Materials and Methods
A retrospective cohort of 1250 nulliparous patients was selected randomly from a computerized database at our innercity university-based hospital between 1988 and 1989. The computer generated a random-ordered list of nulliparous patients. Their charts were then re-reviewed to determine the station at the onset of active labor, as this information was not a part of our standard database. Four hundred forty-seven patients (36% of the total) were excluded from the analysis because of a nonvertex presentation, cesarean delivery before the active phase of labor, multiple gestation, delivery at less than 37 weeks or greater than 42 weeks' gestation, induction of labor, incomplete data, or missing charts. The final study group comprised 803 patients.
For the purpose of this study, we defined active labor as regular contractions, with a cervical dilatation of 3 cm or more. Engagement was defined at station 0 or below. The station at the onset of active labor was recorded.
Statistical analysis was by the χ2 test, and logistic regression analysis was used to adjust for maternal or obstetric criteria that could bias the analysis. These include maternal body mass index (BMI), maternal weight gain, use of oxytocin, birth weight, use of epidural anesthesia, estimated gestational age, race, and maternal age.
Of 803 patients, 567 patients (71%) were unengaged at the onset of active labor. The remaining 236 patients (29% of the total) were engaged at the onset of active labor. Ten percent were at −3 station, 21% at −2, 40% at −1, 23% at 0, 5% at +1, and 1% at +2. A significant difference was found in the cesarean rate: 14% for the unengaged vertex compared with 5% for the engaged vertex group (χ2 = 11.9, P < .001). The cesarean rates for patients entering the active phase at stations −3, −2, and −1 were 13%, 17%, and 12%, respectively, and did not differ statistically. The cesarean rates at stations 0, +1, and +2 were 5%, 5%, and 0%, respectively, and did not differ statistically (Table 1).
The patient demographics showed a number of differences between the engaged and unengaged groups, which might have biased the results (Table 2). Logistic regression analysis was used to control for these confounding conditions. In the adjusted model, an association between engaged vertex at the onset of active labor and a lowered risk for cesarean delivery was demonstrated (odds ratio .512, 95% confidence interval .285, .922, P = .026). All confounders showed similar associations with cesarean delivery, as previously reported in the literature.8–14 Increased risk was associated with increasing maternal BMI, increasing pregnancy weight gain, use of oxytocin, increasing birth weight, use of epidural anesthesia, increasing estimated gestational age at term, and increasing maternal age. Decreased risk was associated with black race. Not all of these associations reached statistical significance in our sample (Table 3).
In response to increasing concerns over the rising rate of cesarean delivery, the National Consensus Conference on Aspects of Cesarean Birth, convened in 1985, made recommendations in three major areas: vaginal birth after cesarean delivery, dystocia, and breech presentation.15 The rate of repeat cesarean delivery decreased subsequently by 15%, with an increase of only 2.6% in vaginal deliveries. The discrepancy may be explained by a shift in the indication for cesareans.16
The leading indication of primary cesarean delivery is the diagnosis of dystocia.7 Station at time of arrest has been identified previously as a risk factor associated with increased rate of cesarean delivery.3 Other known risk factors affecting the rate of cesarean delivery include maternal BMI, pregnancy weight gain, birth weight, gestational age, the use of oxytocin, the use of epidural anesthesia, maternal race, and maternal age.8–14 This study demonstrated that after controlling for these risk factors, nulliparous patients with vertex term infants entering active labor at station 0 or lower have a lower risk for cesarean delivery. Additionally, cesarean rates did not differ among nulliparous patients with vertex term infants entering active labor at stations − 3, −2, and −1.
Evaluation of station is rather difficult, and the findings may vary from individual to individual. As multiple examiners evaluated stations in this study, some errors may have been introduced into the measurement. In spite of this potential error, a striking difference in cesarean rate between engaged and unengaged vertex presentation at the onset of active labor was demonstrated. The findings of a nonengaged vertex in a nulliparous patient with regular contractions and cervical dilation of 3 cm or more calls for careful evaluation of the patient's pelvis. Nonetheless, 86% of such patients in our population delivered vaginally.
Although many obstetricians believe that nulliparous patients present mostly with engaged fetal heads in active labor, this study and others4,5 indicate that most nulliparous patients present with an unengaged fetal head in active labor. Despite the fact that nulliparous patients who present with an unengaged fetal head have a longer first and second stage of labor, the majority deliver vaginally. This finding allows the clinician to be more optimistic regarding vaginal delivery of nulliparous patients who present with an unengaged vertex in active phase of labor and avoid hasty decisions toward cesarean delivery.
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