Background
High station at specific points in the first stage of labor, such as a floating head on admission, or at 4-cm dilation or when arrest of dilation occurs, is associated with higher rates of failure to deliver vaginally. Therefore it could be useful to know if station is within an expected range at a given dilation during first stage. Arrest of descent disorders have been defined thus far on criteria applicable in the second stage. Statistical modeling is an attractive methodology to characterize the relationship between station and dilation because the resulting mathematical expressions could be used as a reference for comparison in the future. In addition, they can be used to produce a finely graded assessment of descent using numerical terms such as percentile rankings. A 2-step approach to potentially improving the assessment of station could be to develop a statistical model that describes the general relationship between station and dilation in the first stage of uncomplicated births and then determine if such a model would have identified births with complications related to poor labor progress. Given the complex nature of labor data, especially the imprecision of dilation and station measurement, it is not immediately evident that such a model is identifiable or what its precision would be.
Objective
We sought to characterize in mathematical terms the relationship of station to dilation during the first stage of labor for nulliparous and multiparous women with spontaneous vaginal births.
Study Design
This retrospective cohort study included 28,121 exams from 5555 women with singleton cephalic presentations at ≥37 weeks’ gestation with electronic fetal monitoring tracings, who delivered vaginally without instrumentation and had 5-minute Apgar scores >6 at 2 academic community referral hospitals in 2012 through 2013. Women with a previous cesarean birth were excluded. We used longitudinal statistical techniques suitable to biological data that were irregularly sampled with repeated measures over time.
Results
A linear relationship was observed between station and dilation. For both nulliparous and multiparous women the final model was a linear regression with random effects for intercept and slope and a first-order autoregressive correlation structure. The 5th-95th range of station at any given dilation spanned about 3-4 cm.
Conclusion
Our results demonstrate a general trend of increasing descent of the presenting part as dilation advances during the first stage of labor in women who delivered vaginally without instrumentation. We propose that the mathematical expressions describing this relationship may be valuable in the assessment of first-stage labor progression.
Introduction
High station in the first stage can be a harbinger of cesarean or difficult birth. Several clinical studies have reported that high station at certain points in the first stage of labor, such as a floating head on admission, or at 4-cm dilation or when arrest of dilation occurs, is associated with higher rates of failure to deliver vaginally. Therefore it could be useful to know if station is within an expected range at specific dilations during the first stage. Although descent begins during the first stage, arrest disorders of descent are essentially pass-fail criteria applicable in the second stage only.
A 2-step approach to potentially improving the assessment of descent could be to develop a statistical model that describes the general relationship between station and dilation in the first stage of labor in uncomplicated births and then determine if such a model could identify births with complications related to labor progress disorders. Statistical modeling is an attractive methodology because the resulting mathematical expressions may be used as a reference for comparison in the future. In addition, they can be used to produce a finely graded assessment of descent using numerical terms such as percentile rankings.
The relationship between dilation and station in the first stage has been shown indirectly by plotting both dilation and station over time on the same graph as in the classic Friedman or contemporary labor curves. A statistical model of the relationship between station and dilation has not been reported previously.
The objective of this study was to create a mathematical expression describing the relationship between station and dilation during the first stage for nulliparous and multiparous women with spontaneous vaginal deliveries.
Materials and Methods
In this retrospective cohort study, deidentified data were extracted from the departmental electronic perinatal database for the clinical variables on all births between Jan. 1, 2012, and Dec. 31, 2013, at 2 acute care, academic community teaching hospitals and regional referral centers in the Baltimore-Washington corridor, MedStar Franklin Square Medical Center and MedStar Washington Hospital Center. The inclusion criteria were all labors with singleton cephalic presentations at ≥37 weeks’ gestation with electronic fetal monitoring tracings, delivered vaginally without instrumentation and with 5-minute Apgar scores >6. Women with a previous cesarean birth were excluded.
All data including cervical dilation, effacement and station and clinical data were extracted from the perinatal electronic medical record, PeriBirth (PeriGen, Cranbury, NJ.) Dilation and station were measured in centimeters where station values could range from –5 to +5.
First we examined the data using scatter plots, trajectory plots, and variograms.
Based on the observation of these plots and inherent biological variation in the process of labor, we chose a modeling approach from the extended linear mixed model family. We fit our data to several models of increasing complexity within the extended linear mixed model family, and chose the final model using the Akaike information criterion.
Since both fetal station and cervical dilation are closely related to time, we also tested the effect of time by adding minutes elapsed as a covariate to the model.
We checked the final model regarding assumptions of normality.
All statistical analyses were conducted using R, Version 3.0.2.
This study was reviewed and approved by the MedStar Research Institute.
Materials and Methods
In this retrospective cohort study, deidentified data were extracted from the departmental electronic perinatal database for the clinical variables on all births between Jan. 1, 2012, and Dec. 31, 2013, at 2 acute care, academic community teaching hospitals and regional referral centers in the Baltimore-Washington corridor, MedStar Franklin Square Medical Center and MedStar Washington Hospital Center. The inclusion criteria were all labors with singleton cephalic presentations at ≥37 weeks’ gestation with electronic fetal monitoring tracings, delivered vaginally without instrumentation and with 5-minute Apgar scores >6. Women with a previous cesarean birth were excluded.
All data including cervical dilation, effacement and station and clinical data were extracted from the perinatal electronic medical record, PeriBirth (PeriGen, Cranbury, NJ.) Dilation and station were measured in centimeters where station values could range from –5 to +5.
First we examined the data using scatter plots, trajectory plots, and variograms.
Based on the observation of these plots and inherent biological variation in the process of labor, we chose a modeling approach from the extended linear mixed model family. We fit our data to several models of increasing complexity within the extended linear mixed model family, and chose the final model using the Akaike information criterion.
Since both fetal station and cervical dilation are closely related to time, we also tested the effect of time by adding minutes elapsed as a covariate to the model.
We checked the final model regarding assumptions of normality.
All statistical analyses were conducted using R, Version 3.0.2.
This study was reviewed and approved by the MedStar Research Institute.
Results
The study sample included data from 5555 labors and 28,121 exams recorded during the 24-hour period ending with spontaneous vaginal birth. Characteristics of the study population are summarized in Table 1 .
| No. | % | |
|---|---|---|
| Nulliparity | 2510 | 45.2 |
| Multiparity | 3045 | 54.8 |
| Diabetes | 251 | 4.5 |
| Hypertension | 546 | 9.8 |
| Induction | 2055 | 37.0 |
| Augmentation | 1121 | 20.2 |
| Epidural anesthesia | 3325 | 59.9 |
| Median | IQR | |
|---|---|---|
| Gestational age | 39.6 | 38.9–40.3 |
| Birthweight | 3320 | 3037–3610 |
| BMI | 30.8 | 27.4–35.3 |
We excluded 210 examinations because either dilation or station values were missing, 441 examinations that were in the second stage, 83 examination with obvious date and time errors, and 13 examinations with obvious sign errors that were influential points. For example, after a labor with progressive descent in a multiparous patient a station value was entered as –4 and the baby delivered vaginally 20 minutes later. The final data set included 5535 labors with 27,374 observations. There were 14,320 observations in 2507 nulliparous births, and 13,054 observations in 3028 multiparous births. Figure 1 shows the percentage of examinations at each level of station.
