Obesity is an expanding public health concern, with more than half of American women aged 20 to 39 years classified as overweight or obese1 and more than one in five women entering pregnancy classified as obese.2 Obesity is associated with a myriad of prepregnancy and antepartum complications,3 and evidence exists that obese women have increased rates of cesarean delivery.3 Cesarean delivery, moreover, poses particular hazards for obese women, placing them at an even higher risk of wound complications, endometritis, and thromboembolic events.3
Studies of the characteristics of labor in the obese gravid woman have been limited. We are aware of only two studies that examined the duration of second stage in a nulliparous population categorized by body mass index (BMI, calculated as weight (kg)/[height (m)]2)4,5 (English language MEDLINE search using the key words “obstetric, labor,” and “obesity”). Obesity was associated with a shorter second stage than normal-weight control group participants in one study,4 whereas the other study demonstrated no difference in duration of second stage compared with normal-weight control group participants.5 Because of the limited data, the association between maternal weight and characteristics of the second stage of labor is uncertain.
The randomized trial of fetal pulse oximetry6 was a randomized controlled trial that assessed whether the use of fetal pulse oximetry resulted in a lower cesarean delivery rate. In this trial, both patient and labor characteristics were prospectively collected. Using the data from this study,6 we examined the relationship between BMI and the characteristics of the second stage of labor in nulliparous women at 36 weeks or more of gestation.
MATERIALS AND METHODS
We performed a secondary analysis of a clinical trial of fetal pulse oximetry conducted at 14 clinical centers of the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Participants were laboring nulliparous women 2 to 6 cm dilated at randomization with a singleton vertex fetus at 36 weeks of gestation or more. Women with a known last menstrual period were dated by first ultrasound examination if a discrepancy existed of more than 7 days at less than 20 weeks, more than 14 days between 20 and 29 6/7 weeks, or more than 21 days beyond 30 weeks. If the last menstrual period was unknown, gestational age was established by the first ultrasound examination using the standard method of ultrasound gestational age determination at that institution. Exclusion criteria for the primary trial included a planned cesarean delivery, maternal fever immediately before randomization, or pre-existing medical conditions such as diabetes mellitus, known human immunodeficiency virus infection, hepatitis virus infection, or heart or renal disease. Pregnancy-associated hypertension was not an exclusion criterion. As part of the research protocol, decisions regarding labor management were left to the discretion of the managing physician. Each participating center and the biostatistical coordinating center had Institutional Review Board approval for the study.
Maternal and neonatal data were collected by trained research nurses who were present during the labor. Because of a previous analysis of the data set detailing duration of second stage of labor in relation to maternal and neonatal outcomes such as chorioamnionitis, third-degree or fourth-degree perineal trauma, postpartum hemorrhage, and neonatal morbidity, outcomes such as these were not examined.7 The trained research nurses obtained prepregnancy weight and height using review of the patient's chart. A prepregnancy BMI of 18.5–24.9 was classified as normal body weight, a prepregnancy BMI of 25–29.9 was classified as overweight, and a BMI 30 or higher was classified as obese.
Women were included in this secondary analysis if they reached the second stage of labor. Duration of the second stage was calculated as the number of minutes from the first cervical examination that revealed full dilation until delivery, irrespective of whether delivery was vaginal or cesarean. Women were excluded if their BMIs were less than 18.5 because the purpose of this analysis was to examine second-stage labor characteristics in normal-weight, overweight, and obese women.
Univariate associations between BMI classification and binary variables (eg, induced compared with spontaneous labor) were analyzed using the Mantel-Haenszel trend test. For variables of three or more categories (eg, maternal race), the Pearson χ2 test was used. Univariate comparisons of continuous variables among the BMI classes used analysis of variance (eg, gestational age, second-stage duration). General linear modeling was used to analyze the association between second-stage duration and BMI class, controlling for regional anesthesia, type of labor, oxytocin administration, birth weight, maternal race, and mode of delivery. This method was also used to test the interaction between induced compared with spontaneous labor and BMI on the duration of the second stage of labor and to test the association between delivery route and BMI class, controlling for regional anesthesia, type of labor, oxytocin administration, birth weight, and maternal race. The time from onset of the second stage to delivery was compared among the three BMI classes using the log-rank test, and Kaplan-Meier survival curves were produced. All tests were two-tailed with P<.05 used to define significance. SAS (SAS Institute, Inc., Cary, NC) was used for the analysis.
Of the 5,341 women in the clinical trial, 5,169 (97%) had prepregnancy BMI recorded. Of these, 4,884 women had BMIs of 18.5 or higher, of which 3,739 reached the second stage of labor. Table 1 displays the demographics of the 3,739 patients included in this analysis. Maternal age, gestational age at delivery, or use of regional anesthesia did not differ statistically among women in the different BMI classes. Women with higher BMIs were significantly more likely to undergo labor induction, be administered oxytocin, have newborns with higher birth weights, and be African American.
