Objective
We sought to assess body mass index (BMI) effect on cesarean risk during labor.
Study Design
The Consortium on Safe Labor collected electronic data from 228,668 deliveries. Women with singletons ≥37 weeks and known BMI at labor admission were analyzed in this cohort study. Regression analysis generated relative risks for cesarean stratifying for parity and prior cesarean while controlling for covariates.
Results
Of the 124,389 women, 14.0% had cesareans. Cesareans increased with increasing BMI for nulliparas and multiparas with and without a prior cesarean. Repeat cesareans were performed in >50% of laboring women with a BMI >40 kg/m 2 . The risk for cesarean increased as BMI increased for all subgroups, P < .001. The risk for cesarean increased by 5%, 2%, and 5% for nulliparas and multiparas with and without a prior cesarean, respectively, for each 1-kg/m 2 increase in BMI.
Conclusion
Admission BMI is significantly associated with delivery route in term laboring women. Parity and prior cesarean are other important predictors.
Amid an epidemic of obesity in the United States, obesity among pregnant women has risen dramatically. The increased perinatal morbidity associated with maternal obesity such as birth defects, preeclampsia, gestational diabetes, stillbirth, abnormal fetal growth, and cesarean deliveries has caught the attention of obstetrician-gynecologists. Long-term adverse outcomes of maternal obesity, including childhood and adolescent obesity for their offspring, are becoming well known. Another critical issue in obstetrics is the rising cesarean rate, estimated at 31.8% in 2007. In addition to the known short-term complications such as infectious morbidity and thromboembolic events, cesarean deliveries are associated with long-term complications such as abnormal placentations and hysterectomies.
Labor management as well as cesarean delivery in the obese gravida presents many clinical challenges. The relationship between body mass index (BMI) and cesarean delivery is well established with some studies showing a direct linear relationship between the two. However, prior studies have not independently evaluated the associations among parity, prior cesarean, BMI, and delivery route. The objective of this study was to characterize the role of BMI at labor admission on cesarean delivery via regression analysis using data from the Consortium on Safe Labor database.
Materials and Methods
This is an analysis of data from the Consortium on Safe Labor. The primary goal of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD)-sponsored Consortium on Safe Labor was to establish a comprehensive database from multiple sites and to characterize labor and delivery in a contemporary group of women experiencing current obstetrical clinical practices. The complete database contained 228,668 deliveries from 2002 through 2008 acquired from electronic obstetrical databases. Twelve clinical centers from 19 distinct hospitals across 9 American College of Obstetricians and Gynecologists districts participated in the Consortium on Safe Labor. The majority (87%) of births occurred from 2005 through 2007. All births at ≥23 weeks were included in the database. Participating institutions extracted detailed information from their electronic medical records on maternal demographic characteristics, medical history, reproductive and prenatal history, labor and delivery summary, postpartum, and newborn information. An in-house obstetrician was available 24 hours per day at 11 of the 12 participating sites. The institutional review boards of all participating institutions, the NICHD, and the Data Coordinating Center (EMMES Corporation, Rockville, MD) approved this project.
For the current cohort study, the inclusion criteria were liveborn cephalic singletons at ≥37 0/7 weeks of gestation with induced or spontaneous labor, defined as those who had a vaginal delivery or those who had at least 2 cervical examinations documented in the obstetrical database. As such, the intent was to exclude patients with a prelabor cesarean delivery. In addition, we included only cases where the maternal height and weight at the time of labor admission were available so as to calculate BMI in kg/m 2 for each patient. Further, after applying the eligibility criteria noted earlier, about 6% of women contributed >1 delivery to the database. To avoid intraperson correlation, we selected the first delivery captured in the study regardless of the woman’s parity. The primary outcome was delivery route (ie, cesarean or vaginal delivery). Independent variables considered in the statistical analyses and adjusted for in the regression analyses included maternal age, race, gestational age, parity, short stature (height <1.50 m), prior cesarean delivery, pregestational or gestational diabetes, cervical dilation on admission (in centimeters), and induction of labor. These independent variables were selected not only because they were available at the time of admission to labor and delivery, but also because they have been shown to be associated with delivery route. The data for maternal age, gestational age, and cervical dilation on admission were analyzed as continuous variables, whereas the other variables were analyzed as categorical except for BMI at admission. The latter was analyzed both continuously and in categories grouped by World Health Organization (WHO) criteria as normal, <25.0 kg/m 2 ; overweight, 25.0-29.9 kg/m 2 ; obese, class I, 30.0-34.9 kg/m 2 ; obese, class II, 35.0-39.9 kg/m 2 ; and obese, class III, ≥40 kg/m 2 . Because parity and prior cesarean are known determinants of cesarean delivery, the relationship between BMI at admission and cesarean was also examined within the 3 subgroups defined by these factors: nulliparas, multiparas with a prior cesarean, and multiparas without a prior cesarean. There were 6025 multiparas in the eligible cohort wherein prior cesarean status was not noted in the electronic medical record. Because the proportion of these cases that had a cesarean (3.5%) was very similar to the proportion that had a cesarean in multiparas without a prior cesarean (4.9%), it was assumed the lack of a comment in the electronic medical record for these cases corresponded to no prior cesarean and therefore this group with missing prior cesarean status was analyzed with those without a prior cesarean.
