Today, several pharmacological agents permit labor to be induced or augmented. Induction implies stimulation of contractions before the spontaneous onset of labor, with or without ruptured membranes. When the cervix is closed and uneffaced, labor induction is often preceded by cervical ripening, a process to soften and open the cervix. This is somewhat of a misnomer in that cervical ripening, per se, may also be a means of labor induction. Augmentation refers to enhancement of spontaneous contractions that are considered inadequate because of failed cervical dilation and poor fetal descent. This chapter presents indications and methods for labor induction or augmentation and for cervical ripening.
In the United States, the incidence of labor induction rose from 9.5 percent in 1991 to 27 percent in 2019 (Martin, 2019). Induction is indicated when the benefits to either mother or fetus outweigh those of pregnancy continuation. The more common indications include membrane rupture without labor, gestational hypertension, oligohydramnios, nonreassuring fetal status, postterm pregnancy, and various maternal medical conditions such as chronic hypertension and diabetes (American College of Obstetricians and Gynecologists, 2019a).
Methods to induce or augment labor are contraindicated by most conditions that preclude spontaneous labor or delivery. Maternal contraindications are an abnormally implanted placenta or a prior uterine incision type that is associated with a high rupture risk. Uncommon conditions are active genital herpes infection, contracted or distorted pelvic anatomy, or cervical cancer. Fetal factors include appreciable macrosomia, severe hydrocephalus, malpresentation, or nonreassuring fetal status.
Some maternal complications associated with labor induction are chorioamnionitis, uterine rupture, and postpartum hemorrhage from uterine atony. Earlier nonrandomized studies suggested a particularly higher risk for cesarean delivery in nulliparas undergoing labor induction (Luthy, 2004; Yeast, 1999). More recent ones, however, cite comparable or even lower rates of cesarean delivery when induced labor is compared with spontaneous labor (Middleton, 2020; Souter, 2019).
These studies used spontaneous labor as the comparator, which may be less suitable than using expectant management as the comparator. In the ARRIVE trial—A Randomized Trial of Induction Versus Expectant Management-more than 6000 low-risk nulliparas at 39 weeks’ gestation were randomly assigned to labor induction or expectant management groups (Table 26-1) (Grobman, 2018; Tita, 2021). Primary outcome, which was the rate of severe neonatal morbidity or death, did not differ between cohorts. However, the cesarean delivery rate in the induction arm was significantly lower than that in the expectantly managed cohort—18.6 versus 22.2 percent, respectively. Rates of chorioamnionitis, postpartum hemorrhage, and peripartum infection were not significantly different. In a metaanalysis of six cohort studies, elective induction at 39 weeks’ gestation had a significantly lower risk of cesarean delivery and these other morbidities compared with expectant management (Grobman, 2019).