Table 2 displays the median duration of the second stage, expressed in hours and classified by BMI. As illustrated in the Table, the median duration of the second stage was not associated with BMI (P=.13). There also was no association between route of delivery and BMI in women who reached the second stage (Table 3). The rates of cesarean delivery in normal weight, overweight, and obese women were 7.1%, 9.6%, and 6.9%, respectively (P=.17). After controlling for potential confounding with regional anesthesia, type of labor, oxytocin administration, birth weight, race, and mode of delivery, maternal BMI still was not associated with duration of second stage (P=.07). Also, in women who reached the second stage, maternal BMI was not associated with route of delivery (P=.08) after controlling for regional anesthesia, type of labor, oxytocin administration, birth weight, and race.
A graph depicting Kaplan-Meier survival curves of the time in the second stage for each of the BMI classes is shown in Figure 1. When examining successive time intervals to delivery from the onset of the second stage, there was no association between time to delivery and BMI (P=.15).
Given the knowledge that obese women have been shown in other studies to be at increased risk for cesarean delivery,8–12 a test was performed to discern whether there was an increased risk for cesarean delivery in any stage of labor in the present cohort. Table 4 demonstrates that there is an increased incidence in cesarean delivery among overweight and obese women, although it appears to be confined to the first stage of labor. Among women reaching the second stage with cesarean delivery, the indications for cesarean delivery did not differ among BMI classes (P=.64).
Induction of labor is a commonly recognized risk factor for cesarean delivery and prolonged labor. Because of a higher frequency of labor induction within this cohort in overweight and obese women compared with their normal-weight counterparts (Table 1), the data were analyzed to determine whether the relationship between BMI and second-stage duration differed between those with induced compared with spontaneous labor. The relationship between BMI and second-stage duration did not differ between those women with induced compared with spontaneous labor (P=.84).
This secondary analysis demonstrates no association between maternal BMI and the length of the second stage in nulliparous women. In addition, maternal BMI in nulliparous women reaching the second stage is not associated with a higher incidence of cesarean delivery.
In a retrospective case-control study including spontaneously laboring women, Verdiales et al5 found no difference between second-stage duration in 51 obese nulliparous women (BMI more than 35) and 60 nulliparous normal-weight control group participants (BMI less than 26). Buhimschi et al13 performed a prospective cohort study in 71 women during the second stage of labor and determined that second-stage duration between obese women and nonobese control group participants were similar. They also found no increase in frequency of operative delivery or perineal lacerations. These data suggest that the portion of labor most significantly affected by increased maternal weight is not the second stage, but rather the first stage of labor. The study by Buhimschi13 did not analyze the effect of parity on outcomes and did not differentiate between induced and spontaneously laboring patients. However, the Vahratian study,4 which contained 612 participants, found that obese nulliparous women had a shorter second stage of labor than normal-weight control group participants, contradicting the theoretical notion that soft tissue dystocia may lead to longer labors in obese women. An investigation by Cheng et al14 examined perinatal outcomes associated with the duration of second stage labor in more than 5,000 multiparous women and found no association between BMI and length of the second stage. Our study supports the findings of Verdiales,5 Buhimschi,13 Vahratian,4 and Cheng14 in that we found no evidence of increased second-stage duration in women with increasing BMI, nor did we find an association between BMI and cesarean delivery in women reaching the second stage. Furthermore, our results mirror those of Fyfe et al12 in that increasing BMI appears to be a risk factor for cesarean delivery in the first stage but not in the second stage of labor.
This study has inherent strengths and weaknesses. The prospective nature of the data collection by research nurses who were present during labor supports the accuracy of the data. This was also a multicenter trial, which means that the practice patterns may better-represent national practices and enhance external validity. Other strengths of our analysis are the large population size and inclusion of only nulliparous parturient women to examine associations between BMI and labor characteristics. Among women reaching the second stage, no association was found between BMI and time interval to delivery, suggesting that there was not a quicker or slower tendency on behalf of providers to perform a cesarean delivery or otherwise expedite delivery based on maternal BMI. Although analysis did not demonstrate that providers were predisposed to more commonly choose a cesarean delivery because of a patient's weight, the lack of standardized second-stage management is a potential limitation of the study, and this study does not address adherence by providers to guidelines for management of a prolonged second stage.15 Additionally, the inclusion of women who had cesarean deliveries after reaching the second stage may introduce bias into the duration of the second stage based on factors that may not have been captured, such as the availability of an operating room. The external validity of our findings also may be limited to populations with high frequencies of women receiving regional anesthesia and undergoing induction of labor. Last, because of recruitment protocol that did not include standardized measures in the enrollment process to avoid weight-based selection bias, the internal validity of the frequency of spontaneous compared with induced labors, as well as the frequency of delivery in the first stage of labor, by BMI, should not be regarded as conclusive.
These data suggest that the characteristics of the second stage of labor may be independent of maternal BMI. The second stage in the nulliparous parturient woman does not appear to be longer or more likely to end in cesarean delivery based on prepregnancy BMI. This knowledge may aid obstetric providers in counseling women about the expected effect of their BMI on their intrapartum course, as well as assist in clinical decision-making in women who reach the second stage of labor.
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