Objective
The purpose of this study was to evaluate the labor curves of patients who undergo preterm induction of labor (IOL) and to assess possible predictors of vaginal delivery (VD).
Study Design
Data from the National Institute of Child Health and Human Development Consortium on Safe Labor were analyzed. A total of 6555 women who underwent medically indicated IOL at <37 weeks of gestation were included in this analysis. Patients were divided into 4 groups based on gestational age (GA): group A, 24-27+6 weeks; B, 28-30+6 weeks; C, 31-33+6 weeks; and D, 34-36+6 weeks. Pregnant women with a contraindication to VD, IOL ≥37 weeks of gestation, and without data from cervical examination on admission were excluded. Analysis of variance was used to assess differences between GA groups. Multiple logistic regression was used to assess predictors of VD. A repeated measures analysis was used to determine average labor curves.
Results
Rates of vaginal live births increased with GA, from 35% (group A) to 76% (group D). Parous women (odds ratio, 6.78; 95% confidence interval, 6.38–7.21) and those with a favorable cervix at the start of IOL (odds ratio, 2.35; 95% confidence interval, 2.23–2.48) were more likely to deliver vaginally. Analysis of labor curves in nulliparous women showed shorter duration of labor with increasing GA; the active phase of labor was, however, similar across all GAs.
Conclusion
Most women who undergo medically indicated preterm IOL between 24 and 36+6 weeks of gestation deliver vaginally. The strongest predictor of VD was parity. Preterm IOL had a limited influence on estimated labor curves across GAs.
Approximately 12% of all deliveries in the United States occur at <37 weeks of gestation. Preterm birth is the leading cause of neonatal death and morbidity, contributing to >35% of total infant health care spending, well >5 billion dollars per year. Spontaneous labor precedes approximately 50% of preterm deliveries, the remainder are guided by medical necessity because of either maternal or fetal indications. Cervical favorability, as assessed by Bishop scoring, cervical length, and maternal parity predicted vaginal delivery after induction of labor (IOL) at term. However, data regarding predictors of vaginal delivery and labor curves in pregnancies that undergo preterm IOL are limited.
Using an interval censored analysis, Zhang et al revisited the median progression of labor at term. Active labor occurred most commonly after 6 cm of dilation, and cervical dilation progressed more slowly than previously thought, especially between 4 and 6 cm. These results represent a departure from the Friedman curve and now inform our clinical knowledge of median labor progression in modern obstetric practice. Additionally, inherent differences in the progress of labor have been attributed to specific patient characteristics or clinical conditions. Maternal obesity, gestational age <37 weeks, and even fetal sex have been shown to influence labor progression. In our study, we examined a large, contemporary US labor database to identify labor curves and predictors of vaginal delivery in pregnant women who underwent medically indicated preterm IOL. We hypothesized that gestational age would influence labor curves in women who undergo preterm IOL.
Materials and Methods
This was a retrospective analysis of deidentified data from the Consortium on Safe Labor (CSL). The CSL is a multicenter, retrospective, observational study with detailed labor and delivery information from electronic medical records at 12 clinical centers (which included a total of 19 US hospitals) from 2002-2008; 87% of the deliveries occurred from 2005-2007. Data collected from electronic medical records included demographics, medical history, labor and delivery information, and obstetric, postpartum, and neonatal outcomes. Patient data were supplemented with maternal discharge International Classification of Diseases, ninth revision, codes for each delivery. Each site transferred data in electronic format to the data coordinating center where data were mapped to common categories for each predefined variable. Validation studies indicated that the electronic medical record data represented the medical charts accurately. This analysis was approved by the Institutional Review Board of MedStar Health Research Institute.
The CSL cohort includes information on 233,844 births from 228,562 pregnancies. IOL was a predefined variable when either the patient’s electronic medical record indicated that there was an induction or a start time was recorded in the patient’s chart. The database included a distinct variable for labor augmentation. The indication for induction was used to identify the precursors of delivery and classified with the use of a previously described hierarchy. One site did not provide indications for induction and was not included in the precursor analysis. Four hospitals did not report methods of induction, and 2 hospitals did not report cervical dilation at admission, which left cases from 13 hospitals available for analysis ( Figure 1 ). Fewer than 20% of the remaining cases had an original Bishop score with all 5 components reported. Therefore, we used the previously described simplified Bishop score comprised of dilation, effacement, and station. We defined an unfavorable cervix as a simplified Bishop score ≤4 because of similar sensitivity and specificity to the original Bishop score ≤6.
