Objective
We sought to estimate the association between fetal gender and first-stage labor curve at term.
Study Design
Within a large, retrospective cohort study of consecutive, singleton term labor patients who delivered in the second stage, we compared the active phase of first-stage labor by fetal gender. The primary outcome was length of active stage 1. Interval-censored regression was used to estimate the effect of fetal gender on the duration of active first stage (4-10 cm) and was adjusted for relevant covariates.
Results
Of 2400 women, 2373 women had complete labor information and were available for this analysis. Male gender was associated with both a statistically significantly longer active first stage of labor (4.6 vs 4.0 hours; P = .002) and stratified analyses by parity and labor type.
Conclusion
Male fetuses are associated with longer active phase of the first stage of labor and, specifically, may need to be considered in the setting of arrest diagnoses.
Male fetal gender has been identified as a risk factor for cesarean delivery. This finding has been interpreted as a surrogate for fetal size because male fetuses tend to be larger on average compared with female fetuses. Animal and pathologic models have provided evidence for a fetal influence on the labor process ; however, the potential impact of fetal gender on the labor curve has gone largely unstudied.
Recently described techniques to analyze the labor curve have highlighted subtle inaccuracies both in what has long been considered normal since the description by Friedman and the need to gain further understanding of the influencing factors on the most commonly encountered clinical entity in obstetrics: labor. With a startlingly high cesarean delivery rate that continues to rise, opportunities to better understand factors that influence the normal progression of labor can help practitioners individualize the diagnoses of arrest disorders.
We sought to estimate the independent impact of fetal gender on the active phase of the first stage of labor at term.
Materials and Methods
The participants in this study were part of the first 2 years of a 4-year retrospective cohort study of all consecutive term deliveries at Washington University Medical Center in St. Louis, MO, between July 2004 and June 2008, which was conducted with institutional review board approval. The parent study was designed to estimate the association between intrapartum electronic fetal heart rate patterns and acidemia at birth in patients who delivered in the second stage of labor. Women were included if their gestational age was at least 37 weeks and 0 days at admission to labor and delivery, if they carried a singleton pregnancy in vertex presentation, and if they had an umbilical cord gas level obtained at delivery. We excluded women who delivered preterm, had fetuses with congenital anomalies, or delivered by cesarean before complete dilation.
We extracted detailed information on maternal sociodemographic, obstetric, and gynecologic history, medical and surgical history, prenatal history, antepartum records, and labor and delivery records. The labor and delivery records included medications, labor type, cervical examination times, dilation and station, length of labor stages, mode of delivery, and postpartum records. Cervical dilation was documented in centimeters that ranged from 0–10 cm. Gender was defined as the gender assignment made by the pediatrician at delivery.
For this study, we compared the duration and curves of the first stage of labor by gender and in stratified analyses that considered parity and type of labor. To model the curvilinear trend of cervical dilation over time, we used a repeated-measures regression with a polynomial function. Because the known fact was achievement of complete dilation, the regression model was executed in a reverse approach by starting at 10 cm and working backward to the first cervical examination. Polynomial equations are formed by taking the independent variable to sequential powers. A 9th-order polynomial in time was the best fit for the dilation values in our data ; we used XTMIXED software (version 11.1; StataCorp, College Station, TX) for the analysis.
The main analysis investigated the time for cervical dilation to increase from 4-10 cm in aggregate, by increments of 2 cm (eg, 4-6 cm). Because of the variability of cervical dilation at first examination and subsequent examination timing, it is not possible to know exactly when a level of cervical dilation is reached. This required us to calculate a time interval between every centimeter of cervical dilation, which gave a possible minimum and maximum time at which the cervical dilation was reached. The time interval assumption fits a log normal distribution; previous publications have demonstrated that the duration of labor often has a right-skewed pattern. We calculated the time interval between each consecutive cervical dilation for all individuals, which gave them an interval-censored value for each level of dilation. We used PROC LIFEREG software (version 9.2; SAS Institute Inc, Cary, NC) to fit a log normal distribution to the time interval and estimated median (5th and 95th percentiles). Multivariate models were built to adjust for relevant confounding factors. Variables that were demonstrated to be both historically relevant and identified in bivariable analyses were considered. Birthweight was considered continuously, dichotomously (>4000 g) and categorically (by increments of 500 g); fetal station was also considered. Final models to estimate the relationship between fetal gender and labor curve were adjusted only for statistically significant factors: parity, regional anesthesia, prostaglandin level, and birthweight. These analyses were repeated to stratify by parity and by labor type (induced vs augmented vs spontaneous).
Results
Of 2400 consecutive term deliveries from the second stage of labor, 2373 deliveries met inclusion criteria for this analysis. Of the 27 women who were excluded, 2 women were excluded for neonatal ambiguous genitalia, and the remaining 25 women were excluded for complete dilation at admission, which prevented the construction of their first-stage labor curve. Women who carried a male fetus were similar, on average, to women who carried a female fetus with respect to maternal age, body habitus, gravidity, labor type, and rates of cesarean delivery ( Table 1 ). They were also similar with respect to rates of maternal comorbidities and rates of alcohol and tobacco exposure. Women who carried a male fetus had a higher rate of operative vaginal deliveries and their infants weighed more, on average, with a greater number of male fetuses being >4000 grams.
