In this issue of American Family Physician, Tenore1 provides a concisely written “how to” article on cervical ripening and labor induction. This is timely information for family physicians practicing maternity care, considering that the prevalence of induction has nearly doubled over the past decade. Several major shifts have occurred during this period that are worth highlighting.
The Epidemiology of Induction Has Changed
The increase in the frequency of term labor induction has been well established,2–4 yet the change in incidence rates varies considerably by indication. Macrosomia has increased the most as an indication, 22.5-fold since 1980, despite evidence that induction for suspected macrosomia has shown potential benefit only in women with type 1 diabetes mellitus.5,6 Post-term pregnancy, the most common reason for labor induction (10 percent of live births), had only a 2.3-fold increase. At the same time, the average gestational age at delivery of post-term pregnancies declined from 41.9 to 41.0 weeks,6 corresponding with data showing a decreased risk of stillbirth when induction is done at 41 weeks' gestation.7 Although still controversial, induction at 41 weeks' gestation is medically justifiable. Finally, there has been a 15-fold increase in elective induction (e.g., no known medical indication).
Of note, induction rates have shown large variations across maternal classes, with higher induction rates being found in white, non-Hispanic women (25.3 percent), women with more than 12 years of education (24.6 percent), and women with private insurance (24.5 percent).2 Higher induction rates are found in community hospital settings (increased elective inductions), compared with university or federally controlled hospitals (increased inductions because of medical conditions).8
Elective Induction of Labor Is More Common
The rationale for elective induction is mutual convenience, allowing a pregnant woman to handle logistic issues such as child care and transportation, and to know that her expected birth attendant will be present for delivery. Given that most induced births occur between 10 a.m. and 8 p.m., it is reasonable to presume that the physician and staff will be alert and better able to respond to an emergency. However, elective induction is not without potential risks, including iatrogenic prematurity, uterine hyperstimulation, nonreassuring fetal heart rate tracing, and greater likelihood of operative delivery, shoulder dystocia, and postpartum hemorrhage. While these complications are rare in multiparous women, nulliparous women have significantly higher rates of cesarean delivery, instrumented delivery, epidural analgesia, and neonatal intensive care unit admission.9,10 Because the risk of cesarean delivery with elective induction is potentially as high as 2.8 times that for spontaneous labor, it is difficult to advocate elective induction in a nulliparous woman.10–12
Indicators of Induction Success Have Been Refined
Bishop's original cervical scoring system from 1964 demonstrated that the success of induction is related to the state of the cervix, and his work continues to influence the practice of labor care today.13 Since Bishop's work, others have demonstrated that the most important predictor of success is cervical dilation. Effacement, station, and consistency each have about one half the effect, and position has little effect at all.14
It is important to note that Bishop's research centered on elective induction, with a study population that included only multiparous white women at 36 weeks' gestation or more, with a vertex presentation and normal obstetric history. No such scoring system has been validated in nulliparous women, a population known to have higher rates of induction failure and subsequent cesarean delivery.9
Newer predictors of induction success are emerging. Transvaginal ultrasonography holds promise but, to date, has not been shown to outperform digital examination and cervical scoring.15,16 Fetal fibronectin, initially studied as a way of predicting preterm labor, has been evaluated in relation to induction success. Women with fetal fibronectin in their cervicovaginal secretions had a better cervical response to prostaglandins, required fewer doses for induction of labor, and experienced a shorter time from induction to delivery.17
Pharmacologic Agents Have Revolutionized Cervical Ripening
The literature is filled with reports of safe and effective use of prostaglandin preparations for cervical ripening.18–21 Dinoprostone (Cervidil) and misoprostol (Cytotec) lead the list, yet their similarities seem to stop at the fact that they are both prostaglandins. Dinoprostone, approved for single-dose ripening, was shown to be less effective than other prostaglandins in comparative trials.22,23 It is removed more easily than misoprostol, making it preferable in managing hyperstimulation.
Misoprostol, on the other hand, is inexpensive, stable, and more easily stored than dinoprostone preparations. The low cost (less than $1 per dose) offers an economic advantage over dinoprostone, which ranges from $150 to $200 per dose. Numerous prospective, randomized clinical trials have demonstrated the efficacy of misoprostol for cervical ripening and labor induction in term pregnancy.18,20,21,24,25 Maternal and neonatal outcomes have not been shown to be significantly different, despite higher rates of uterine contraction abnormalities with misoprostol use. However, misoprostol is not approved by the U.S. Food and Drug Administration for cervical ripening and induction, and patients should be informed of this before it is used.
Vaginal Birth After Cesarean Delivery and Induction: To Induce or Not To Induce? Successful vaginal birth after cesarean delivery attempts occur far more often in women who experience spontaneous labor (77.1 percent) than in those who have induced labor (57.9 percent).26 Concomitantly, the occurrence of uterine scar separation has been reported to be highest (7 percent) in induced labor, followed by repeat cesarean delivery.
Uterine rupture rates have been demonstrated to be as high as 24.5 per 1,000 with prostaglandin use for cervical ripening or induction after a previous cesarean delivery.27 This is significantly higher than the rates occurring after spontaneous labor or labor induced without prostaglandins (5.2 per 1,000 and 7.71 per 1,000, respectively). This finding has led the Committee on Obstetric Practice of the American College of Obstetricians and Gynecologists to issue an opinion stating that use of prostaglandin cervical ripening agents is discouraged in women with a uterine scar.28
Because of the increase in frequency of labor induction, it is more important than ever that family physicians select appropriate candidates and understand the various methods of achieving a successful induction. Safe and effective agents are available when the need for induction arises. There is a place for elective induction, although the case for choosing it in nulliparous women has less support because of the greater risk of induction failure and cesarean delivery.