics and obstetric practices. In contemporary cohort studies, the parturients tended to be more racially diverse, older, and more overweight/obese than Friedman's parturients. Oxytocin and epidural were utilized more frequently, and episiotomy and instrument-assisted vaginal birth were performed less frequently in contemporary studies than in Friedman's parturients. Many clinicians believe that the Consortium of Safe Labor data should now inform evidence-based labor management since these robust contemporary data better reflect contemporary parturients and labor practices than Friedman's initial data, which were based on labors in only 500 nulliparous and 500 parous patients with different maternal characteristics and managed at a single institution by different obstetric norms. The characteristics of normal labor progression remain unclear and controversial. Friedman and Cohen have not accepted the revision of the classic labor curve, arguing that the shape of the contemporary curve may have been influenced by selection biases, confounders, and statistical methods [12-16]. They argue that the change in guidelines has not confidently demonstrated that their implementation has safely and consistently reduced the rate of cesarean birth. Friedman (historic) observations — Emanuel Friedman conducted his now classic studies defining the spectrum of normal labor by evaluating the course of labor of 500 nulliparous and 500 parous patients admitted to the Sloane Hospital for Women in New York in the mid-1950s [4,17,18]. Many more patients were studied in subsequent years to validate the criteria. The norms established by these data, called the "Friedman curve" ( figure 1), were widely accepted as the standard for assessment of normal labor progression for decades. Key findings were: First stage The rate of cervical dilation is slow until approximately 3 to 4 cm (ie, latent phase), at which time there is transition to more rapid dilation (ie, active phase). The statistical minimum rates (fifth percentile) of normal cervical dilation during the active phase for nulliparous and parous patients were 1.2 and 1.5 cm/hour, respectively. There is a deceleration phase in cervical dilation at approximately 9 cm. Second stage – The statistical maximum duration (95th percentile) for the second stage also differs by parity: Nulliparous patients: 2.9 hours Parous patients: 1.1 hours Contemporary observations — The Consortium on Safe Labor collected detailed information from electronic medical records of >228,000 births from 19 hospitals across the United States from 2002 to 2008. To establish criteria for normal labor progress, Zhang and colleagues evaluated a subset of these data, which included information on 62,415 singleton pregnancies with spontaneous onset of labor, cephalic vaginal birth (≥88 percent were spontaneous), and normal neonatal outcome. Key findings were: First stage Once labor enters the active phase, cervical dilation is at least 1 to 2 cm/hour by both historic and contemporary criteria. However, over 50 percent of patients in the Consortium on Safe Labor database did not dilate by 1 to 2 cm/hour until they reached 6 cm. Indeed, many nulliparous and parous patients who went on to have a spontaneous vaginal birth took over six hours to dilate from 4 to 5 cm and over three hours to dilate from 5 to 6 cm ( table 1), without an abrupt change in the rate of cervical dilation indicating a clear transition from the latent to active phase. These findings suggest that the normal rate of cervical change between 4 and 6 cm dilation can be much slower than that described by Friedman (see 'Friedman (historic) observations' above) and that slow cervical dilation between 4 and 6 cm reflects the shallow slope of the latent phase, not a protracted active phase [7,19,20]. Nulliparous and parous patients appeared to progress at a similar pace before 6 cm. Beyond 6 cm dilation, the cervix dilated more rapidly in both nulliparous and parous patients (but faster in parous than nulliparous patients), suggesting that the active phase begins by 6 cm in all patients and that slow cervical dilation (ie, less than approximately 1 to 2 cm/hour) beyond this point is a deviation from the slope of the contemporary normal labor curve and abnormal if it persists. A deceleration phase at the end of the first stage was not observed. The median (95th percentile) times for labor duration from 4 to 10 cm in nulliparous and parous patients were 5.3 hours (16.4) and 3.8 hours (15.7), respectively. In contrast, Friedman reported the corresponding mean (95th percentile) durations in nulliparous and parous patients were 4.6 hours (11.7) and 2.4 hours (5.2), respectively. The increase in labor duration in contemporary studies persisted after adjustments were made for maternal and pregnancy characteristics , suggesting that changes in labor practice patterns may be the primary reason for the increase. Although epidural use has increased dramatically since the 1960s, increased use of epidural anesthesia does not fully account for the difference. Further study is required. Second stage – The median (95th percentile) duration of the second stage was ( table 1) : Nulliparous patients without epidural anesthesia: 0.6 hours (2.8) Nulliparous patients with epidural anesthesia: 1.1 hours (3.6) Parous patients without epidural anesthesia: 0.2 hours (1.3) Parous patients with epidural anesthesia: 0.4 hours (2) In addition to type of anesthesia, other characteristics such as diabetes, preeclampsia, fetal size, chorioamnionitis , duration of the first stage , maternal height, and station at complete dilation may also play a role in predicting the duration of the second stage, but standards that account for these other characteristics are not available. The effect of induction is discussed below. Contemporary management — The most recent recommendations from ACOG and SMFM , and the revised World Health Organization (WHO) partograph [26,27] have all incorporated changes based