Induction of Labor
The goal of induction of labor is to achieve vaginal delivery by stimulating uterine contractions before the spontaneous onset of labor. According to the National Center for Health Statistics, the overall rate of induction of labor in the United States has increased from 90 per 1,000 live births in 1989 to 184 per 1,000 live births in 1997 (1). Generally, induction of labor has merit as a therapeutic option when the benefits of expeditious delivery outweigh the risks of continuing the pregnancy. The benefits of labor induction must be weighed against the potential maternal or fetal risks associated with this procedure. The purpose of this bulletin is to review current methods for cervical ripening and induction of labor and to summarize the effectiveness of these approaches based on appropriately conducted outcomes-based research. These practice guidelines classify the indications for and contraindications to induction of labor, describe the various agents used for cervical ripening, cite methods used to induce labor, and outline the requirements for the safe clinical use of the various methods of inducing labor.
In 1948, Theobald and associates described their use of the posterior pituitary extract, oxytocin, by intravenous drip for labor induction (2). Five years later, oxytocin was the first polypeptide hormone synthesized by du Vigneaud and associates (3). This synthetic polypeptide hormone has since been used to stimulate uterine contractions. Other methods used for induction of labor include membrane stripping, amniotomy, and administering prostaglandin E (PGE) analogues.
If induction is indicated and the status of the cervix is unfavorable, agents for cervical ripening may be used. The status of the cervix can be determined by the Bishop pelvic scoring system (Table 1) (4). If the total score is more than 8, the probability of vaginal delivery after labor induction is similar to that after spontaneous labor.
Acceptable methods for cervical ripening include mechanical cervical dilators and administration of synthetic prostaglandin E1 (PGE1) and prostaglandin E2 (PGE2) (5-9). Mechanical dilation methods are effective in ripening the cervix and include hygroscopic dilators, osmotic dilators (Laminaria japonicum), the 24-French Foley balloon, and the double balloon device (Atad Ripener Device) (10-15). Laminaria ripen the cervix but may be associated with increased peripartum infections (6, 16).
|Table 1. Bishop Scoring System|
|Score||Dilation (cm)||Effacement (%)||Station *||Cervical Consistency||Position of Cervix|
|*Station reflects a . 3 to +3 scale.|
|Modified from Bishop EH. Pelvic scoring for elective induction. Obstet Gynecol 1964;24:267|
Misoprostol, a synthetic PGE1 analogue, can be administered intravaginally or orally and is used for both cervical ripening and induction. It currently is available as a 100-mcg or 200-mcg tablet, and can be broken to provide 25-mcg or 50-mcg doses. Misoprostol currently is approved by the U.S. Food and Drug Administration (FDA) for the prevention of peptic ulcers, but not for cervical ripening or induction of labor.
Two PGE2 preparations are commercially available: a gel available in a 2.5-mL syringe containing 0.5 mg of dinoprostone and a vaginal insert containing 10 mg of dinoprostone. Both are approved by the FDA for cervical ripening in women at or near term. The vaginal insert releases prostaglandin (PG) at a slower rate (0.3 mg/h) than the gel. Both the gel and the vaginal insert have been reported to increase the probability of successful initial induction, shorten the interval from induction to delivery, and decrease the total and maximal doses of oxytocin needed to induce contractions (17).
Other pharmacologic methods for cervical ripening include continuous intravenous oxytocin drip, extraamniotic saline infusion, vaginal recombinant human relaxin, and intracervical purified porcine relaxin. The safety and efficacy of these latter methods are unclear.
Methods of Labor Induction
In addition to oxytocin and misoprostol, other agents can be used for induction of labor. The progesterone antagonist mifepristone (RU 486) is one such suitable and effective induction agent (18). Nonpharmacologic methods of labor induction include stripping the amniotic membranes, amniotomy, and nipple stimulation.
|Confirmation of Term Gestation
Oxytocin, an octapeptide, is one of the most commonly used drugs in the United States. The physiology of oxytocin-stimulated labor is similar to that of spontaneous labor, although individual patients vary in sensitivity and response to oxytocin. Based on pharmacokinetic studies of synthetic oxytocin, uterine response ensues after 3-5 minutes of infusion, and a steady state of oxytocin is achieved in plasma by 40 minutes (19). The uterine response to oxytocin depends on the duration of the pregnancy; there is a gradual increase in response from 20 to 30 weeks of gestation, followed by a plateau from 34 weeks of gestation until term, when sensitivity increases (20). Cervical dilation, parity, and gestational age are predictors of the dose response to oxytocin for labor stimulation (21).
Stripping the amniotic membranes is commonly practiced to induce labor. However, several studies have yielded conflicting results regarding the efficacy of membrane stripping (22-24). Significant increases in phospholipase A2 activity and prostaglandin F2a (PGF2a) levels occur from membrane stripping (25). Stripping membranes appears to be associated with a greater frequency of spontaneous labor and fewer inductions for postterm pregnancy. In a randomized trial of 195 normal pregnancies beyond 40 weeks of gestation, two thirds of the patients who underwent membrane stripping labored spontaneously within 72 hours, compared with one third of the patients who underwent examination only (26).
Artificial rupture of the membranes may be used as a method of labor induction, especially if the condition of the cervix is favorable. Used alone for inducing labor, amniotomy can be associated with unpredictable and sometimes long intervals before the onset of contractions. However, in a trial of amniotomy combined with early oxytocin infusion compared with amniotomy alone, the induction-to-delivery interval was shorter with the amniotomy-plus-oxytocin method (27).
What are the indications and contraindications to induction of labor?
Indications for induction of labor are not absolute but should take into account maternal and fetal conditions, gestational age, cervical status, and other factors. Following are examples of maternal or fetal conditions that may be indications for induction of labor:
Generally, the contraindications to labor induction are the same as those for spontaneous labor and vaginal delivery. They include, but are not limited to, the following situations: