Background
New labor curves have challenged the traditional understanding of the general pattern of dilation and descent in labor. They also revealed wide variation in the time to advance in dilation. An interval of arrest such as 4 hours did not fall beyond normal limits until dilation had reached 6 cm. Thus, the American College of Obstetricians and Gynecologists/Society for Maternal–Fetal Medicine first-stage arrest criteria, based in part on these findings, are applicable only in late labor. The wide range of time to dilate is unavoidable because cervical dilation has neither a precise nor direct relationship to time. Newer statistical techniques (multifactorial models) can improve precision by incorporating several factors that are related directly to labor progress. At each examination, the calculations adapt to the mother’s current labor conditions. They produce a quantitative assessment that is expressed in percentiles. Low percentiles indicate potentially problematic labor progression.
Objective
The purpose of this study was to assess the relationship between first-stage labor progress- and labor-related complications with the use of 2 different assessment methods. The first method was based on arrest of dilation definitions. The other method used percentile rankings of dilation or station based on adaptive multifactorial models.
Study Design
We included all 4703 cephalic-presenting, term, singleton births with electronic fetal monitoring and cord gases at 2 academic community referral hospitals in 2012 and 2013. We assessed electronic data for route of delivery, all dilation and station examinations, newborn infant status, electronic fetal monitoring tracings, and cord blood gases. The labor-related complication groups included 272 women with cesarean delivery for first-stage arrest, 558 with cesarean delivery for fetal heart rate concerns, 178 with obstetric hemorrhage, and 237 with neonatal depression, which left 3004 women in the spontaneous vaginal birth group. Receiver operating characteristic curves were constructed for each assessment method by measurement of the sensitivity for each complication vs the false-positive rate in the normal reference group.
Results
The duration of arrest at ≥6 cm dilation showed poor levels of discrimination for the cesarean delivery interventions (area under the curve, 0.55–0.65; P < .01) and no significant relationship to hemorrhage or neonatal depression. The dilation and station percentiles showed high discrimination for the cesarean delivery–related outcomes (area under the curve, 0.78–0.93; P < .01) and low discrimination for the clinical outcomes of hemorrhage and neonatal depression (area under the curve, 0.58–0.61; P < .01).
Conclusions
Duration of arrest of dilation at ≥6 cm showed little or no discrimination for any of the complications. In comparison, percentile rankings that were based on the adaptive multifactorial models showed much higher discrimination for cesarean delivery interventions and better, but low discrimination for hemorrhage. Adaptive multifactorial models present a different method to assess labor progress. Rather than “pass/fail” criteria that are applicable only to dilation in late labor, they produce percentile rankings, assess 2 essential processes for vaginal birth (dilation and descent), and can be applied from 3 cm onward. Given the limitations of labor-progress assessment based solely on the passage of time and because of the extreme variation in decision-making for cesarean delivery for labor disorders, the types of mathematic analyses that are described in this article are logical and promising steps to help standardize labor assessment.
Since the landmark publications of Friedman and others to quantify labor progress over time, clinicians have used “normal” labor curves to help identify “abnormal” labor. More recently, the publication of several contemporary labor curves that were based on data with current management and interventions has renewed interest in the evaluation of labor progress and contributed to new definitions of arrest disorders. The observed range of time to advance 1 cm in dilation or station was very long in normal labor, especially when dilation was <6 cm or station was less than +2. Therefore, it was only late in the first stage that an arrest of dilation that lasted 4 hours fell beyond the 95th percentile that is seen in normal labor.
The wide range of time to dilate is not surprising because cervical dilation has neither a precise nor direct relationship to time. Cervical dilation is a direct response to contractions and that response is modified by many others factors, such as pelvic size and shape or biomechanical factors like cervical compliance and occiput position. Furthermore, most of these factors change over time. Consequently, it is inevitable that the relationship of dilation or descent to time will show wide variation, especially in early labor when contractions and cervical compliance can be very different among mothers. Even the current first-stage arrest definitions, which are applicable only at ≥6 cm dilation when many of the influential factors listed earlier have reached a stable state, require extra criteria based on Montevideo units to account for the influence of different contraction patterns.
Fortunately, there is a broad base of mathematical modeling techniques that can be applied to the problem of the assessment a process with multiple determinants that evolve over time. A stimulus-response function is a mathematical equation that describes the quantitative response to a measured stimulus. Stimulus-response functions in physiology are complex because there are often many factors that influence the response and because the factors are in constant flux. Statistical models are necessary because the response will cover a range because of natural biologic variation, the effect of measurement inaccuracy, and the effect of unmeasured factors.
An adaptive multifactorial model of dilation in response to contractions was developed previously, and the range of variation in the response appeared adequate for clinical utility. Because station has a defined relationship to dilation, it is possible to use that information to estimate expected station at specific points during the first stage of labor. Using these models, one can construct “expected” dilation and station curves for an individual mother. A mother’s actual dilation and station can then be compared with the “expected” ranges and described in percentiles. Percentile ranking are understood easily in clinical medicine; in this application, percentiles summarize the effect of many factors succinctly in a single number. Low percentiles indicate lower than expected dilation and station, taking in to account all the factors in the model including contractions and effacement.
