The study of normal labor is fraught with uncertainties, the least of which includes the definition. By strict definition, labor begins with uterine contractions that bring about demonstrable effacement and dilatation of the cervix.1,2 A landmark study of this process was conducted by Friedman in the 1950s when he developed a labor curve using statistical observations with division of labor into first and second stages.3,4 In the 1960s and 1970s, Friedman defined the onset of labor as the point at which the woman perceives regular uterine contractions and provided definitions for the expected duration of the stages of labor.5,6 Abnormal progression in active labor was defined as cervical dilatation less than 1.2 cm per hour in nulliparous women and less than 1.5 cm per hour in multiparous women. Arrest of labor was defined as no appreciable change in cervical dilatation for more than 2 hours. These limits have been the paradigm for labor management for more than 50 years. Zhang and colleagues have evaluated labor times to characterize labor patterns in contemporary obstetric practice.7,8 They found that the rate of cervical dilatation accelerated after 6 cm and that progress from 4 cm to 6 cm was slower than previously described by Friedman.
Despite more than 50 years of study, only Hellman and Prystowsky9 have described a proportional relationship between the first and second stages of labor. The purpose of this study was to evaluate the relationship, if any, of the length of the second stage of labor compared with the length of the first stage of labor.
PATIENTS AND METHODS
Parkland Hospital is a tax-supported institution serving Dallas County. The obstetric service is staffed by house officers, certified nurse midwives, and faculty of the Department of Obstetrics and Gynecology at the University of Texas Southwestern Medical Center. Our service maintains a computerized database of selected obstetric and neonatal outcomes for all women delivering at Parkland. Nurses attending each delivery complete an obstetric data sheet, and research nurses assess the data for completeness and consistency before electronic storage. The data sheets included the obstetric estimate of gestational age that was used to manage the care of women during the intrapartum period. Briefly, the reported date of the last menstrual period was accepted to be correct if the fundal height measured between 18 and 30 weeks of gestation corresponded to the predicted gestational age. Women with discrepancies underwent ultrasonography. Neonatal outcomes are abstracted from discharge records. The data are entered into a secure password-protected file server accessible only by the departmental epidemiologist (D.M.). Deidentified data can then be extracted for analysis. This was an observational study performed without a priori sample size calculations. This analysis was approved by the institutional review board of the University of Texas Southwestern Medical Center at Dallas.
Between January 1, 2001, and June 30, 2012, a total of 172,522 women delivered neonates at our hospital. Nulliparous women with a live singleton fetus in cephalic presentation and in spontaneous labor at or beyond 37 0/7 weeks of gestation were studied. This study was limited to women with uterine contractions and cervical dilatation 3–4 cm on arrival to the labor unit with or without ruptured membranes. Women with more advanced cervical dilatation were excluded. Women with prior cesarean delivery or known fetal malformations were also excluded as were those with any obstetric or medical complications. Such complications included placental abruption, placenta previa, diabetes (gestational or pregestational), any hypertensive disorders, and disorders of amniotic fluid volume. The onset of the first stage of labor was defined as admission at 3- to 4-cm cervical dilatation in the presence of uterine contractions with or without rupture of membranes. The second stage was defined as the length of time from observed complete cervical dilatation to birth.
We use a standardized protocol for labor management at our hospital based on the implementation of specific, sequential interventions when inadequate labor is suspected.1,2 This approach is used in both complicated and uncomplicated pregnancies. These guidelines stipulate that pelvic examinations should be performed approximately every 2–3 hours. Ineffective labor is suspected when the cervix fails to dilate at least 1 cm per hour within 2–3 hours of admission. Amniotomy is then performed if the fetal membranes are still intact and an intrauterine pressure catheter is placed to assess uterine contractions. Hypotonic contractions and no cervical dilation after an additional 2–3 hours is managed with stimulation of labor using an oxytocin regimen previously described.10,11 Uterine activity of 200–250 Montevideo units is expected for 2–4 hours before dystocia is diagnosed. Dilatation rates of 1–2 cm per hour are accepted as evidence of progress after satisfactory uterine activity has been established with oxytocin.
The characteristics studied included time intervals for the durations of the first and second stages of labor, maternal habitus (defined as maternal body mass index [BMI, calculated as weight (kg)/[height (m)]2]), use of epidural analgesia, route of delivery, and neonate birth weight. Each woman was analyzed based on the specific lengths of her first and second stages of labor. Put another way, only paired observations were used for each woman. The change in the mean length of the second stage in hours by the increasing length of first stage measured in seven categories was evaluated using analysis of variance with the alternative hypothesis of increasing second stage times with increasing first stage times. For comparisons of frequencies between cohorts, Pearson’s χ2 test was used when appropriate. For those outcomes with small cell sizes, Fisher’s exact test was substituted. The Mantel-Haenszel test for trend was used to examine increases in frequency of prolonged second stage (2 or more hours) by first stage times stratified by selected obstetric characteristics. Analysis was performed using SAS 9.2 and P<.05 was judged statistically significant.
A total of 172,522 women were delivered during the study period and 12,523 (7.3%) met the inclusion criteria. Selected demographic characteristics are shown in Table 1. The women were predominantly Hispanic, and a large proportion had BMIs of 30 or more. Shown in Table 2 is an overview of the first and second stages of labor in the study cohort. Thirty-eight percent of women received labor augmentation, and 61% received epidural analgesia (Table 2). Only 4% of parturients required cesarean delivery. Because women had to reach the second stage to be included in this analysis, this cohort by definition only includes those cesarean deliveries performed during the second stage (Table 2). The most common indication for second-stage cesarean delivery was dystocia (91%).