After excluding data from women with any contraindication to vaginal delivery (ie, vasa previa, complete placenta previa, breech presentation, previous myomectomy or classical cesarean delivery), multifetal gestation, spontaneous labor, and gestational age (GA) <24 weeks, our cohort included 6555 pregnant women who underwent IOL at 24+0 to 36+6 completed weeks ( Figure 1 ). Outcomes were grouped and analyzed by GA that was determined from the labor and delivery admission records: 24+0 to 27+6 weeks (group A), 28+0 to 30+6 weeks (group B), 31+0 to 33+6 weeks (group C), and 34+0 to 36+6 weeks (group D), with further comparison to a control group of women who underwent IOL at 37+0 to 41+6 weeks (group E). Analysis of variance and pairwise comparisons were used to assess differences between GA groups in demographic characteristics and rates of vaginal delivery. Multiple logistic regression analysis that controlled for maternal age, parity, body mass index (BMI), cervical effacement, cervical dilation, and fetal station was used to determine which clinical characteristics that were available at the time of admission were most associated with subsequent vaginal delivery after preterm IOL. A repeated measures analysis with an 8-degree polynomial model was used to determine average labor curves for live births in each GA group. This method takes into account both the interval-censored and repeated-measure nature of cervical dilation data. Stillbirth cases were excluded from the labor curve analysis because of expected variation in clinical management of these cases, especially at an early GA. Because we sought only to describe labor patterns by GA, we did not perform any statistical comparisons of the labor curves among various groups. Significance was considered at a probability value of < .05.
Results
Age, prepregnancy BMI, and current BMI were overall similar across GA groups. The earliest GA group (24-27+6 weeks) had a higher proportion of African American women, parous women, cases with a history of cesarean delivery, and cases with a previous preterm delivery. Within each group, more than one-half of subjects delivered vaginally after IOL. Vaginal delivery rates differed among most GA groups. Rates of vaginal live births were similar in groups A and B then increased gradually and significantly with GA, from 57% (group A) to 80% (group E; Table 1 ).
| Variable | Gestational age, wk | P value a | ||||
|---|---|---|---|---|---|---|
| Group A: 24–27+6 (n = 258) | Group B: 28–30+6 (n = 339) | Group C: 31–33+6 (n = 902) | Group D: 34–36+6 (n = 5056) | Group E: 37–42 (n = 68,965) | ||
| Age, y b | 28.3 ± 6.8 | 26.9 ± 6.7 | 26.6 ± 6.8 | 27.4 ± 6.7 | 27.7 ± 6.1 | < .001 (1,2,3,7,8,9,10) |
| African American, % | 51.7 | 48.9 | 43.8 | 36.4 | 22.0 | < .001 (2,3,4,6,7,8,9,10) |
| Prepregnancy body mass index, kg/m 2 b | 26.9 ± 7.4 | 26.6 ± 7.3 | 27.1 ± 7.9 | 26.6 ± 7.3 | 25.6 ± 6.2 | < .001 (4,9,10) |
| Current body mass index, kg/m 2 b | 30.5 ± 7.7 | 30.8 ± 7.2 | 31.8 ± 7.9 | 32.0 ± 7.6 | 31.2 ± 6.3 | < .001 (2,3,6,10) |
| Nulliparous, % | 44.2 | 58.1 | 53.0 | 50.8 | 46.8 | < .001 (1,2,3,6,7,9,10) |
| Previous cesarean delivery, % | 7.4 | 3.5 | 4.6 | 3.7 | 3.2 | < .001 (1,3,4,9,10) |
| Previous preterm delivery, % | 16.7 | 13.0 | 15.4 | 13.0 | 4.5 | < .001 (4,7,9,10) |
| Vaginal delivery rate (live and stillbirths, % | 70.5 | 64.0 | 69.1 | 77.4 | 80.3 | < .001 (3,4,6,7,8,9,10) |
| Vaginal delivery rate (live births only), % | 56.9 | 54.2 | 66.7 | 77.1 | 80.2 | < .001 (2,3,4,5,6,7,8,9,10) |
a Pairwise comparisons: overall difference across all gestational age groups assessed by analysis of variance; additional result of pairwise comparisons is listed in the parentheses and denoted by the following numbers: 1: A≠B; 2: A ≠C; 3: A ≠D; 4: A ≠E; 5: B ≠C; 6: B ≠D; 7: B ≠E; 8: C ≠D; 9: C ≠E; 10: D ≠E
Hypertensive disease was the precursor indication for preterm IOL in 35% of cases in group A, 51% in group B, 53% in group C, and 41% in group D ( Table 2 ). Within this category, the most common underlying pathophysiologic condition was preeclampsia, followed by chronic hypertension. Fetal anomalies (25-33%) and antepartum stillbirth accounted for up to one-third of preterm inductions at <31 weeks of gestation. By comparison, hypertensive disease and fetal (25%) and maternal (24%) conditions were the most common indications for delivery in GA groups ≥31 weeks of gestation. Premature rupture of membranes preceded 20-25% of preterm labor inductions. Chorioamnionitis was noted in up to 15% of cases at <31 weeks of gestation but occurred less often at >34 weeks of gestation (3%). Rates of gestational and preexisting diabetes mellitus were similar across GA groups. Unspecified fetal and maternal reasons were the most common precursors to induction in the term IOL control group.
