Materials and Methods
We conducted a 4 year retrospective cohort study of all consecutive term (gestation of ≥37 weeks) deliveries at Washington University School of Medicine (St. Louis, MO) from July 2004 to June 2008 who reached 10 cm dilation. Institutional board review approval was obtained from Washington University School of Medicine.
Women were included if their gestational age was at least 37 0/7 weeks’ gestation at admission to labor and delivery, carried a singleton pregnancy in vertex presentation, and had an arterial umbilical cord gas obtained at delivery. Women were excluded if they had a prior cesarean, delivered preterm, had fetuses with congenital anomalies, or delivered by cesarean before complete dilation.
Detailed information on maternal sociodemographic, obstetric and gynecological history, medical and surgical history, prenatal history, antepartum history, and labor and delivery course was extracted from the medical charts. The labor and delivery records included medications, labor type, cervical examinations, cervical examination times, length of labor stages, mode of delivery, and postpartum record. All data were extracted using close-ended forms by trained research assistants who underwent regularly scheduled training.
Because the first stage of labor can be defined in many ways, receiver-operator characteristic (ROC) curves were generated to determine the definition of the first stage of labor most closely associated with maternal and neonatal outcomes. The first stage of labor was defined as the time from admission to complete dilation, time from 4 cm of dilation to complete, dilation and time from 6 cm of dilation to complete dilation.
Women presenting with cervical examinations greater than 4 cm or greater than 6 cm were assigned times based on the time of the first cervical examination to complete. The areas under the ROC curves were calculated for each definition of the first stage. ROC curves were created to visually evaluate the relationship between the length of the first stage and maternal and neonatal outcomes, measured as a composite of any of the maternal and neonatal outcomes of interest.
Maternal outcomes considered were cesarean delivery in the second stage, operative vaginal delivery (forceps and vacuum), postpartum hemorrhage (as documented by the delivery physician), prolonged second stage (specific for parity and regional anesthesia use), and maternal fever. Neonatal outcomes considered were analyzed as a composite of the following: 5 minute Apgar less than 3, arterial cord pH less than 7.0, cord base excess –12 or less, admission to a level 2 or 3 nursery, or shoulder dystocia. Shoulder dystocia was documented by the delivery physician and at our institution is typically defined as requiring at least 1 maneuver to deliver the anterior shoulder.
Thereafter the first stage of labor was defined as the time from 4 cm to complete dilation. The exposure group was defined as having a first stage of labor less than the 90th percentile, between the 90th and 94th percentile, between the 95th and 96th percentile, or the 97th percentile or greater. Percentiles were determined for parity (nulliparous vs multiparous) and labor type (induced vs spontaneous) ( Figure 1 ). Maternal and neonatal outcomes were considered as a composite and individually. Because shoulder dystocia is a potentially debilitating complication that can be prevented with cesarean delivery, the number of cesarean deliveries performed to prevent 1 shoulder dystocia was determined for each cutoff of abnormal labor.
Study groups were compared using a Student t test or Mann-Whitney U test for continuous or χ 2 for categorical variables as appropriate. Potentially confounding variables of the exposure-outcome association were identified in the stratified analyses. Multivariable logistic regression models were then developed to better estimate the effect of the length of the first stage of labor on maternal and neonatal outcomes while adjusting for potentially confounding effects. Clinically relevant covariates for initial inclusion in the models were selected using the results of the stratified analyses, and factors were removed in a backward stepwise fashion, based on significant changes in the likelihood ratio test. Factors considered included parity, race, body mass index, birthweight, and use of oxytocin. All analyses were completed using Stata SE, version 11 (StataCorp, College Station, TX).
Results
Of 5388 women in the cohort, 5030 were included in the analysis (11 excluded for incomplete time data, 347 for prior cesarean). The ROC curves were created to visually estimate the association between the length of the first stage and adverse outcomes using 3 different definitions of the first stage: time from admission to complete dilation, active phase of labor defined as starting at 4 cm, and the active phase of labor defined as starting at 6 cm ( Figure 2 ). All 3 ROC curves had an area under the curve of 0.64-0.66, demonstrating a moderate association between length of labor and the composite of adverse maternal and neonatal outcomes. No curve demonstrated a clear cut point that could be used as a threshold for determining abnormal labor.
