Materials and Methods
Between December 1, 1997, and April 4, 2000, women whose pregnancies reached 41 completed weeks were seen in a special post-term clinic held at Parkland Hospital. Information about each patient's pregnancy, labor course, and neonatal outcome was prospectively entered into a computerized database maintained by a research nurse. This study was limited to women with singleton cephalic presentations. Women with an anomalous fetus, diabetes, prior cesarean delivery, or other medical or obstetric indications for delivery were excluded. Gestational age was confirmed to be 41 weeks if the stated last menses agreed with an ultrasound examination prior to 26 weeks or the last menses was supported by fundal height measurements between 18 and 30 weeks' gestation.11
The management protocol for women who reach 41 weeks at our institution begins with referral to a specialized clinic staffed by a maternal-fetal medicine specialist (JA). Women identified to be 41 weeks based on gestational age landmarks or ultrasound were scheduled for a two-stage induction attempt beginning at 41–67 weeks. All women received a cervical examination by either an MD or MD-supervised midwives and nurse practitioners during the clinic visit. The first stage included installation of 0.5 mg prostaglandin E2 gel into the cervix the afternoon before scheduled induction of labor with oxytocin (stage 2) the next morning. Women who did not develop sustained uterine contractions with intracervical prostaglandin received oxytocin according to a previously published schedule.12 Briefly, an oxytocin infusion was begun at 6 mU/min and increased by 6 mU/min every 40 minutes to a maximum of 42 mU/min. Labor management was standardized and included cervical examinations every 2–3 hours, with amniotomy when cervical dilatation reached 3–4 cm, followed by internal uterine and fetal heart rate (FHR) monitoring. The uterine activity goal for labor stimulation was more than 200 Montevideo units. Failure to progress was diagnosed and cesarean delivery performed when cervical dilatation or fetal descent ceased for 2–4 hours despite adequate uterine activity.
Statistical analysis was by Pearson chi-square, Mantel–Haenszel chi-square for trend,13 Student t test, and multiple logistic regression. The variables entered into the logistic regression model were selected a priori as variables known to be related to cesarean delivery. These included cervical dilation (modeled as zero or larger), parity (nulliparity to multiparous), epidural (present or absent), gestational age (a continuous variable of completed integral weeks), and induction (induced or spontaneous). Results are presented as means ± standard deviation, number and percent, and odds ratios (OR) with 95% confidence intervals (CI). Wilcoxon rank sum methods were used for non-normally distributed data and are shown as median values with quartiles. All P values are two-sided and were considered statistically significant if less than .05. SAS version 8 (SAS Institute, Cary, NC) was used.
A total of 1325 women with pregnancies 41 to 41–67 weeks were prospectively enrolled in this observational study. A total of 687 (52%) women entered spontaneous labor before their scheduled inductions and the remainder underwent labor induction. Shown in Table 1 are selected demographic characteristics for women who had spontaneous labor compared with those whose labor was induced. There were no significant differences except for nulliparity, which was noted for 54% of induced pregnancies compared with 49% of those with spontaneous labor. As expected, gestational age at delivery was approximately 4 days less, on average, in women who entered spontaneous labor before their scheduled inductions, compared with those who required labor induction (Table 2). Labor was longer and epidural analgesia was more frequent in women who underwent induction than in those with spontaneous labor. Cesarean delivery was significantly increased in women with inductions; this increase was limited to cesareans for failure to progress.
Table 3 shows the cesarean births stratified by cervical dilatation before the onset of labor. This examination occurred at the clinic visit where the induction was scheduled. Using trend analysis, cesarean deliveries were related significantly to cervical dilatation in both study groups. Cervical dilatation was used in this analysis because it was more predictive of cesarean delivery than cervical effacement or fetal head station when analyzed using receiver operator characteristic curves. Further analysis, using chi-square, showed that the increase in cesarean delivery associated with induction of labor was attributable to the subgroup of induced women with undilated cervices.
