Objective
The purpose of this study was to quantify the relationship between class of obesity and rate of failed induction of labor.
Study Design
Using the Ohio Department of Health’s birth certificate database from January 1, 2006, through December 31, 2007, we performed a population-based cohort study that compared failed induction of labor rates between obese and normal-weight women.
Results
The rate of induction is associated with increasing body mass index from 28% in normal-weight women to 34% in class III obese women (body mass index, ≥40 kg/m 2 ). Induction failure rates are also associated with increasing obesity class from 13% in normal-weight women to 29% in class III obese women. Women with class III obesity without a previous vaginal delivery and a macrosomic fetus had the highest rate of failed induction at 80%.
Conclusion
Obesity is associated with an increased risk of failed labor induction that appears to be related directly to increasing class of obesity.
Obesity is an epidemic that complicates the delivery of health care in the United States, which includes the care for pregnant women. The 2007-2008 National Health and Nutrition Examination Survey found that 68% of adults who were >20 years old were overweight, that 33.8% were obese, and that 5.7% were extremely obese (body mass index [BMI] ≥40 kg/m 2 ). More than one-third of reproductive aged women were obese, and 7.6% of those women were extremely obese. Obese women are more likely to have medical, surgical, and obstetrics complications that include higher rates of induction, dysfunctional labor patterns and higher rates of cesarean delivery. The exact mechanism of dysfunctional labor in the obese woman is not completely understood. Elevated cholesterol level has been shown to decrease uterine contractility, and obese women are more likely to have elevated cholesterol levels than are normal-weight women. This elevation in cholesterol may contribute to the higher incidence of dysfunctional labor in obese women and subsequent cesarean delivery.
Although many reviews list obesity as a risk factor for failed induction, this risk has yet to be quantified; previous studies have not examined the rate of failed induction according to obesity classification. One study of 1273 women who were undergoing induction of labor with prostaglandins found longer induction to delivery time, higher requirements of oxytocin, and higher cesarean delivery rates in obese women compared with lean women (BMI <30 kg/m 2 ). The goal of our study was to report the strength of the association between obesity and failed induction and to examine whether this association directly correlates with increasing class of obesity.
Materials and Methods
The protocol for this study was approved by the Ohio Department of Health and was exempt from review by the Institutional Review Board of the University of Cincinnati. Using the Ohio Department of Health’s birth certificate database from January 1, 2006, through December 31, 2007, we performed a population-based cohort study to compare the rate of failed induction of labor between obese and normal-weight women. We excluded births with gestational ages of <20 weeks, multiple gestations, and infants with major congenital anomalies. We included all women who were coded as having an induction of labor. Augmentation of labor was a separate category; however, induction and augmentation of labor are not mutually exclusive in this database. Women who were coded as both an induction and an augmentation of labor were included in the study group. Maternal height and prepregnancy weight are included in the birth certificate data that were collected and were used to calculate BMI (kilograms per square meter). We stratified this population by maternal obesity classification as defined by the National Institute of Health: underweight (BMI <18.5 kg/m 2 ), normal (BMI 18.5-24.9 kg/m 2 ), overweight (BMI 25-29.9 kg/m 2 ), obesity class I (BMI 30-34.9 kg/m 2 ), obesity class II (BMI 35-39.9 kg/m 2 ), and obesity class III (BMI ≥40 kg/m 2 ).
The primary outcome of the study was the rate of failed induction of labor, which was defined as delivery by cesarean delivery after an attempted induction, compared between obese and normal-weight women. Odds ratios were calculated by obesity classification, history of a vaginal delivery, primiparity, and birthweight. Secondary outcomes included composite neonatal morbidity that included assisted ventilation for >6 hours, neonatal transport to a tertiary care center, seizures, birth injury, and a 5-minute Apgar score of <7.
Statistical analyses were performed with Stata Statistical Software (release 10; Stata Corp, College Station, TX). Demographic characteristics between the groups were compared with the use of the Student t test for continuous variables and chi-square test for categoric variables. Indicators of low socioeconomic status included women who received Medicaid benefits or participated in the Women, Infants, and Children Program. The primary outcome was calculated by chi-squared calculation. Odds ratios were calculated with normal weight (BMI 18.5-24.9 kg/m 2 ) as the referent. A multiple logistic regression model was built that included demographic and obstetric variables, which were significantly different between study groups. The final model was constructed with a backward elimination method to include variables with a >10% effect on the adjusted odds ratio, such as parity, or biologic plausibility, which included medical comorbidities and maternal age. A sensitivity analysis was performed to incorporate the women with missing data to evaluate the possibility of bias in our study conclusion.
