Before the 1970s, Cragin's dictum “once a cesarean, always a cesarean” guided obstetric practice in the United States. Until that time the overall cesarean delivery rate was about 5%. As techniques, such as a low transverse scar, that increased the safety of cesarean delivery were introduced, indications for cesarean birth were liberalized. Medicolegal pressures on physicians also increased, and after declining for many years in the 1990s, the cesarean rate peaked at 27.6% in 2003, the highest percentage ever reported in the United States.1
Vaginal birth after cesarean delivery (VBAC) has been successfully practiced in this country since the late 1950s and was considered a feasible method by the early 1960s after data became available indicating the safety of this approach.2 Large multicenter trials have shown that the average VBAC success rate is between 60% and 80% in appropriate candidates, with decreased hospital stays and complication rates in the VBAC group.3,4 In the past decade, the VBAC discussion has turned toward adverse maternal and neonatal outcome related to uterine rupture during trial of labor after a previous cesarean,5,6 and although we are still trying to identify which factors are associated with VBAC success, there is little information available regarding obesity. Excessive weight gain and obesity have an increasing relevance in obstetrics. This is especially true considering that the results of the National Health and Nutrition Examination survey show that 64% of adults in the United States are either overweight or obese.7
Before 2004, a PubMed search for “weight gain, trial of labor” or “weight gain, VBAC” revealed no previous publications in English, and there were only a few publications on obesity in pregnancy. There is a paucity of reports on the management of subsequent pregnancy among obese women with a prior cesarean delivery. Chauhan et al8 found that women weighing more than 300 lb at the first perinatal visit had a VBAC success rate of 13% and had higher infectious morbidity than the elective repeat cesarean group. Carroll et al9 also compared successful VBAC and infectious morbidity by dividing patients into 3 different weight classes. Patients were classified as lean (< 200 lb), intermediate-to-average (200–300 lb), and morbidly obese (> 300 lb) at the first prenatal visit. Obese women undergoing trial of labor had infectious morbidity of 53.3% compared with 17.8% on average and 9% in lean women. Also, as weight increased, the VBAC success rate decreased from 82% in the lean to 13% in the morbidly obese group.
The purpose of this study is to estimate whether excessive weight gain in pregnancy is a risk factor affecting VBAC success. We will compare patients with excessive weight gain with those who are normal in weight to identify variables that are predictive for successful VBAC.
MATERIALS AND METHODS
A chart review using International Classification of Diseases, 9th Revision (ICD-9) codes ‘‘VBAC'’ and ‘‘non-primary C-section'’ and review of logbooks on labor and delivery from January 1, 1996, to December 31, 2000, identified patients attempting VBAC at Mount Sinai Medical Center, a tertiary care referral center. We identified 1,741 patients from ICD-9 codes. An additional 184 cases were identified from the logbooks. Thus, a total of 1,925 patients were identified for possible inclusion. Exclusion criteria included multiple gestation, more than one previous cesarean delivery, previous uterine scar other than low transverse, malpresentation in the current pregnancy, presence of an intrauterine fetal demise, delivery at less than 36 weeks of gestation (< 36 0/7 weeks), and incomplete information for a patient. Using these criteria, 709 patients were excluded. In addition, only 3 patients were excluded because their prepregnancy weight data were not in their charts. Height was documented for all patients. After excluding these patients, the total number available for analysis was 1,213. Patients were divided into the following groups: underweight (body mass index [BMI] < 19.8), normal weight (BMI 19.8–26), overweight (BMI 26.1–29), and obese (BMI > 29) based on the categories described by the Institute of Medicine.10 Each patient was assigned to a BMI group based on her prepregnancy weight. Excessive weight gain was defined as a weight gain of more than 40 lb, based on the maximum amount of weight gain for underweight patients suggested by the Institute of Medicine.10 Pregnancy weight gain was defined as the difference between the prepregnancy weight (data provided by the patient) and the weight at the last prenatal visit. Variables of interest included diabetes, previous successful VBAC, previous successful normal spontaneous vaginal delivery, history of gestational or pregestational diabetes, birth weight, labor induction, and the presence of a recurrent indication for cesarean delivery.
Nonrecurrent indications for cesarean delivery included malpresentation, nonreassuring fetal heart tracing, elective primary cesarean delivery, macrosomia, placenta previa, placental abruption, cord prolapse, and active herpes simplex virus infection. Recurring indications included cephalo-pelvic disproportion and failure to progress (dilatation and/or descent). Complications included maternal chronic medical conditions, intrauterine growth restriction, preeclampsia/eclampsia, oligo/polyhydramnios, preterm labor, and placenta previa.