The objective of this study was to assess the relationship between first-stage labor progress and labor-related complications with the use of 2 different labor assessment methods. The first assessment method was based on the first-stage arrest definitions published by the American College of Obstetricians and Gynecologists and the Society for Maternal–Fetal Medicine in 2012. The second method used percentile ranking of dilation and station, based on adaptive multifactorial models. We selected 4 labor-related complications. Two complications were related to intervention: cesarean delivery (CD) for first-stage arrest without fetal heart rate concerns and CD for fetal heart rate concerns. Two complications were actual clinical conditions: obstetric hemorrhage and neonatal depression.
Materials and Methods
We included all deliveries with singleton cephalic presentations at ≥37 weeks gestational age that occurred between January 1, 2012, and December 31, 2013, at 2 acute care academic community hospitals and regional referral centers in the Baltimore-Washington corridor. Both spontaneous and induced labors were included. All cases included patients who had electronic fetal monitoring (EFM) for a least 1 hour before birth and umbilical artery gases measured at birth. Mothers with a previous cesarean birth were excluded. De-identified data were extracted from the departmental electronic perinatal database for the clinical variables, from the laboratory database for the paired cord blood gases and from the EFM archiving system for digital versions of the tracings. Clinical examination data was obtained by clinicians with different levels of experience using standard digital vaginal exam techniques. No specific protocols dictated precise clinical management with respect to CD for labor disorders although arrest of dilation was widely accepted to require at least 2 hours or arrest of dilation in the active phase of labor. This approach allows observation of actual practice rather than practice based on a prospective protocol.
Selecting a marker of abnormal labor progression is challenging because intervention is generally performed to prevent complications, such as fetal or maternal trauma. Prevention has a paradoxic effect; it hinders us from seeing the potential adverse outcome. Moreover, there is no postoperative test to always confirm that a labor-progress complication was truly impending. An experiment in which labor is allowed to take its natural course without intervention is both impractical and unethical. To address these limitations, we choose 4 different outcome markers. Two outcomes were related to the decision for CD intervention, and 2 outcomes were objective patient conditions.
CD for a first-stage labor-progress disorder included all women with a cesarean birth before 10-cm dilation where any of the listed indications included a reference to a disorder of labor progress and there were no references to concerning fetal heart rate patterns, abruption, or cord prolapse.
CD for fetal heart rate concerns included all women with a CD in which any of the listed indications included reference to concerning fetal heart rate patterns or abruption or cord prolapse. The 2 CD outcome groups were mutually exclusive.
The neonatal depression outcome included all babies with any 1 of the following events: umbilical artery base deficit >12 mmol/L or intubation or cardiopulmonary resuscitation in the delivery room. We chose to focus on base deficit because it does indicate that the baby had an exposure to hypoxemia and because it does not fluctuate as rapidly as pH. A pH value alone could represent recent respiratory acidosis developing at the end of the second stage.
Recognizing that there are different definitions of postpartum hemorrhage, obstetric hemorrhage was defined as an estimated blood loss of >500 mL with a vaginal delivery or >1000 mL with CD. These values corresponded to the 95th percentiles of estimated blood loss recorded with vaginal and CD births in these institutions and are consistent with the definition of postpartum hemorrhage provided in the most recent American College of Obstetricians and Gynecologists practice bulletin on postpartum hemorrhage.
The normal reference group included spontaneous vaginal births without neonatal depression or obstetric hemorrhage. Separate analyses were conducted for nulliparous and multiparous mothers.
First-stage labor disorders develop during labor and become increasingly apparent over time. The clinical decision to intervene generally is finalized at the time of the last examination in the first stage. Therefore, we noted dilation, duration of arrest of dilation, the percentile ranking of dilation, and station at the last examination of the first stage. These observations were used to construct receiver operating characteristic (ROC) curves and measure the area under the curve (AUC).
ROC curves were constructed by measuring the sensitivity in each of the complication groups vs the false-positive rate in the normal reference group at varying levels of percentiles or the duration of arrest at ≥6 cm dilation.
Dilation percentile rankings were calculated with a stimulus-response model of cervical dilation. This multifactorial model estimates expected cervical dilation and the 5th to 95th percentile range, based on the variables in its equation. The 5 variables are contraction counts (estimated automatically), dilation, effacement and station at the previous examination (from clinical examinations), and the presence or absence of epidural anesthesia. Calculations are updated at each examination, which allowed the curve to adapt to changing conditions. Nulliparous and multiparous mothers have different equations. These stimulus response models for dilation have been cleared by the Food and Drug Administration for clinical use. The models were developed with the use of longitudinal statistical techniques for unbalanced repeated measures. The development data set included 7731 examinations from 1341 women with uncomplicated spontaneous vaginal deliveries in 3 different hospitals. In addition, they have been tested prospectively and retrospectively. The general structure of the model is shown given:
D i l a t i o n t = ω + α C o n t r a c t i o n s t + β D i l a t i o n t − 1 + γ E f f a c e m e n t t − 1 − ϑ S t a t i o n t − 1 + φ E p i d u r a l t ± r a n g e
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