Based on this information, we elected to use the time from 4 cm to complete dilation to define the first stage of labor, and this definition was used in the remainder of the analyses. The cutoffs used to define the 90th, 95th, and 97th percentiles from 4 cm to complete dilation by parity and the type of labor can be found in Figure 1 ; the cutoffs ranged from 10 to 18 hours.
Of the 5030 women, 4534 women experienced a first stage 90th percentile or less for parity and labor type, 251 experienced a first stage between the 90th and 94th percentiles, 102 experienced a first stage between the 95th and 96th percentiles, and 143 experienced a first stage at the 97th percentile or greater. The groups were similar with respect to maternal age, insurance status, and the presence of maternal hypertensive disorders ( Table 1 ). Women with a longer first stage, adjusted for parity and whether labor was induced, were more likely to be white, nulliparous, obese, receive oxytocin, have diabetes, have been induced, and have a macrosomic infant.
| Variable | <90th percentile (n = 4534) | 90th-94th percentile (n = 251) | 95th-96th percentile (n = 102) | ≥97th percentile (n = 143) | P value |
|---|---|---|---|---|---|
| Age, y | 24.7 ± 5.9 | 24.5 ± 5.9 | 24.6 ± 5.5 | 25.0 ± 6.0 | .86 |
| Nulliparous | 1790 (39.5%) | 102 (40.6%) | 39 (38.2%) | 59 (41.3%) | .94 |
| Race | .04 | ||||
| Black | 3298 (72.7%) | 185 (73.7%) | 80 (78.4%) | 100 (69.9%) | |
| White | 749 (17.0%) | 45 (17.9%) | 16 (15.7%) | 35 (34.5%) | |
| Hispanic | 292 (6.4%) | 14 (5.6%) | 4 (3.9%) | 3 (2.1%) | |
| Insurance | .10 | ||||
| Private | 644 (14.2%) | 24 (9.6%) | 10 (9.8%) | 23 (16.1%) | |
| Public | 3890 (85.8%) | 227 (90.4%) | 92 (90.2%) | 120 (83.9%) | |
| Obese (BMI ≥30 kg/m 2 ) | 2295 (52.5%) | 152 (62.0%) | 62 (61.4%) | 88 (62.4%) | < .01 |
| Hypertension | 112 (2.5%) | 5 (2.0%) | 5 (4.9%) | 7 (4.9%) | .12 |
| Preeclampsia | 298 (6.6%) | 14 (5.6%) | 10 (9.8%) | 8 (5.6%) | .50 |
| Diabetes | 46 (1.0%) | 8 (3.2%) | 0 | 6 (4.2%) | < .01 |
| Induced | 1397 (43.1%) | 77 (78.6%) | 32 (88.9%) | 43 (82.1%) | < .01 |
| Received oxytocin | 2375 (52.4%) | 216 (86.1%) | 91 (89.2%) | 122 (85.3%) | < .01 |
| Birthweight | 3229 ± 522 | 3375 ± 571 | 3349 ± 553 | 3440 ± 719 | < .01 |
| Macrosomia | 221 (4.96%) | 26 (10.4%) | 9 (8.8%) | 21 (14.7%) | < .01 |
Maternal and neonatal outcomes were examined at each percentile division ( Table 2 ). The risk of a prolonged second stage and maternal fever ( P < .01) increased as the first stage length increased, although the risk of cesarean, operative vaginal delivery, and postpartum hemorrhage remained unchanged. The composite adverse neonatal outcome, shoulder dystocia, and admission to a higher-level nursery increased as the length of the first stage increased ( P < .01). Apgar score less than 3 at minutes, cord pH less than 7.0, and base excess of –12 or less were not associated with increasing length of the first stage.