The majority of inductions were performed with prostaglandins. Until 1999, misoprostol was regularly used for induction at our institution in previously scarred uteri. Oxytocin was the other commonly used induction agent. Birth weight was categorized as either 4,000 g or more or less than 4,000 grams. Uterine rupture was defined as a symptomatic, complete separation of the previous uterine scar, with extrusion of fetal parts into the peritoneal cavity.
Statistical analysis was performed with χ2 or Fisher exact tests for the dichotomous variables. The Student t test was used for continuous variables. Analysis of variance in means was performed with analysis of variance testing. Univariate and multivariable logistic regression analyses were then performed to evaluate predictors of VBAC success. Finally, linear regression determination of a Pearson correlation coefficient was performed to examine the relationship between BMI and VBAC success. The study was approved by the Mount Sinai Institutional Review Board.
We identified 1,213 patients who met inclusion criteria. The different patient characteristics are listed in Table 1. The percentage of patients within the BMI categories of less than 19.8, 19.8–26.0, 26.1–29.0, and greater than 29.0 were 13.2%, 58.9%, 11.3%, and 16.6%, respectively (Table 2). Our patients were similar in their gestational age of delivery, indication for the previous cesarean delivery, percentage with a previous successful VBAC, and percentage undergoing induction (Table 1). Some notable, yet expected, differences were that the overweight and obese patients were more likely to have gestational or pregestational diabetes (16.0% and 20.0% versus 4.0% and 6.0% in the underweight and normal weight groups, respectively, P < .001), and the birth weights of their infants were greater than those in the normal weight and underweight patients (Table 1).
Overall, the VBAC success rate was 77.2%. The VBAC success rates for each BMI category, from less than 19.8 to greater than 29.0 were 83.1%, 79.9%, 69.3%, and 68.2%, respectively, P < .001. The prepregnancy BMI was similar in those who succeeded or failed at VBAC (24.1 ± 5.2 and 25.8 ± 5.6, respectively, P = .71) (Table 3). However, the BMI at the time of the delivery was significantly higher in those who failed VBAC (31 ± 5.9) than in those who were successful (29.0 ± 4.9, P < .001). There were more patients with a BMI greater than 29 in the failed VBAC group (61.4%) than in the successful group (41.7%, P < .001). When controlling for previous normal spontaneous vaginal delivery, previous VBAC, indication for previous cesarean delivery, birth weight, and diabetes, those patients with a BMI greater than 29 were still almost 50% less likely to have VBAC success than their underweight counterparts (odds ratio [OR] 0.53, 95% confidence interval [CI] 0.29–0.98, P = .043) (Table 4). A linear regression analysis showed that, as BMI increased, VBAC success decreased (r = –0.182, P < .001.)
Interestingly, uterine rupture rates were higher in the overweight group (3.6%) than in the underweight (0.6%), normal weight (1.8%), and obese (0%) patients, P = .041. However, when controlling for the number of layers of closure of the uterine scar, this no longer held true (OR 5.08, 95% CI 0.53–48.79, P = .159). Information from the operative note about the number of layers of uterine closure was available in 947 cases (or 78.1%). A single-layer closure was significantly associated with uterine rupture (OR 10.13, 95% CI 2.46–41.68, P = .001). However, there were only 35 patients whose uteri were closed with a single layer, and there may be confounders that we have not yet identified that contribute to this observation. The uterine rupture rate for single-layer closures was 8.6%, whereas for double-layer closures it was 1.4%, P < .001. Overall, the uterine rupture rate was 1.6%. Until 1999, misoprostol and other prostagladins were routinely used for induction of labor for VBAC.
With respect to pregnancy weight gain, the VBAC success rate was 79.1% in those who gained less than 40 lb and 66.8% for those who gained more than 40 lb during the pregnancy, P < .001. Patients who gained more than 40 lb were almost 40% less likely to be successful at VBAC than those who gained less than that amount (OR 0.63, 95% CI 0.42–0.97, P = .034) (Table 5). A higher VBAC success rate was still noted in the group gaining less than 40 lb when controlling for previous normal spontaneous vaginal delivery, previous VBAC, indication for previous cesarean delivery, birth weight, and diabetes (OR 1.58, 95% CI 1.04–2.40, P = .034). A history of a previous normal spontaneous vaginal delivery (OR 2.28, 95% CI 1.51–4.52, P = .018) or a previous VBAC (OR 6.32, 95% CI 3.46–11.55, P < .001) were the 2 factors predictive of success for those women who gained more than 40 lb (Table 5). Those who were successful at VBAC gained significantly less weight than those who failed VBAC (29 ± 10.1 lb and 31.4 ± 12.7 lb, respectively, P = .005). There were fewer patients who gained more than 40 lb in the successful VBAC group (13.6%) than in the failed VBAC group (22.7%, P < .001). Although there were more uterine ruptures in the group that gained more than 40 lb (2.1% versus 1.5%), this difference was not statistically significant, P = .515.