The relationship of the first and second stages of labor was analyzed by dividing the cohort into seven subgroups according to the length of the first stage in hours plotted against each woman's corresponding length of the second stage (Fig. 1). The length of the second stage significantly increased concomitant with increasing length of the first stage (P<.001 for trend). We then examined three factors that might affect the relationship of the first and second stages of labor. These included neonate macrosomia defined as birth weight 4,000 g or more, epidural analgesia, and maternal BMI 30 or more. As shown in Figures 2, 3, and 4, as the length of the first stage increased, so also did the second stage regardless of neonate macrosomia, epidural analgesia, or maternal BMI 30 or more. This relationship also persisted in those with interventions during the first stage of labor to include artificial rupture of membranes and oxytocin augmentation as well as those in whom received operative vaginal delivery (data not shown).
The study cohort was then divided into subgroups to further analyze the relationship of the lengths of the first and second stages of labor (Table 3). We selected the 95th percentile value for the lengths of the observed first and second stages of labor and subdivided the cohort into those less than the 95th percentile and greater than the 95th percentile. The 95th percentile was 15.6 and 2.9 hours for the first and second stages, respectively. As shown in Table 3, women with prolonged first stages of labor had approximately a fourfold increased rate of a prolonged second stage. That is, women with first stages greater than the 95th percentile had a 16.3% rate of a second stage length greater than the 95th percentile compared with 4.5% (P<.001) in women with first stages less than the 95th percentile. Neonate outcomes of the women with first stage lengths greater than the 95th percentile were then analyzed according to the length of the second stage (Table 4). None of these neonate conditions were significantly increased in those women with first and second stage lengths of labor greater than the 95th percentile when compared with those with second stage lengths of labor less than the 95th percentile.
There are two findings in this analysis of the relationship of the lengths of the first and second stages of labor. First, the length of the second stage increased as the length of the first stage increased. Moreover, women with first stages exceeding the 95th percentile (15.6 hours) had a fourfold increased rate of second stages exceeding the 95th percentile (2.9 hours). Second, amniotomy and oxytocin augmentation for inadequate progress in the first stage as well as intrapartum factors such as epidural analgesia, fetal macrosomia, and maternal BMI 30 or more did not significantly affect the relationship of the first and second stages of labor.
That the duration of the second stage might be increased when the first stage is increased seems to be almost an intuitive expectation. However, we were able to find only one mention of the explicit description of the dependent relationship of the duration of the first and second stages that was by Hellman and Prystowsky in their important 1952 article on the maternal and neonate consequences of prolonged second stages of labor9 (English language MEDLINE search using the key words “relationship” or “duration,” and “first stage” or “second stage,” and “labor”). They wrote, “prolongation of the first stage can be seen to be definitely correlated with a significant and progressive increase in the duration of the second stage.” Sixty years later, we find the same correlation. This correlation could be interpreted to signify that abnormalities in the first stage are premonitory for second-stage abnormalities in at least some women with prolonged first stages. However, what effect does this relationship of the first and second stages have on neonate outcomes? Our results, although showing a relationship of the lengths of the first and second stage, did not reveal a neonate effect, albeit this may have been related to sample size.
The management of labor has increasingly come under scrutiny as a result of the rapidly escalating use of primary cesarean delivery.12 For example, Spong and colleagues12 have recommended that adequate time for normal first stages of labor and for the second stages should be allowed and that the adequate time for each of these stages appears to be longer than traditionally estimated. The duration of the second stage has been of special interest in the obstetric history primarily as a result of concerns about the welfare of the neonate. We recently reviewed the literature between 1952 and 2009 on second-stage safety and found that few series included large numbers of second stages greater than 3 hours, which is the current American College of Obstetricians and Gynecologists threshold for second-stage duration in nulliparous women with epidural analgesia.13,14 In fact, there have been only three series reported that contained more than 1,000 women reaching a 3-hour second-stage threshold. One of these series included 1,489 women with second stages exceeding 3 hours and delivered at Parkland.13 In this series, we found that most women reaching 4 hours into the second stage did so because they were awaiting a cesarean delivery that had been decided at the 3-hour mark. We concluded that current reports on the safety of the second stage likely include large proportions of unintentionally prolonged second stages calling into question the notion that very prolonged second stages can be intentionally advocated as safe for the mother and neonate. Moreover, the three large series of second stages exceeding 3 hours all found either or both significantly increased maternal or neonatal morbidity.13,15,16 We emphasize these series to again underscore the importance of sample size when assessing low-frequency adverse events such as neonate morbidity at term. Sample size considerations, if applied to the results we now report on the time-dependent relationship of the first and second stages of labor, would stipulate that to determine the effects on neonate condition at birth measured using a composite of 5-minute Apgar scores of 3 or less or umbilical artery blood pH less than 7.1 would require a sample size of 860,000 total deliveries for 80% power because the composite ranged between 1.3% and 2.9% in our cohort. Clearly, this greatly exceeds the sample available to us for this analysis of the relationship of the lengths of the first and second stages of labor.
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© 2013 by The American College of Obstetricians and Gynecologists.
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