Results
During this time period, there were 279,521 singleton pregnancies, of which 89,131 (31.9%) underwent induction of labor. For final analysis, we included 80,887 deliveries that underwent induction of labor and had complete information for prepregnancy height and weight, obstetric history, and route of delivery. The BMI class of women who underwent an induction of labor is given in Figure 1 . Of the obese women, 54% were class I; 27% were class II, and 19% were class III (12%, 7%, and 5% of the total study population, respectively). Maternal demographics for normal-weight and obese women are listed in Table 1 . Obese women were more likely to have medical complications that included chronic hypertension and diabetes mellitus. Obese women were more likely to be African American, to have received treatment for infertility, to be of a low social economic status, and to be slightly older. Obese women were less likely to have had a teen pregnancy or to have been married.
| Demographic | Normal (n = 38,110) | Obese | P value | ||
|---|---|---|---|---|---|
| Class I (n = 10,547) | Class II (n = 5355) | Class III (n = 3657) | |||
| Maternal age, y a | 26.8 ± 6.0 | 27.1 ± 5.6 | 27.3 ± 5.5 | 27.4 ± 5.4 | < .005 |
| Parity, n a | 1.9 ± 1.2 | 2.1 ± 1.4 | 2.1 ± 1.4 | 2.1 ± 1.4 | < .005 |
| Previous vaginal delivery a | 20,245 ± 53 | 6172 ± 59 | 3063 ± 57 | 2015 ± 55 | < .005 |
| Birthweight, g a | 3337 ± 494 | 3407 ± 505 | 3418 ± 518 | 3393 ± 538 | < .005 |
| Gestational age, wk a | 38.8 ± 1.7 | 38.8 ± 1.7 | 38.7 ± 1.7 | 38.6 ± 1.8 | < .005 |
| Maternal race, n (%) | |||||
| White | 33,547 (88) | 8760 (83) | 4447 (83) | 2880 (79) | < .005 |
| Black | 3632 (10) | 1680 (16) | 881 (16) | 758 (21) | < .005 |
| Hispanic | 40 (0.1) | 18 (0.2) | 5 (0.1) | 0 | .05 |
| Other | 931 (2.4) | 107 (1.0) | 27 (0.5) | 19 (0.5) | < .005 |
| Treatment of infertility, n (%) | 178 (0.5) | 77 (0.7) | 31 (0.6) | 36 (1.0) | < .005 |
| Married, n (%) | 23,834 (63) | 6344 (60) | 3237 (61) | 2160 (59) | < .005 |
| Indicators of low socioeconomic status: composite variable, n (%) b | 16,102 (44) | 540 (53) | 2911 (56) | 2164 (61) | < .005 |
| Cigarette smoking, n (%) | 6960 (18) | 2073 (20) | 1057 (20) | 686 (19) | < .005 |
| Education <12 y, n (%) | 5400 (14) | 1494 (14) | 668 (13) | 495 (14) | .007 |
| No prenatal care, n (%) | 288 (0.8) | 77 (0.8) | 33 (0.7) | 27 (0.8) | .729 |
| Limited prenatal care: ≤5 visits, n (%) | 1890 (5.4) | 545 (5.6) | 222 (4.5) | 180 (5.4) | .042 |
| Teen pregnancy: <18 y, n (%) | 1548 (4.1) | 190 (1.8) | 66 (1.2) | 20 (0.6) | < .005 |
| AMA: ≥35 y, n (%) | 4395 (12) | 1180 (11) | 628 (12) | 447 (12) | .371 |
| Infant male sex, n (%) | 19,447 (51) | 5325 (50) | 2772 (52) | 1861 (51) | .496 |
| Medical comorbidities, n (%) | |||||
| Chronic hypertension | 431 (1.1) | 366 (3.5) | 294 (5.5) | 345 (9.4) | < .005 |
| Pregestational diabetes mellitus | 159 (0.4) | 141 (1.3) | 97 (1.8) | 104 (2.8) | < .005 |
| Pregnancy complications, n (%) | |||||
| Induction of labor | 3740 (28.0) | 39,611 (30.4) | 20,759 (32.5) | 20,409 (34.0) | < .005 |
| Gestational diabetes mellitus | 1170 (3.1) | 1000 (9.5) | 598 (11.2) | 512 (14) | < .005 |
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