We performed a secondary analysis of a clinical trial of fetal pulse oximetry conducted at 14 clinical centers of the NICHD Maternal Fetal Medicine Units Network. Participants were laboring nulliparous women 2 to 6 centimeters dilated at randomization with a singleton vertex fetus at ≥36 weeks of gestation. Women with a known last menstrual period (LMP) were dated by first ultrasound if a discrepancy existed of greater than 7 days at less than 20 weeks, greater than 14 days between 20 and 29+6 weeks, or greater than 21 days beyond 30 weeks. If the LMP 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 preexisting medical conditions such as diabetes mellitus, known human immunodeficiency virus infection, hepatitis virus infection, heart or renal disease. Pregnancy-associated hypertension was not an exclusion criterion. As part of the research protocol, decisions on 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. Due to a prior analysis of this data set detailing duration of second stage of labor in relation to maternal and neonatal outcomes such as chorioamnionitis, third/fourth degree perineal trauma, postpartum hemorrhage, and neonatal morbidity, outcomes such as these were not examined7
. The trained research nurses obtained pre-pregnancy weight and height utilizing review of the patient’s chart. BMI was calculated with the following formula: BMI= weight (kilograms)/(height [meters])2
. A pre-pregnancy BMI of 18.5-24.9 kg/m2
was classified as normal body weight, a pre-pregnancy BMI of 25-29.9 kg/m2
was classified as overweight, and a BMI 30 kg/m2
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 BMI was < 18.5 kg/m2, as the purpose of this analysis was to examine second stage labor characteristics in normal body weight, overweight and obese women.
Univariate associations between BMI classification and binary variables (e.g., induced vs. spontaneous labor) were analyzed using the Mantel-Haenszel trend test. For variables of three or more categories (e.g., maternal race), the Pearson chi-square test was used. Univariate comparisons of continuous variables among the BMI classes used Analyis of Variance (e.g., 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, birthweight, maternal race and mode of delivery. This method was also used both to test the interaction between induced versus 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, birthweight 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<0.05 used to define significance. SAS software (SAS Institute, Inc., Cary, NC) was used for the analysis.