CHAPTER 40 Carolyn M. Zelop Ultrasound and Perinatal Research, Division of MFM and Department of Obstetrics and Gynecology, The Valley Hospital, Ridgewood, NJ, USA Department of Obstetrics and Gynecology, NYU School of Medicine, New York, NY, USA Controversy regarding the optimal mode of delivery has plagued the first decade of the new millennium. During this time period, the cesarean birth rate has skyrocketed to an unprecedented height close to 33%. Not only has the primary rate accelerated, but the repeat cesarean rate has risen while the vaginal birth after previous cesarean (VBAC) rate has plummeted to less than 10% [1]. New concerns regarding complications stemming from multiple uterine events mainly abnormal placentation leading to maternal hemorrhage and possible hysterectomy have rekindled an interest in Trial of Labor (TOL) after previous cesarean birth [1]. One major impediment to implementing a more liberal policy of TOL after previous cesarean is fear of uterine rupture. While this devastating complication occurs in roughly 1% of all TOL after previous cesarean birth, the maternal and neonatal morbidity and even mortality may be significant [2]. We now understand through clinical experience and epidemiological research that all TOLs are not alike. Ideally, the most appropriate candidates eligible for TOL after previous cesarean will have the highest chance of success and the lowest risk of uterine rupture. The purpose of this chapter is to explore the factors that have been suggested to alter the risk of uterine rupture during TOL after previous cesarean and analyze the quality of evidence surrounding each variable. When assessing the risk of uterine rupture during TOL after previous cesarean, factors to be considered include: obstetrical history, demographics, factors that impact the integrity of the scar, antepartum and intrapartum factors. Thorough evaluation of these risk factors will enable the patient and her provider to develop a care plan for choosing the optimal route of delivery given her history of prior cesarean birth. Before considering the quality of evidence surrounding each factor, it is important to recognize that there are no randomized controlled trials comparing planned elective repeat cesarean versus planned TOL after previous cesarean birth. While the diversity and caliber of available literature is noteworthy, the limitations of non‐randomized studies must be acknowledged since these studies cannot adjust for the inherent clinical insight required for each delivery plan formulated by the patient and her care provider. Additionally, studies that identify uterine rupture only by ICD‐9/10 codes may be biased by misclassification of this important outcome variable and lead to overestimation of the association. Embarking upon a discussion of uterine rupture requires a clarification of its definition since there have been some inconsistencies in the literature regarding uterine events. Uterine rupture as defined by the recent National Institute of Health (NIH) Consensus Development Conference entitled “Vaginal Birth After Cesarean: New Insights” is a complete anatomic separation of the uterine wall regardless of the presence of symptoms with or without extrusion of the fetal placental unit [3]. Another uterine event, uterine dehiscence is a partial or less severe variant with at least the serosa intact. While these entities should not be grouped together, some authorities view uterine dehiscence as a near miss. The true prevalence of uterine dehiscence is also difficult to ascertain since its asymptomatic nature may cause it to go unrecognized. In addition, uterine rupture, may not always involve the actual healed hysterotomy. Factors other than the integrity of the healed scar must also play a part in the mechanism of uterine rupture when the separation occurs in other locations remote from the previous hysterotomy. Prior preterm cesarean birth There are at least five studies [4–8] in the literature that assess the risk of uterine rupture associated with prior preterm cesarean birth and a subsequent TOL. Four are retrospective cohort or case control studies with variable sample sizes. The studies vary in the definition used for prior preterm delivery and the actual number of patients delivered at a particular prior preterm gestational age. The largest prospective observational study [8] controlling for multiple confounders affecting the rate of uterine rupture reported a minimally statistically significant increase risk for uterine rupture during a TOL with a prior preterm cesarean birth (Odds Ratio (OR) 1.6 95%CI (1.01–2.50); p = 0.043). Two retrospective cohort studies [9, 10] in the literature focused upon the primary effect of previous vaginal delivery upon the risk of uterine rupture during a TOL after previous cesarean birth. Additionally, a large prospective observational trial [11] focused upon the effect of previous VBAC upon a subsequent TOL. De Lau et al. [12] performed a systematic review exploring the effect prior vaginal delivery upon this outcome, but they included data sets abstracted as subsets from manuscripts where prior vaginal delivery and uterine rupture were not the main thrust of the study. Their conclusion echoed the conclusions of the primary studies detailed here. Prior vaginal