Although a linear association was noted with BMI and VBAC success when the 4 BMI categories were included in a multivariable regression analysis that controlled for excessive weight gain, we found that it was the weight gain specifically that decreased VBAC success (OR 0.65, 95% CI 0.42–0.98, P = .042). Also, those patients whose prepregnancy BMI was greater than 29 showed a trend toward decreased VBAC success (OR 0.54, 95% CI 2.93–1.01, P = .053).
Our data show that obese patients and those who gain more than 40 lb during the pregnancy are less likely to have VBAC success. A review in English of the MEDLINE database from 1966 to April 2005, using the search terms “BMI, weight gain, and VBAC” and “BMI, weight gain, and cesarean,” reveals only one other published study that addresses BMI and VBAC success, a study by Durnwald et al,11 and they had findings similar to ours. Consistent with our findings, they showed a stepwise decrease in VBAC success with increasing BMI. Also similarly, they found that VBAC success was less likely if the patient was obese, but not overweight or normal weight. Our work differs, however, in that we also looked at patients who had excessive weight gain during the pregnancy. This cohort is also less likely to be successful.
Several authors have noted the link between obesity, cesarean, and adverse pregnancy outcome. Cedergren12 reviewed the pregnancy outcomes of 3,480 morbidly obese women (defined as BMI > 40) and found that they were at increased risk for cesarean delivery, stillbirth, instrumental delivery, shoulder dystocia, fetal death, and early neonatal death. Weiss and colleagues reviewed the records of 16,102 patients to determine whether obesity was associated with pregnancy complications and cesarean delivery.13 They found that the primary cesarean delivery rate was 20.7% for the normal weight patients (BMI < 30), 33.8% for the obese patients (BMI 30–34.9), and 47.4% for the morbidly obese patients (BMI > 35), P < .01. They also found significant associations between obesity and gestational hypertension, preeclampsia, gestational diabetes, and birth weight greater than 4,000 g.
The association between labor dystocia and obesity has also been evaluated. Vahratian and coworkers14 analyzed data from 612 nulliparous women who participated in the Pregnancy, Infection, and Nutrition Study to evaluate preterm birth from 1995 to 2000. They separated these women into the same BMI categories used in our study, based on the recommendations of the Institute of Medicine, and followed their labor curves. Although they found that cesarean delivery rates were similar in the normal weight group compared with the overweight and obese groups, they found that the progression of labor was significantly slower in the overweight (7.52 hours) and obese (7.94 hours) women when compared with normal weight (6.20 hours) women, P < .01.
With the association of obesity and cesarean delivery, it is logical to hypothesize that obese women are also less likely to have successful VBAC. However, should they be counseled against attempting a vaginal trial of labor after cesarean? Edwards et al15 tried to answer this question when they reviewed VBAC success rates in 122 obese (BMI > 40) mother-infant pairs. Of those, 61 had a planned VBAC, and 61 had a planned repeat cesarean. They found a 57% VBAC success rate in those attempting vaginal trial of labor. The VBAC group had significantly higher rates of chorioamnionitis (13.1% versus 1.6%, P = .02) and composite puerperal infection (24.6% versus 8.2%, P = .01) than the planned cesarean group. Of the complications reviewed in the study, the only one more common in the planned cesarean group was blood loss. The authors also performed a cost analysis to evaluate whether VBAC was more cost-effective. They found that VBAC was more cost-effective only if it was successful. When comparing the planned VBAC to the planned cesarean groups, the costs were similar for the mothers, the infants, and the mother-infant pairs.
We have demonstrated that an inversely proportional relationship exists between VBAC success rates and prepregnancy BMI (Table 2). However, those in the obese category had a 68.2% success rate, even although they are 50% less likely to succeed when compared with underweight patients. Similarly, patients who gained more than 40 lb in the pregnancy had a 66.8% VBAC success rate, although they were almost 40% less likely than those who gained a normal amount of weight to be successful. When the weight gain during the pregnancy was evaluated in conjunction with the pregnancy BMI, it was actually the weight gain that made patients less likely to have VBAC success. However, when comparing the success rates of those who gained more than 40 lb, or of obese patients, with the traditionally quoted VBAC success rate of 60–80%, they are within an acceptable range.
Given the known risks associated with attempting VBAC, patient selection to optimize VBAC success and minimize complications becomes of paramount importance. Although obese candidates may succeed, they do so with longer labors, increased complication rates, and a baseline increased risk for cesarean. Obesity is not a contraindication for VBAC, but obese patients need appropriate counseling to understand the risks of attempting a vaginal trial after previous cesarean.
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