Logistic regression was used to adjust for cervical dilatation, gestational age, nulliparity, and epidural analgesia (Figure 1). The OR for cesarean delivery associated with labor induction was 1.1 (95% CI 0.9, 1.2). Unlike labor induction, an undilated cervix, nulliparity, and epidural analgesia remained significantly associated with cesarean delivery. A power analysis was performed to determine how many patients would be required to show no effect of labor induction on cesarean delivery with a β of .8 and an α of .05. Approximately 5200 patients would be required, taking an estimated 28 years to accrue at our institution.
There are three central findings in this analysis of the role labor induction plays in cesarean delivery. Induction of labor, compared with spontaneous labors in demographically comparable study groups, was associated with a 40% increase in overall cesarean delivery rates (from 14% to 19%) and an increase in cesarean deliveries for failure to progress, but not fetal distress. Associated risk factors for cesarean delivery included an undilated cervix, epidural analgesia, more advanced gestational age, and nulliparity. Most importantly, when the analysis was corrected for these confounding risk factors, labor induction per se was not related to excess cesarean delivery. This result suggests that it is the patient's circumstances, for example, undilated cervix, that increase the risk of cesarean delivery rather than the induction itself.
It has long been accepted that induction of labor increases the risk of cesarean delivery. We were able to find eight reports published in the last decade that specifically deal with the effects of labor induction on cesarean delivery.2–9 Two of these reports described randomized trials and the others were retrospective studies. Hannah et al3 randomized 3407 women with uncomplicated pregnancies at 41 weeks' gestation or longer to induction of labor or expectant management. Induction resulted in a lower cesarean rate. However, the increase in cesarean births in the expectantly managed group was primarily due to abnormal FHR patterns during antepartum fetal testing, making it difficult to isolate the effect of labor induction, per se, on cesarean rates. In the other randomized trial, 440 pregnancies at 41 weeks' gestation were randomized, and induction of labor had no significant effect on cesarean delivery but the sample size was deemed insufficient to measure this outcome.2 Shown in Table 4 is a summary of the six retrospective studies published during the 1990s specifically addressing the effect of labor induction on cesarean delivery.4–9 Induction was linked to excess cesarean births in four reports and unrelated in two others. The populations studied were heterogeneous in terms of medical and obstetric complications as well as demographic factors. Frequently identified risk factors for induction-related cesarean births included nulliparity,4–9 unfavorable cervical dilatation,4–7,9 and epidural analgesia.4,6 Our results are similar to those of Prysak and Castronova,6 who performed a case-control study involving 461 pairs of women. The increased rate of cesarean delivery in women undergoing labor induction was explained by nulliparity and undilated cervices and not by induction per se.
Our purpose was to attempt to separate the effects of labor induction on cesarean delivery from the patient characteristics that intrinsically impact cesarean births. As previous studies have shown, this objective is difficult to achieve. The complication for which induction is performed may greatly affect the effectiveness of labor stimulation and have an impact on the associated cesarean rate. For example, in the case of hypertensive disorders due to pregnancy, the clinical exigencies concerning the severity of this disease would undoubtedly influence the obstetrician's readiness to proceed or abandon labor induction in favor of cesarean delivery. Similarly, gestational age is a significant modifier of the success of labor induction with greater success linked to advancing gestational age. The multiplicity of confounding factors such as these makes it difficult to ascertain whether induction of labor based on the patient's characteristics account for the associated increase in cesarean deliveries. Our study cohort was chosen to exclude as many of these confounding factors as possible and was mostly but not entirely successful. For example, a 3-day difference in gestational age was statistically linked to the cesarean delivery rate in our study groups, even though their maximum gestational age difference could not exceed 6 days. Such small incremental differences make it difficult to separate the effects of intrinsic patient characteristics on cesarean delivery distinct from the induction of labor itself. We are of the view, however, that the study cohort we selected was as homogeneous as possible short of a likely unfeasible randomized trial. Specifically, to perform a randomized trial of induction compared with expectant management in women with important risk factors for cesarean delivery including undilated cervix, nulliparity, and epidural analgesia would require 1728 women. This represents less than 5% of our prolonged pregnancy study population and would require ascertainment of over 30,000 pregnancies, a 40-year study at our institution. In the absence of such a randomized trial, we are left to conclude that patient circumstance, as opposed to labor induction itself, leads to increased cesarean delivery in prolonged pregnancy.
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