| Variable | <90th percentile (n = 4534) | 90th-94th percentile (n = 251) | 95th-96th percentile (n = 102) | ≥97th percentile (n = 143) | P value |
|---|---|---|---|---|---|
| Maternal outcomes | |||||
| Cesarean in second stage | 68 (1.5%) | 3 (1.2%) | 3 (2.9%) | 3 (2.1%) | .60 |
| Operative vaginal delivery | 559 (12.5%) | 39 (15.7%) | 13 (13.1%) | 23 (16.4%) | .27 |
| Prolonged second stage | 194 (4.3%) | 13 (5.2%) | 7 (6.9%) | 21 (14.7%) | < .01 |
| Postpartum hemorrhage | 108 (2.4%) | 9 (3.6%) | 3 (2.9%) | 0 | .16 |
| Maternal fever | 162 (3.6%) | 26 (10.4%) | 11 (10.8%) | 19 (13.3%) | < .01 |
| Neonatal outcomes | |||||
| Composite neonatal outcome | 427 (9.5%) | 45 (18.0%) | 15 (14.7%) | 31 (21.7%) | < .01 |
| Shoulder dystocia | 213 (4.7%) | 20 (8.0%) | 8 (7.8%) | 12 (8.4%) | .01 |
| Admission to level 2-3 nursery | 195 (4.3%) | 25 (10.0%) | 8 (7.8%) | 19 (13.3%) | < .01 |
| Apgar <3 | 3 (0.1%) | 0 | 0 | 1 (0.7%) | .06 |
| pH <7.00 | 5 (0.11%) | 0 | 1 (1.0%) | 0 | .08 |
| Base excess –12 or less | 48 (1.1%) | 3 (1.2%) | 1 (1.0%) | 2 (1.4%) | .98 |
Table 3 displays the relative risks and adjusted odds of adverse outcomes at each percentile: less than 90th vs 90th percentile or greater, less than 95th vs 95th percentile or greater, and less than 97th vs 97th percentile or greater. At each percentile examined, a significant increased risk existed for a prolonged second stage, maternal fever, the composite neonatal outcome, and admission to a level 2-3 nursery. The risk of shoulder dystocia was significantly increased for exceeding the 90th and 95th percentiles, although this did not reach significance at the 97th percentile. The relative risk and adjusted odds ratios do not appear to be significantly different between each percentile, as signified by the overlapping confidence intervals.
| Variable | ≥90th percentile compared with <90th percentile | ≥95th percentile compared with <95th percentile | ≥97th percentile compared with <97th percentile | |||
|---|---|---|---|---|---|---|
| RR (95% CI) | AOR (95% CI) | RR (95% CI) | AOR (95% CI) | RR (95% CI) | AOR (95% CI) | |
| Maternal outcomes | ||||||
| Cesarean in second stage | 1.21 (0.61–2.41) | — | 1.65 (0.72–3.76) | — | 1.39 (0.44–4.34) | — |
| Operative vaginal delivery | 1.23 (0.99–1.54) | 1.22 (0.93–1.61) a | 1.19 (0.87–1.62) | 1.19 (0.82–1.73) a | 1.29 (0.88–1.89) | 1.30 (0.81–2.08) a |
| Prolonged second stage | 1.93 (1.40–2.67) | 1.78 (1.23–2.56) b | 2.64 (1.82–3.84) | 2.53 (1.64–3.90) b | 3.35 (2.21–5.08) | 3.19 (1.94–5.26) b |
| PPH | 1.01 (0.56–1.83) | 0.97 (0.53–1.78) c | 0.50 (0.16–1.56) | 0.47 (0.15–1.48) c | — | — |
| Maternal fever | 3.16 (2.37–4.22) | 3.26 (2.34–4.54) d | 3.12 (2.17–4.48) | 3.08 (2.00–4.74) d | 3.26 (2.10–5.07) | 3.01 (1.76–5.15) d |
| Neonatal outcomes | ||||||
| Composite neonatal outcome | 1.94 (1.58–2.38) | 1.88 (1.45–2.44) e | 1.89 (1.44–2.49) | 1.70 (1.20–2.41) e | 2.16 (1.56–2.98) | 1.90 (1.23–2.93) e |
| Shoulder dystocia | 1.72 (1.24–2.38) | — | 1.67 (1.08–2.60) | — | 1.70 (0.98–2.97) | — |
| Admission to level 2-3 nursery | 2.42 (1.81–3.24) | — | 2.38 (1.63–3.47) | — | 2.83 (1.82–4.38) | — |
| Apgar <3 | 3.05 (0.32–29.2) | — | 6.51 (0.68–62.4) | — | 11.4 (1.19–108.8) | — |
| pH <7.0 | 1.83 (0.21–15.6) | — | 3.92 (0.46–33.44) | — | –– | — |
| Base excess –12 or less | 1.14 (0.49–2.66) | — | 1.15 (0.36–3.67) | — | 1.32 (0.33–5.38) | — |
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