The cesarean rate in the United States peaked in 1988 at 24.7%. National data from 1996 indicate a decline in the rate of cesarean birth to 20.7%.1 During this interval, the primary cesarean rate dropped to 14.6%, and the vaginal birth after cesarean (VBAC) rate (vaginal deliveries per 100 women with a prior cesarean) rose to 28.3%. Similarly, the cesarean rate at Northwestern Memorial Hospital has declined from a peak of 27.3% in 1986 to 15.4% in 1996. Contributing factors include greater emphasis on vaginal birth after a prior cesarean delivery, circulating the cesarean rate of every obstetrician to each attending physician, and the use of the active management of labor protocol.2
The American College of Obstetricians and Gynecologists has taken a proactive stance on vaginal birth after prior cesarean delivery.3 Accordingly, patients with one previous low transverse cesarean should be counseled and encouraged to undergo a trial of labor unless contraindications exist. Neither two prior lower-segment uterine incisions nor the inability to document the type of uterine incision should be construed as absolute contraindications to labor. Despite this position and greater encouragement of VBAC nationwide, there remain conflicting opinions as to its safety for mother and fetus.4–12
This study was undertaken to examine the fetal risk associated with our advocacy position towards VBAC. Prior investigations have focused on Apgar scores as a measure of neonatal morbidity. We have expanded our analysis to also include the incidence of fetal acidemia in patients attempting VBAC compared with those patients delivered by elective repeat cesarean. Further, we have included two additional comparison groups: patients experiencing a vaginal birth without a previous cesarean delivery and patients undergoing a primary cesarean after labor. These comparison groups were chosen to assess more completely the fetal risk associated with a prior uterine incision as opposed to that risk posed by labor per se.
Materials and Methods
Northwestern Memorial Hospital is a primarily private university teaching hospital. Approximately one-fourth of the patients are medically indigent and are managed by resident house staff under the supervision of full-time faculty. The remainder are private patients of the volunteer faculty or full-time faculty. Most obstetricians encourage VBAC after a single prior low transverse uterine incision or if the uterine incision is unknown but most likely a low transverse one. Most also support VBAC for a well-motivated patient with two prior low transverse incisions, but discourage VBAC for patients with three or more uterine incisions or a vertical uterine scar.
The Apgar score at 5 minutes and umbilical cord arterial pH measurements were retrieved from the perinatal database for all deliveries between January 1, 1991, and December 31, 1996. Exclusion criteria included antepartum stillbirth, birth weight less than 750 g, multiple gestation, primary cesarean without labor (eg, breech, placenta previa, herpes), or a repeat cesarean if the intent for a trial of labor was unclear. Our current database does not differentiate patients according to the number of prior cesarean deliveries or whether they have experienced a successful VBAC previously.
The following groups of patients who delivered a live-born singleton with birth weight greater than 750 g were therefore identified: 2082 patients with one or more prior cesareans allowed a trial of labor, 1677 (80.5%) of whom delivered vaginally and 405 of whom were delivered by repeat cesarean; 920 patients delivered by elective repeat cesarean without a trial of labor; 22,863 patients without a prior cesarean delivered vaginally; and 2432 patients delivered by primary cesarean after laboring. Umbilical cord arterial blood gases were obtained in 88.3% of these deliveries (range 87.5–95.3%).
Comparisons of Apgar scores at 5 minutes and umbilical cord arterial pH measurements were made between groups with χ2 or Fisher exact test, and odds ratios (ORs) were calculated. The incidences of pH measurements less than 7.1 and less than 7.0 were chosen for comparison because the former represents two standard deviations below the mean,13 and the latter, together with an Apgar score less than 4 at 5 minutes, may be indicative of potentially damaging fetal acidemia.14
Pregnancy characteristics of the patient population are listed in Table 1. Mean maternal age was greatest in the elective repeat cesarean group, and mean gestational age and birth weight were greatest in the unsuccessful VBAC group. Those who attempted VBAC were least likely to be white or private patients.
There were no significant differences in the incidence of a low Apgar score at 5 minutes or a low umbilical cord arterial pH measurement between those patients with one or more prior cesareans who were allowed a trial of labor and those patients who were delivered by elective repeat cesarean (Table 2). Two intrapartum stillbirths occurred in the former group, but both were secondary to congenital anomalies incompatible with life. Although there was not an increase in potentially damaging fetal acidemia (ie, umbilical cord arterial pH less than 7.0 and Apgar score less than 4 at 5 minutes) in our patients attempting VBAC, we still reviewed each of these three deliveries, and they are summarized in Table 3.
The patients with one or more prior cesareans who delivered vaginally are compared with those patients who delivered vaginally but without a prior cesarean in Table 4. The VBAC patients were more likely to have an Apgar score of less than 7 but not less than 4 at 5 minutes. Similarly, they were more likely to have an umbilical cord arterial pH less than 7.1, but not less than 7.0.
In Table 5, the patients with an unsuccessful attempt at VBAC are compared with those patients who underwent a primary cesarean after laboring. There were no significant differences in Apgar scores or umbilical cord arterial pH measurements between these groups.