on data from Zhang et al. However, trials to date comparing these two approaches to labor management are limited and it remains unclear if one approach is superior to the to the other. The Labour Progression Study (LaPS) attempted to determine whether use of Friedman criteria to diagnose normal versus abnormal labor progress resulted in better labor outcomes than use of contemporary criteria. In this multicenter cluster-randomized trial in Norway comparing labor outcome in patients managed with a WHO partogram based on Friedman data with those managed with a partogram based on contemporary (Zhang) data, intrapartum cesarean birth rates and adverse outcomes were similar for the two groups. Interestingly, both study groups demonstrated a reduced frequency of intrapartum cesarean birth during the study period when compared with the frequency prior to the study, which supports the theory that an increased focus on labor progress results in reduced rates of intrapartum cesarean birth. Another cluster-randomized trial demonstrated a small but statistically significant reduction in rate of cesarean birth with implementation of the new guidelines. In the intervention group, the rate of cesarean birth decreased from 22.5 to 21.8 percent with the intervention and in the control group, the rate of cesarean birth increased from 23.2 to 23.5 percent (odds ratio for incremental change over time, adjusted for hospital and patient characteristics, 0.90; 95% CI 0.80 to 0.99; p = 0.04; adjusted risk difference, -1.8%; 95% CI, -3.8 to -0.2). Normal progression in induced labors — The latent phase (defined as dilation 1600 term pregnancies that reached the second stage of labor. Active phase and second stage — The durations of the active phase (time to dilate from 6 to 10 cm) and the second stage in induced labors appear to be similar to or shorter than in spontaneous labors ( table 2A-B) [30-34]. ASSESSMENT OF LABOR PROGRESS Digital examination — Cervical examinations to document cervical dilation, effacement, and fetal station are usually performed: On admission At two- to four-hour intervals in the first stage Prior to administering analgesia/anesthesia When the parturient feels the urge to push (to determine whether the cervix is fully dilated) At one- to two-hour intervals in the second stage (to evaluate descent) If fetal heart rate abnormalities occur (eg, to check for cord prolapse or a change in station due to uterine rupture, to assess fetal position and station for possible vacuum- or forceps-assisted vaginal birth) More frequent examinations are warranted when there is a concern about labor progress, but they increase the risk of contaminating the intrauterine contents with vaginal flora. (See "Clinical chorioamnionitis", section on 'Risk factors'.) A limitation of digital examination is that it is imprecise, which is not a problem when monitoring most labors, but is a concern when the clinician is trying to determine whether cervical dilation and fetal descent are advancing too slowly or not at all. In a study that evaluated the accuracy of digital measurement of cervical dilation with a position-tracking system, when cervical dilation was >8 cm, the mean error of digital examination was 0.75±0.73 cm, and when cervical dilation was 6 to 8 cm, the mean error was 1.25±0.87 cm, which is substantial. However, a nondigital method for examining cervical dilation is not available for clinical use. Partogram — Results of cervical examinations may be documented on a partogram (also called a partograph), which enables graphic comparison of the curve of the patient's cervical dilation over time with the curve representing the expected lower limit of normal progress. The partogram in the figure ( figure 2) is based on cervical dilation at admission and shows the curves for the minimum rate of labor progress achieved by 95 percent of nulliparous patients with singleton term pregnancies in spontaneous labor who had a vaginal birth and normal neonatal outcome. Right deviation from this curve suggests a protraction or arrest disorder. Recently, the World Health Organization (WHO) published a revised partograph based on the revised global recommendations for intrapartum care published in 2018 [26,27]. Although useful for visualizing labor progress, routine use of a partogram has not been proven to significantly improve obstetric outcome, and no partogram has been proven to be superior to others in comparative trials [36-38]. Ultrasound — Ultrasound cannot be used to measure cervical dilation without the use of special equipment. Although not widely used clinically, it can document fetal position and descent, the presence and extent of caput, and rotation (when performed serially) in the second stage. For assessing fetal position and station, it appears to be more objective and reproducible than digital examination. One technique is transperineal ultrasound (TPUS) measurement of the angle between the symphysis pubis and the leading part of the fetal skull (called the angle of progression [AoP]) ( figure 3) between contractions. Station is then determined from AoP using a formula or a table. When the ultrasound is performed at the beginning of the second stage, this technique may be used to predict the likelihood of spontaneous vaginal birth. In a metaanalysis (8 studies, 887 pregnancies), AoPs from 108 to 119 degrees yielded the highest sensitivity (94 percent) and AoPs from 141 to 153 degrees yielded the highest specificity (82 percent) for predicting spontaneous vaginal birth. Another approach is to measure the head to perineum distance (HPD) serially to assess descent over time; however, station cannot be determined because the HPD measurement does not account for the curvature of the birth canal. In a study of nulliparous patients in spontaneous labor at term, HPD of 30 mm and AoP 125 degrees each predicted birth within three hours (95% CI 2.5-3.8 hours and 2.4-3.7 hours, respectively) in those who went on to have a vaginal birth.