Analyses included descriptive and univariate statistics (χ 2 and the nonparametric Wilcoxon rank sum test) for describing the relationship between the independent variables and delivery route. The Cochran-Armitage trend test was used to assess the linear trend relationship in the proportion with cesarean delivery by BMI category according to WHO criteria. Modified Poisson regression methodology (with robust error variance) estimated the unadjusted and adjusted relative risk and 95% confidence intervals of a cesarean delivery. Predicted means generated from the overall Poisson regression multivariate model, with BMI at admission as a continuous variable, were used to calculate predicted probabilities of a cesarean delivery for each delivery. After rounding the BMI to the closest integer, the predicted probabilities were averaged for each BMI at admission value between the range of 21-50 kg/m 2 , which represents the 1st and 99th percentiles of the data. A locally weighted scatterplot smoothing regression method was applied to the average predicted probabilities to generate a smoothed line to visually display the relationship between BMI at admission and probability of a cesarean section. Statistical analysis was performed using Statistical Analysis Software (version 9.2; SAS Institute Inc, Cary, NC).
Results
The entire Consortium on Safe Labor database consisted of 228,668 deliveries. After exclusions (18% prelabor cesareans, 4% multiple gestations, 14% deliveries <37 weeks, 21% missing BMI data, or a combination of these factors), 132,165 met the eligibility criteria for the current study. After removing 7776 deliveries of multiple pregnancies from the same mother and retaining the first delivery, 124,389 patients remained in the analysis data set, of which 17,434 (14.0%) had a cesarean delivery performed during labor. Table 1 describes the demographic data of the current study, grouped by delivery route and stratified by parity and prior cesarean status. Maternal age ≥35 years, short stature, black or Hispanic race, nulliparity, less dilated cervices on labor admission, diabetes, and induced labor were more common in cesarean compared to vaginal deliveries. Only 4.3% of the total group had a prior cesarean, and of these, 63% delivered vaginally. The intrapartum cesareans among the 12 different participating sites ranged from 9.2–26.5%.
| Characteristic | Total | Parity group | ||||||
|---|---|---|---|---|---|---|---|---|
| Nulliparas | Prior cesarean | No prior cesarean a | ||||||
| Mode of delivery | Mode of delivery | Mode of delivery | Mode of delivery | |||||
| Cesarean | Vaginal | Cesarean | Vaginal | Cesarean | Vaginal | Cesarean | Vaginal | |
| Total, n (%) | 17,434 (14.0) | 106,955 (86.0) | 12,500 (21.8) | 44,730 (78.2) | 1977 (37.4) | 3311 (62.4) | 2957 (4.8) | 58,914 (95.2) |
| Maternal age, y b | ||||||||
