Interpretation
Two of the previously published landmark studies regarding induction of labor at 39 weeks gestation versus expectant management were the randomized trials by Walker et al. and the ARRIVE trial.20, 21 The Walker et al. trial compared labor induction at 39 weeks gestation with expectant management in women 35 years of age or older.20 The primary outcome of the study was to determine the rate of cesarean delivery in each group. Secondary outcomes evaluated maternal and neonatal morbidity and serious neonatal complications. Their results indicated that induction of labor at 39 weeks did not result in a significant difference in adverse short-term effects on maternal or neonatal outcomes, including cesarean section. In contrast our study demonstrated that induction of labor at 39 weeks gestation resulted in a significantly lower risk of maternal triple I, cesarean section, and neonatal morbidity as compared to expectant management through 42 weeks. This difference may be explained by the fact that the Walker et al. study was relatively small and homogeneous sample size (619 subjects), looked only at women of 35 years of age or older, and was powered to primarily test the hypothesis that induction of labor would reduce the rate of cesarean section. In contrast our cohort study with the much larger sample size of 1,885,694 subjects including women of all ages may have allowed detecting between-group differences of individual maternal and neonatal morbidities.
The ARRIVE trial was designed to test the hypothesis that elective induction of labor at 39 weeks would result in a lower risk of a composite outcome of perinatal death or severe neonatal complications than expectant management among low-risk nulliparous women.21 Their results indicated that induction of labor at 39 weeks did not result in a significantly lower frequency of a composite adverse perinatal outcome, but it did result in a significantly lower frequency of cesarean delivery. Consistent with the data, our study demonstrates there is a significantly lower rate of cesarean section in the induction group compared to the expectant-management group. In contrast to the ARRIVE trail, however, our study shows a significantly lower frequency of adverse neonatal outcomes between the induction of labor and the expectant management group. These differences may be explained by the fact that the ARRIVE trial took place in a solely academic center and therefore may have had a relatively homogeneous subject population. In addition, despite its sample size, the trial was not powered to detect differences in rare adverse outcomes.
A novel finding of our study is the increased rate of cesarean hysterectomy in the induction of labor group. This new finding was identified due to the large study population, which is powered to detect rare adverse outcomes such as this. The increased rate of cesarean hysterectomy cannot be explained exclusively by differences in the rates of uterine rupture or postpartum hemorrhage, the two most common indications. Previous research has demonstrated that induction of labor and the methods used for induction of labor are associated with an increased risk for uterine rupture. Induction agents, such as prostaglandins and oxytocin, have been associated with risk for uterine atony and rupture, which are the two most common indications to proceed with cesarean hysterectomy.30-34 Due to the nature of this study, the indication for cesarean hysterectomy could not be determined in this cohort, however, the overall rate is consistent with previous rates seen with induction of labor. It is likely that a combination of these factors led to an increased rate of cesarean hysterectomy among women undergoing induction of labor. With this new finding in mind, it is important to note that induction of labor is not without consequence.