The most serious complication of trial of labor after cesarean delivery is the risk of symptomatic uterine rupture. Overall, the risk of uterine rupture during trial of labor is approximately 1%.1 Certain subgroups of parturients have been identified as being at higher risk of uterine rupture, including those with multiple prior cesarean deliveries2,3 and induction with oxytocin.4 There might be other subgroups that are also at increased risk of uterine rupture during a trial of labor after cesarean delivery. We evaluated interdelivery interval in this regard.
Materials and Methods
The medical records of all women admitted to Brigham and Women's Hospital for trials of labor after cesarean delivery over 12 years (July 1984 to June 1996) were reviewed. Specific findings were reported previously.1,5 Our data set included the month and year of the prior cesarean delivery. The current analysis was limited to women with one prior cesarean delivery and no previous vaginal deliveries. We included only women who delivered singletons at term and whose medical records included the month and year of the prior delivery. Of 2825 records, 416 lacked adequate information regarding the date of the prior delivery, leaving 2409 for the present analysis.
Uterine rupture was defined as complete disruption of the cesarean scar with at least one of the following symptoms: hemorrhage, need for hysterectomy, bladder damage, extrusion of any portion of the fetus or placenta from the uterus, or cesarean delivery for nonreassuring fetal status or suspected uterine rupture. Asymptomatic uterine dehiscences were not included because they were not associated with maternal or neonatal morbidity.
The frequency of uterine rupture was related to the interdelivery interval. The interdelivery interval, calculated as the time in months between the index trial of labor and the prior delivery, was categorized as up to 18 months and 19 months or more. Demographic data, clinical characteristics, and uterine rupture rates were compared for the groups. Statistical significance was assessed using χ2 for categoric variables or Fisher exact test, as appropriate; continuous variables were compared with Student t test. P < .05 was considered statistically significant. Multiple logistic regression analysis was used to assess the association of interdelivery interval with uterine rupture, while controlling for potential confounding factors.
Two thousand four hundred nine women had trials of labor after one prior cesarean delivery and had the indicated date of the prior delivery noted in the medical record. There were 29 uterine ruptures (1.2%) for the entire population. The rupture rate was 2.25% (seven of 311) for women with an interdelivery interval of up to 18 months and 1.05% (22 of 2098) for women with intervals of 19 months or more; this difference was not statistically significant (P = .07).
Demographic and clinical characteristics of both groups were compared (Table 1). Women with shorter interdelivery intervals were on average younger (mean 28.5 years versus 31.4 years) and were more likely to be receiving public assistance (19.3% versus 12.7%). They were also less likely to have a gestational age of at least 41 weeks (16.7% versus 21.8%).
The characteristics of women with and without uterine rupture were compared (Table 2). Women with uterine rupture were more likely to have had labor induced with oxytocin and were on average 2.6 years older than women without uterine rupture. Because older women were also more likely to have a longer interdelivery interval (which is associated with a lower uterine rupture rate), maternal age represented a potentially important confounder of the association between interdelivery interval and uterine rupture.
To control for such potential confounding, multiple logistic regression analysis was done. In that model, we assessed the risk of uterine rupture for women with interdelivery intervals of up to 18 months, compared with those 19 months or more, while controlling for the potential confounding factors of maternal age, public assistance, length of labor, gestational age of at least 41 weeks, and induction or augmentation of labor with oxytocin. After controlling for this confounding, short interdelivery interval was a significant predictor of uterine rupture. Women with interdelivery intervals of up to 18 months had an odds ratio (OR) of 3.0 (95% confidence interval [CI], 1.2, 7.2) for a symptomatic uterine rupture (Table 3).
Our data indicate that among women with one prior cesarean delivery who then have a trial of labor, the risk of uterine rupture may be higher for women with interdelivery intervals of up to 18 months. This correlates with an interpregnancy interval of approximately 9 months or less, because we included only women with term gestations in the index pregnancies. Women who have such short interdelivery intervals have a threefold increased risk of uterine rupture during a trial of labor after cesarean delivery. Although the crude association did not reach statistical significance (P = .07), this resulted from substantial confounding by maternal age. Older women were more likely to have had uterine rupture and more likely to have had longer interdelivery intervals. Multivariate analyses were needed to control for these imbalances, and after adjustment there was a significant association of interdelivery interval with uterine rupture. The risk of uterine rupture with interdelivery intervals of 19 months or more did not vary according to the time from prior cesarean delivery and remained constant at approximately 1%.
One possible explanation for our findings is incomplete healing of the uterine scar from the previous cesarean delivery. Healing of the hysterotomy has been evaluated by magnetic resonance imaging (MRI).6 The zonal anatomy of the uterine hysterotomy site from uncomplicated low transverse incisions was restored by 6 months postpartum as evaluated by MRI. Complete involution of the uterus and restoration of uterine anatomy required at least 6 months and possibly 9 months.6 We hypothesized that pregnancy occurring within this relatively short period postpartum, before complete uterine healing, might be associated with an increased risk of uterine rupture during a subsequent trial of labor after cesarean delivery. Therefore, by defining our short interdelivery interval as deliveries at term with interdelivery intervals of 18 months or less, the higher rate of uterine rupture in this subgroup might be from incomplete healing of the prior hysterotomy site.
It was not known whether the hysterotomy site healed by regeneration of the myometrium or through the more common means of scar formation. Some investigators have argued for the former because of the frequent failure to identify the prior cesarean delivery wound during subsequent surgery. However, pathologic review of a recent cesarean delivery wound that was incompletely healed found the presence of granulation tissue and fibrosis, suggesting scar formation.7 A rat model developed to assess the strength of uterine wound healing8 might prove helpful for further investigations.
Despite the frequent use of a trial of labor after cesarean delivery, there is little information regarding the effect of short interdelivery interval on uterine rupture after prior hysterotomy. A first-trimester rupture of a prior classical cesarean incision has been described in a woman with an interpregnancy interval of approximately four months.9 In a cohort of women who had pregnancy termination by hysterotomy and then a subsequent pregnancy after a short interval, with 42% having an interval of 6 months or less and 58% 1 year or less, there were three uterine ruptures (7.9%) or impending ruptures in 38 women with pregnancies reaching the third trimester. The scars were described as thin in 45.2% of women.10 Although prior pregnancy termination via hysterotomy cannot be compared directly with prior cesarean delivery, the short interpregnancy intervals could have led to incomplete healing and therefore the higher rate of uterine rupture in subsequent pregnancies. Further study is required to determine etiologic mechanisms.
We did not examine the occurrence of asymptomatic dehiscences in this study and instead focused on symptomatic ruptures. As previously stated, we defined uterine rupture as complete disruption of the prior incision, with associated maternal or neonatal sequelae.1,5 Asymptomatic dehiscences do not, by our definition, lead to adverse sequelae and therefore are less clinically significant in the index pregnancy. In addition, study of these lesions from retrospective data is difficult because they frequently go undetected.
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