| <35 | 14,596 (83.7) | 93,673 (87.6) | 10,884 (87.1) | 41,791 (93.4) | 1574 (79.6) | 2607 (78.7) | 2138 (72.3) | 49,275 (83.6) |
| ≥35 | 2829 (16.2) | 13,188 (12.3) | 1612 (12.9) | 2896 (6.5) | 402 (20.3) | 700 (21.1) | 815 (27.6) | 9592 (16.3) |
| Mean (SD) | 27.6 (6.5) | 27.1 (6.0) | 26.5 (6.4) | 24.6 (5.8) | 29.4 (5. 8) | 29.8 (5.5) | 30.6 (6.0) | 28.8 (5.4) |
| Maternal height, m | ||||||||
| <1.5 | 747 (4.3) | 2215 (2.1) | 519 (4.2) | 862 (1.9) | 106 (5.4) | 101 (3.1) | 122 (4.1) | 1252 (2.1) |
| ≥1.5 | 16,687 (95.7) | 104,740 (97.9) | 11,981 (95.8) | 43,868 (98.1) | 1871 (94.6) | 3210 (96.9) | 2835 (95.9) | 57,662 (97.9) |
| Mean (SD) | 1.62 (0.07) | 1.64 (0.07) | 1.62 (0.07) | 1.64 (0.07) | 1.62 (0.08) | 1.63 (0.07) | 1.62 (0.07) | 1.64 (0.07) |
| Maternal race | ||||||||
| Black | 4457 (25.6) | 20,931 (19.6) | 2969 (23.8) | 8651 (19.3) | 503 (25.4) | 746 (22.5) | 985 (22.5) | 11,534 (19.6) |
| Hispanic | 3557 (20.4) | 19,102 (17.9) | 2386 (19.1) | 7525 (16.8) | 473 (23.9) | 614 (18.5) | 698 (23.6) | 10,963 (18.6) |
| White | 7443 (42.7) | 56,784 (53.1) | 5669 (45.4) | 23,524 (52.6) | 771 (39.0) | 1655 (50.0) | 1003 (33.9) | 31,605 (53.6) |
| Other | 1977 (11.3) | 10,138 (9.5) | 1476 (11.8) | 5030 (11.8) | 230 (11.6) | 296 (8.9) | 271 (9.2) | 4812 (8.2) |
| Parity | ||||||||
| Nulliparas | 12,500 (71.7) | 44,730 (41.8) | 12,500 (100.0) | 44,730 (100.0) | 0 | 0 | 0 | 0 |
| Multiparas | 4934 (28.3) | 62,225 (58.2) | 0 | 0 | 1977 (100.0) | 3311 (100.0) | 2957 (100.0) | 58,914 (100.0) |
| BMI category, kg/m 2 | ||||||||
| <25.0 | 1327 (7.6) | 16,903 (15.8) | 1011 (8.1) | 8102 (18.1) | 130 (6.6) | 393 (11.9) | 186 (6.3) | 8408 (14.3) |
| 25.0-29.9 | 5579 (32.0) | 44,005 (41.1) | 4172 (33.4) | 19,381 (43.3) | 616 (31.2) | 1275 (38.5) | 791 (26.8) | 23,349 (39.6) |
| 30.0-34.9 | 5157 (29.6) | 28,084 (26.3) | 3676 (29.4) | 10,998 (24.6) | 583 (29.5) | 920 (27.8) | 898 (30.4) | 16,166 (27.4) |
| 35.0-39.9 | 2941 (16.9) | 11,496 (10.7) | 1994 (16.0) | 4051 (9.1) | 363 (18.4) | 468 (14.1) | 584 (19.7) | 6977 (11.8) |
| >40.0 | 2430 (13.9) | 6467 (6.0) | 1647 (13.2) | 2198 (4.9) | 285 (14.4) | 255 (7.7) | 498 (16.8) | 4014 (6.8) |
| Mean (SD) | 32.9 (6.9) | 30.2 (5.7) | 32.6 (6.8) | 29.6 (5.4) | 33.2 (6.9) | 31.1 (5.9) | 33.8 (7.0) | 30.5 (5.8) |
| Gestational age, wk | ||||||||
| Mean (SD) | 39.5 (1.2) | 39.2 (1.1) | 39.6 (1.2) | 39.4 (1.1) | 39.0 (1.2) | 39.2 (1.1) | 39.3 (1.2) | 39.1 (1.1) |
| Dilation at admission, cm c | ||||||||
| Mean (SD) | 2.2 (1.7) | 3.5 (2.2) | 2.0 (1. 7) | 3.1 (2.1) | 2.3 (1.8) | 3.9 (2.3) | 2.7 (1.8) | 3.8 (2.1) |
| Prior cesarean | 1977 (11.3) | 3311 (3.1) | 0 | 0 | 1977 (100.0) | 3311 (100.0) | 0 | 0 |
| Diabetes | 1102 (6.3) | 3988 (3.7) | 718 (5.7) | 1288 (2.9) | 116 (5.9) | 172 (5.2) | 268 (9.1) | 2528 (4.3) |
| Induction | 10,261 (58.9) | 44,109 (41.2) | 7833 (62.7) | 18,688 (41.8) | 665 (33.6) | 868 (26.2) | 1763 (59.6) | 24,553 (41.7) |
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