delivery significantly reduces the risk of uterine rupture even when it occurs prior to the index cesarean birth. Additionally, Mercer et al. [11] demonstrated that prior VBACs do not increase the risk of uterine rupture. In other words, the healed uterus does not appear to be more susceptible but rather less susceptible to subsequent pregnancies that test the integrity of the prior hysterotomy. Prior vaginal delivery, including VBAC also increases the success of TOL after previous cesarean. TOL beyond the estimated day of delivery ( EDD ) The literature contains four retrospective cohort studies [13–16] that examine the effect of gestational age after the EDD and the risk of uterine rupture. All studies attempted to control for possible confounders that might affect the rate of uterine rupture during a TOL after previous cesarean especially for birth weight and induction of labor. The results are divergent; however, the studies vary by sample size. The two largest studies, Coassolo et al. [13] and Zelop et al. [14] do not demonstrate an increased rate of uterine rupture during a TOL after the EDD reporting a rate of 1.1–1.3 versus 0.8–1.0% in patients less than or equal to EDD. Post EDD TOL is less successful than a TOL prior to or at the EDD. Induction of labor, however, prior to or at the EDD does not improve the outcome. Survey of the literature reveals two retrospective cohort studies [16, 17] and one prospective observational study [18] in women with prior cesarean birth that demonstrate a decrease rate of uterine rupture and comparable or higher success rate of a TOL after previous cesarean during a preterm gestation in the current pregnancy. There are three retrospective cohort studies [19–21] that evaluate the association between maternal age and the risk of uterine rupture. The studies vary according to the manner in which patients are grouped in certain age strata. Shipp et al. [19] examines the association using a dichotomous categorization of less than 30 years of age and greater than or equal to 30 years of age while Bujold et al. [20] and Srinivas et al. [21] employed a three tiered approach. All three studies substantiate decrease success in TOL in older women. All studies attempted to control for possible confounders. Shipp et al. reported a threefold increased risk of uterine rupture among women at least 30 years of age. Bujold et al. did not confirm this increased risk, however, a smaller sample size may have led to a Type II error. Srinivas analyzed VBAC related complications which included uterine rupture and revealed an increased risk in women 35 years of age or older. The proposed mechanism of increased maternal age interfering with wound healing including hysterotomy is plausible. However, analysis of age at index cesarean would further substantiate the basis of this theory. Management of women with previous cesarean delivery and elevated BMI presents a true clinical dilemma. Repeat surgery has multiple risks, but is large maternal BMI associated with an elevated risk of uterine rupture during a TOL after previous cesarean. While there are several studies examining this question, the sample sizes are variable, some studies compare outcomes of women undergoing TOL after previous cesarean versus elective repeat cesarean delivery (ERCD) while others compare maternal outcomes stratified by increasing levels of maternal BMI attempting a TOL [22–26]. Some researchers have presented analyses of the same population using these two study designs [23, 25]. Hibbard et al. [22] employing the Maternal‐Fetal Medicine (MFM) units of prospectively collected data provides the most comprehensive analysis and conclusions. Data was stratified across increasing categories of maternal BMI to examine the effect of maternal BMI on the risk of uterine rupture during TOL after prior cesarean. Uterine disruptive events increased in the morbidly obese (2.1% versus 0.9%; p = 0.03) compared with normal BMI women. Uterine rupture was elevated in morbidly obese women compared to women with normal BMI, but this rate was not statistically different (1.2% vs. 0.6%; p = 0.12). This study confirmed the decrease success associated with larger BMI compared to normal BMI in women undergoing TOL after prior cesarean. However, there is a lingering question whether a higher rate of induction in obese women with prior cesarean may bias these results [23]. Ideally, the impact of the estimated fetal weight (the passenger) upon the risk of uterine rupture is the variable of interest. However, birth weight has been utilized as a proxy for this assessment in the literature. Three retrospective cohort studies [27–29] which controlled for potential confounders including prior indication for previous cesarean analyzed the association between increasing birth weight and the risk of uterine rupture during a TOL after previous cesarean birth. Birth weight greater than or equal to 4000 g was associated with an increased risk of uterine rupture reaching statistical significance in two of the three studies and highest in those without a previous vaginal delivery (3.2–3.6%). Success of TOL after previous cesarean birth decreases with increasing birth weight. Overall rates of success have been affected by confounders with the lowest success rates (41%) observed in women