The safety of vaginal birth after cesarean (VBAC) has been demonstrated in many studies, and it has been accepted as a way to lower the overall cesarean delivery rate.1 Although there is strong consensus that a trial of labor is appropriate for most women with prior low-transverse cesarean sections, increased experience with VBAC indicates that there are potential problems. Reports have implicated numerous maternal and neonatal complications associated with an unsuccessful trial of labor.2,3
There is little information about the healing of the lower uterine segment cesarean scar. It appears that healing occurs mainly by fibroblast proliferation, and as the scar shrinks, connective tissue proliferation becomes less obvious.4,5 It is plausible that a short interval between deliveries may not allow complete healing of the uterine scar, causing ineffective uterine contractility, poor lower uterine segment thinning, and cervical effacement in labor, or increased potential risk of uterine dehiscence or rupture.
The objective of this study was to determine whether a short interdelivery interval between prior cesarean delivery and trial of labor of the index pregnancy is associated with an increase in maternal and perinatal morbidity and a decrease in the rate of success of VBAC.
MATERIALS AND METHODS
A retrospective cohort study of deliveries from January 1, 1997, to December 31, 2000, from the University of California at Irvine and Long Beach Memorial Medical Center was conducted. Institutional Review Board approval was obtained from both locations to abstract data from the patient charts. Patients with previous low-transverse cesarean section who had undergone a trial of labor were identified. Maternal inpatient and prenatal records were examined in all eligible subjects. Demographic data and neonatal information were ascertained. We also obtained data on maternal or neonatal complications, such as uterine rupture (complete disruption of previous cesarean scar, causing maternal or neonatal morbidity), sepsis, hemodynamic or coagulation derangement, respiratory impairment, and neurologic damage. The analysis was limited to patients at or greater than 37 weeks with one cesarean section, and the interdelivery interval was calculated as time in months between the index trial of labor and prior cesarean delivery. The principle outcome assessed was the interdelivery interval and the rate of successful VBAC. We categorized the subjects into two groups: group 1 with interdelivery interval of less than 19 months, and group 2 with interdelivery interval greater than or equal to 19 months. Because the postpartum restoration of the lower-segment hysterotomy may require at least 6–9 months, as suggested by a magnetic resonance imaging study, we selected an interdelivery interval of 19 months (9 months postpartum plus 10 months of gestation).6
The differences between the groups were analyzed. χ2 and Fisher exact tests were used for categoric variables, and the Student t test was used for continuous variables. A P value of < .05 was considered significant. Multivariate logistic regression analysis was used to evaluate the association of VBAC success with interdelivery interval and other potential confounding factors. Statistical analysis was accomplished using SPSS 8.0 (SPSS Inc., Chicago, IL).
A total of 1516 patients who attempted trials of labor at term after one prior cesarean delivery were identified among 24,162 deliveries. We excluded 331 patients because of non-low-transverse hysterotomy incision with prior cesarean section or missing data. Complete information was available in 1185 cases. Maternal demographics and clinical characteristics are listed in Table 1. The VBAC success rate was 79.0% (64 of 81) for patients with interdelivery interval less than 19 months compared with 85.5% (943 of 1104) for patients with interdelivery interval greater than or equal to 19 months (P = .12). Power existed to detect a 12% difference between the groups (α = .05, β = .80).
In the subanalysis of subjects who underwent spontaneous labor, we detected no difference in the rate of VBAC success between the two groups (85.2% [63 of 74] versus 85.2% [757 of 888]; P = .98). However, in subjects whose labors were induced, patients with interdelivery interval less than 19 months were less likely to have successful VBAC when compared with those with longer intervals (14.3% [1 of 7] versus 86.1% [186 of 216]; P < .01; odds ratio 37; 95% confidence interval 4.2, 849.5; sufficient power [β = .95] existed to detect a 64% difference between the groups [α = .05]). We did not find a difference in the rate of VBAC success among patients with no prior vaginal deliveries (70.4% [38 of 54] versus 77.5% [386 of 498]; P = .24).
Our multivariate logistic regression analysis found no association between VBAC success, interdelivery intervals, and other potential confounding factors. These include: maternal race; gestational age at delivery; gravidity or parity; history of spontaneous or elective abortions; obstetric service (private versus resident); usage, duration, or dosage of pitocin; indication of cesarean section for prior or index pregnancy; ruptured amniotic membrane; epidural use; evidence of intrapartum chorioamnionitis; meconium stained amniotic fluid; 1- or 5-minute Apgar scores; and neonatal gender and birth weight.
There were three cases of symptomatic uterine rupture, all in the group with interdelivery interval greater than 19 months, but this difference was not significant (P = 1.00). This rate of uterine rupture is consistent with those reported in the literature.1 We found no other significant maternal or neonatal morbidity. No data were available with respect to clinical practice or uterine closure. However, no significant changes in labor management were evident at the two centers during the study period.
Currently, about 24% of the births in the United States are cesarean deliveries.1 Although the safety of vaginal birth after a previous low-transverse cesarean section has been established in many studies, there is increasing evidence that a failed attempt of VBAC is associated with various maternal and neonatal complications.2,3 These include chorioamnionits, postpartum endometritis, uterine rupture requiring hysterectomy, blood transfusion, perinatal and neonatal deaths, and neonatal neurologic impairment. Furthermore, these patients are at greater risk for complications compared with those with elective repeat cesarean section without labor.7
Incomplete healing of the uterine scar from a previous cesarean delivery, as a result of short interdelivery interval, has been suggestive as a risk factor for uterine rupture during a trial of VBAC. Shipp et al supported this idea, in a report of increased rate of uterine rupture during a trial of labor, in VBAC patients with interdelivery interval less than 18 months compared with those with longer intervals.8 Esposito et al also observed an increase in the rate of uterine rupture with short interdelivery intervals.9 Potentially, a short interdelivery interval can adversely affect uterine activity during labor as a result of the inadequate postpartum healing of the previous cesarean scar.
Characteristics associated with successful trials of labor after cesarean delivery have been extensively studied.10–12 However, no study has been conducted to specifically address the potential relationship between interdelivery interval and the rate of VBAC success, according to a MEDLINE search from 1966 to August 2001 using the terms “interdelivery interval” and “VBAC.” In our study, the rate of VBAC success did not appear to be affected by the interdelivery interval between prior cesarean delivery and the index pregnancy. Furthermore, there was no increase in the rate of symptomatic uterine rupture in those patients with shorter interdelivery intervals, although this finding may be limited by the sample size.
A short interdelivery interval was associated with a decrease in the rate of successful VBAC in patients whose labors were induced, a difference not found in those who underwent spontaneous labor. Perhaps spontaneous labor represents the complete healing of the uterine scar, enabling the uterus to adequately respond to the hormonal milieu that causes the spontaneous progression of labor. We must be cautious when interpreting this information, however. Because of the small number of patients in the group with interdelivery interval less than 19 months, the significance of this association is unclear.
Although it appears that the healing of the lower uterine segment cesarean scar occurs promptly postpartum in most patients, and does not seem to interfere with the normal physiologic and anatomic changes that transpire during subsequent labor and delivery, the safety of VBAC remains debatable. We demonstrated a potential association between interdelivery interval and success of VBAC, specifically in patients who undergo labor induction. Nevertheless, the clinical importance cannot be definitely established by our retrospective study design. Obstetricians should continue to counsel patients regarding trial of VBAC based on current established guidelines.
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