During this 6-year period, 38 patients were coded in the perinatal database to have experienced a uterine dehiscence or rupture. Twenty-six patients had undergone a trial of labor, and 12 patients had not. Thirty patients were delivered by repeat cesarean, and eight patients delivered vaginally. The nine pregnancies in which either the Apgar score was less than 7 at 5 minutes or the umbilical cord arterial pH was less than 7.0 are listed in Table 6.
A number of factors have contributed to the rise in cesarean rate over the last two decades. These include electronic fetal monitoring,15 the change in philosophy regarding breech presentation and the use of forceps,16 epidural anesthesia,17,18 the medicolegal climate,19 and physician practice styles.20,21 The most common obstetric indications for abdominal delivery are now dystocia and prior cesarean birth, with malpresentation and fetal heart rate (FHR) abnormalities contributing to a lesser degree.22 Strategies to decrease operative delivery for dystocia range from patience and patient support in labor23 to the active management of labor protocol.24 Repeat operative rates will decline by decreasing primary cesareans and encouraging VBAC in appropriate candidates.
We chose to expand our analysis beyond those patients who attempted VBAC compared with those patients who were delivered by elective repeat cesarean. By also comparing successful VBAC attempts to vaginal births without a previous cesarean and unsuccessful VBAC attempts to primary cesarean deliveries after labor, a more complete assessment of the fetal risk associated with a prior uterine incision, as opposed to labor per se, can be made. The only statistically significant differences we could identify were a greater risk for an Apgar score less than 7 at 5 minutes or an umbilical cord arterial pH less than 7.1 in successful VBAC attempts. These neonates were not, however, at greater risk for an Apgar score less than 4 or an umbilical arterial pH less than 7.0.
Ascertainment of umbilical cord arterial pH was high in all groups. As the clinical utility and desire to obtain these measurements are greatest for depressed newborns, it is unlikely that the incidence of a low pH was significantly affected by incomplete ascertainment.
Although the incidence of potentially damaging fetal acidemia was not increased in our study population, we reviewed the hospital course for each of the three neonates born with both an Apgar score less than 4 at 5 minutes and an umbilical cord arterial pH less than 7.0 after an attempt at VBAC. In one patient, a neonatal death followed an unsuccessful VBAC attempt, but the abdominal incision–to-delivery interval was 24 minutes, attributable to dense adhesions. Whether the outcome would have been different with delivery by elective repeat cesarean is uncertain. The second case was complicated by a prolonged FHR deceleration during an attempt at vacuum extraction because of a low scalp pH. It is unclear whether the hypoxic ischemic encephalopathy was related to the attempt at VBAC or the management of the second stage of labor. In the third case, in which the second stage of labor was shortened by the use of forceps, the newborn was acidemic, but its course was not complicated by any signs of encephalopathy or multiorgan system dysfunction.
We also reviewed each pregnancy complicated by a uterine dehiscence or rupture during this 6-year period in which the Apgar score was less than 7 at 5 minutes or the umbilical cord arterial pH was less than 7.0. Nine pregnancies met these criteria, all delivered by cesarean. Five deliveries followed a trial of labor, and four deliveries did not. Aside from the aforementioned neonate with hypoxic ischemic encephalopathy (case 2), two other neonates had a complicated newborn course but had a normal neurologic examination by the time of discharge. One stillbirth occurred at 26 weeks secondary to an abruptio placenta that progressed while the patient was being evaluated for abdominal pain of unclear etiology.
Our analysis was focused on very specific outcome measures, the Apgar score at 5 minutes and umbilical cord acidemia. We did not examine labor characteristics, such as use of oxytocin, length of labor, choice of labor analgesia or delivery anesthesia, or birth weight, as the value of this analysis from a perinatal database would be limited, particularly as the primary outcome variables were similar in the comparison groups. We also did not analyze maternal morbidity or neonatal outcome measures such as intensive care unit admissions and length of hospitalization. Our findings add to the experience and are consistent with that of other authors who have evaluated newborn Apgar scores and perinatal mortality and who have been proponents of VBAC.4,5,9,10,12 Recognizing that a retrospective analysis from a perinatal database suffers the limitation of a lack of long-term follow-up, our policy toward VBAC was associated with only one neonatal death and one newborn with hypoxic ischemic encephalopathy that might have been preventable by elective repeat cesarean. Avoidance of this risk by performing more elective repeat cesarean deliveries would increase cost and also potentially increase maternal morbidity.
Whereas our findings should be encouraging for the clinician counseling a patient regarding a possible attempt at VBAC, it should be noted that we would need a much larger sample size in any of our comparisons to be able to detect a two-fold increase in risk of potentially damaging fetal acidemia. For example, if we grouped all patients attempting VBAC and compared them with all patients laboring with an unscarred uterus, we would still need a population size of almost 400,000 patients. Similarly, to demonstrate that a policy promulgating elective repeat cesarean section would halve the incidence of potentially damaging fetal acidemia, 85,000 patients would be required. It also must be remembered that the preponderance of the published literature on VBAC, including this report, comes from institutions in which obstetricians, anesthesiologists, and operating personnel are readily available. Nonetheless, we believe that in settings where appropriate personnel are available, VBAC poses acceptable risk to the fetus and its use is an important component of efforts to lower the rate of cesarean birth.
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