undergoing labor induction with no prior vaginal delivery and birth weights greater than or equal to 4000 g. Using logistic regression and adjusting for potential confounders, Peaceman et al. reported the odds of success decreased by 3.8% for each increase of 100 g in birth weight in a TOL relative to the index cesarean birth weight employing the prospectively collected MFM unit’s observational trial [30]. The sonographic measurement of the thickness of the lower uterine segment (LUS) has been proposed as a method to assess the risk of uterine rupture during a TOL after previous cesarean birth [31, 32]. A systematic review performed by Jastrow et al. [33] examined the diagnostic accuracy of sonographic measurement of the LUS thickness near term in predicting LUS disruption. They concluded that sonographic LUS thickness correlated inversely with the risk of uterine rupture during a TOL after previous cesarean. However, lack of standardization of the measurement technique and the heterogeneity of the studies precludes the determination of an ideal cut‐off value that is clinically useful. Since this publication, Martin et al. [34] have suggested that ultrasonographic measurement of the LUS muscular thickness transvaginally appears more reliable than a transabdominal full thickness measurement. Three‐ dimensional approach may offer promise for off‐line analysis allowing patients to be evaluated from geographically remote areas. One of the difficulties that plagues this approach is that uterine rupture remote from the uterine scar cannot be predicted from this technique [35]. Three studies, two retrospective cohorts [36, 37] and one prospective observational study [38] analyze the influence of greater than one previous cesarean delivery upon the risk of uterine rupture during a TOL after prior cesarean birth. The prospective observational study patients were derived from patients managed from 1999 to 2002 whereas patients from the other two studies (Caughey et al. [36] and Macones et al. [37] were from older cohorts dating back to 1984 but published since 1998. The patients managed more recently may have benefited from knowledge regarding uterine rupture risk that may have influenced their ultimate planned mode of delivery. The two retrospective cohort studies demonstrate a 2–4.8 fold increase risk of uterine rupture in women during a TOL with an absolute rate of 1.8–3.7% risk of rupture. All three studies attempted to control for confounders through logistic regression analysis. While all three studies demonstrated a statistically increased risk of morbidity in those attempting VBAC with greater than one previous hysterotomy, absolute risks remained small and those with prior vaginal delivery had the lowest risk of uterine rupture (0.36 OR; 95% CI (0.08–0.88)). Four retrospective cohort studies [39–42] (two using similar databases from the same institutions using four more years of data) [40, 41] and one case control study [43] have evaluated the effect of interdelivery interval upon the risk of uterine rupture during a TOL after previous cesarean birth. The studies actually examined different intervals: interpregnancy and or interdelivery but attempted to control for confounders using multiple logistic regression. Consistently, a shorter interval was associated with an increased risk of uterine rupture. Shipp et al. [39] reported a threefold increase risk of uterine rupture associated with interdelivery interval of less than or equal to 18 months (95% CI 1.2, 7.2) after controlling for potential confounding. One possible mechanism that might explain the increased risk of rupture with a short interdelivery interval is incomplete repair or healing of the uterine hysterotomy. The literature suggests that complete uterine involution and restoration of zonal anatomy may require at least six months or longer as evaluated by Magnetic resonance imaging (MRI) [39]. Most of the literature analyzes the risk of uterine rupture during a TOL of a prior low transverse hysterotomy or an unknown scar location presumed to be a low transverse incision based upon clinical context of the delivery. The largest cohort examined retrospectively by Shipp et al. [44] reported a similar rate of uterine rupture between women with prior low vertical (0.8%) versus low transverse (1.0%; p > 0.999). This study possessed an 80% power to detect an increase from 1 to 3%; comparable to uterine rupture risk with two prior cesarean births undergoing TOL. Two retrospective cohort studies [45, 46] and one case control study [47] evaluated the risk of uterine rupture during a subsequent TOL after previous cesarean birth. The cohort studies reported conflicting results, but may be weakened by small sample sizes. The case control study does utilize multivariable analysis to control for confounders and reported a modest association (OR = 2.69; 95% CI 1.37–5.28) between uterine rupture and TOL after previous cesarean where a single closure was utilized. Factors such as postoperative infection complicating the index cesarean delivery, suture material and method of closure may also compromise the integrity of the hysterotomy scar.
Vaginal birth after cesarean delivery
Background
Critical review of the literature and clinical questions
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