| Precursor a | Gestational age, wk | P value b | ||||
|---|---|---|---|---|---|---|
| Group A: 24–27+6 (n = 258) | Group B: 28–30+6 (n = 339) | Group C: 31–33+6 (n = 902) | Group D: 34–36+6 (n = 5056) | Group E: 37–42 (n = 68,965) | ||
| Premature rupture of membranes | 19.8 | 19.2 | 25.3 | 19.6 | 5.5 | < .001 |
| Chorioamnionitis | 15.1 | 10.9 | 5.8 | 2.9 | 3.3 | < .001 |
| Decidual hemorrhage/abruption | 8.9 | 12.1 | 7.3 | 3.3 | 0.8 | < .001 |
| Hypertensive disease (overall category) | 35.3 | 51.0 | 52.7 | 40.9 | 14.2 | < .001 |
| Gestational hypertension | 8.1 | 10.6 | 12.5 | 13.2 | 6.1 | < .001 |
| Preeclampsia | 20.5 | 36.9 | 39.8 | 26.7 | 6.5 | < .001 |
| Superimposed preeclampsia | 12.4 | 17.7 | 16.6 | 8.8 | 1.4 | < .001 |
| Eclampsia | 2.3 | 2.1 | 1.4 | 0.7 | 0.1 | < .001 |
| Chronic hypertension | 16.3 | 20.9 | 21.1 | 13.6 | 3.98 | < .001 |
| Unspecified hypertensive disease | 2.3 | 3.8 | 2.3 | 3.0 | 1.3 | < .001 |
| Maternal medical condition c | 17.8 | 20.7 | 23.7 | 24.0 | 13.8 | < .001 |
| Maternal pregestational diabetes mellitus | 7.4 | 5.0 | 8.0 | 7.4 | 2.4 | < .001 |
| Maternal gestational diabetes mellitus | 5.0 | 5.3 | 8.4 | 9.3 | 6.1 | < .001 |
| Fetal anomaly | 32.6 | 24.8 | 16.1 | 9.7 | 5.7 | < .001 |
| Antepartum stillbirth (intrauterine fetal death) | 31.4 | 15.9 | 7.7 | 1.9 | 0.4 | < .001 |
| Fetal condition d | 28.7 | 28.6 | 25.4 | 24.7 | 15.4 | < .001 |
| Maternal fever on admission | 14.0 | 10.0 | 6.8 | 5.4 | 6.1 | < .001 |
| Admission for fetal reason, not otherwise specified e | 0.4 | 0.3 | 0.1 | 0.3 | 0.5 | .136 |
| Admission for maternal reason, not otherwise specified e | 1.6 | 1.2 | 1.8 | 4.9 | 18.2 | < .001 |
| History of maternal/obstetric condition f | 0.4 | 0 | 0.2 | 0.2 | 0.3 | .870 |
| History of fetal condition | 3.9 | 2.7 | 2.0 | 3.4 | 2.9 | .125 |
| Previous uterine scar | 7.8 | 3.8 | 4.6 | 3.8 | 3.2 | < .001 |
a Sum of precursors can exceed 100% because women could have >1 precursor
b From analysis of variance assessment of overall relationship between variable and gestational age category
c Maternal medical problems (the percentage of women with diabetes melllitus is listed)
d Included conditions such as intrauterine growth restriction and abnormal antenatal testing
e Included only if there was no other pregnancy condition; 1 of the 2 categories that were exclusive of other indications
f Included pregnancy complications in a previous pregnancy (eg, history of fetal death or traumatic first delivery, respectively); 1 of the 2 categories that were exclusive of other indications.
From 28+0 to 36+6 weeks of gestation, nulliparous and parous women who delivered vaginally after IOL had a higher median simplified Bishop score when compared with those who subsequently required cesarean delivery ( Table 3 ). Intravenous oxytocin infusion was the most common method of induction, regardless of GA, parity, or cervical favorability (data not shown). Overall, misoprostol and prostaglandin E2 were used more commonly than mechanical methods to ripen an unfavorable cervix, whereas the use of mechanical ripening was similar in nulliparous women, both at term and preterm (data not shown).
