Induction of Labor

Physicians have turned to induction of labor as a means of preventing complications from conditions such as prolonged rupture of membranes, fetal demise in utero or severe pre-eclampsia, or a post-term pregnancy. For example, post-term delivery beyond 42 weeks' gestation poses risks to both mothers and babies, including increased perinatal death, increased cesarean deliveries, perineal injuries from macrosomia, and labor dystocias. The onset of labor can be stimulated in different ways. Less interventional approaches, such as stripping of the amniotic membranes, may be effective but not predictable and thus may not be sufficient in women who need prompt delivery. Stripping of the membranes requires the cervix to be dilated sufficiently to introduce the examining finger. Potential risks include rupture of membranes, infection, and bleeding. However, this technique appears safe, in particular for those gravidas approaching postdates (El-Torkey and Grant, 1992).

In most cases the pharmacologic induction of pregnancy should be reserved for a medical or obstetric indication, such as hypertension, diabetes, premature rupture of membranes, postdates, fetal demise, and IUGR. Logistic and psychosocial factors, such as distance from the hospital and rapid labors, are also indications. Absolute contraindications for induction of labor are similar to those for spontaneous labor and vaginal delivery. In pregnancy, at term before induction of labor, fetal maturity should be confirmed. Gestational age of at least 39 weeks should be confirmed by early positive pregnancy test, early ultrasound, or early fetal heart tone auscultation. In some cases, fetal lung maturity studies from amniotic fluid may be required.

Success of induction is mainly a factor of parity and cervical readiness at the time of induction. A scoring system developed by Bishop in 1964 to assess the cervix has become popular (Table 21-13). A Bishop's score of 9 or greater is favorable for induction. Women with low scores benefit from cervical ripening with prostaglandins before induction.

Prostaglandin E2 is now approved by the FDA for cervical ripening. Dinoprostone (Prepidil) is 0.5 mg of modified PGE2 in 2.5 mL of gel that is placed intracervically and repeated once in 6 to 12 hours if the Bishop's score does not change appreciably. Intracervical dinoprostone may or may not cause uterine contractions. Its effect on cervical consistency appears unrelated to the number of contractions. Alternately, dinoprostone may be given intravaginally (Cervidil) as a slow-release, 12-hour vaginal insert that may be removed if uterine hyperstimulation occurs. Previous cesarean section is not a contraindication for use. If spontaneous labor does not ensue, an oxytocin infusion may then be used.

Misoprostol (Cytotec), a synthetic PGEj FDA approved for prevention of gastric ulcers, has been used off-label for cervical ripening and labor induction. Several protocols have been developed. It is a potent uterotonic agent that may be associated with increased incidence of uterine rupture. Miso-prostol is best not used in women with prior cesarean section or uterine surgery.

EVIDENCE-BASED SUMMARY

• Patients should be counseled that walking during labor does not enhance or improve progress in labor and that it is not harmful (SOR: A).

• Continuous support during labor from caregivers should be encouraged because it is beneficial for women and their newborns (SOR: A).

• Active management of labor may shorten labor in nulliparous women, although it has not consistently been shown to reduce the rate of cesarean delivery (SOR: B).

• Amniotomy may be used to enhance progress in active labor but may increase the risk of maternal fever (SOR: B).

• Intrauterine pressure catheters may be helpful in the management of dystocia in select patients, such as obese women (SOR: C).

• Women with twin gestations may undergo augmentation of labor (SOR: C).

KEY TREATMENT

Prostaglandin E (PGE) analogs are effective for cervical ripening and inducing labor (SOR: A).

Low-dose or high-dose oxytocin regimens are appropriate for women in whom induction of labor is indicated (SOR: A) Before 28 weeks' gestation, vaginal misoprostol appears to be the most efficient method of labor induction regardless of Bishop's score, although high-dose oxytocin infusion is also acceptable (SOR: A).

Approximately 25 ^g of misoprostol should be considered as the initial dose for cervical ripening and labor induction; frequency of administration should be no more than every 3 to 6 hours (SOR: A). Intravaginal PGE2 for induction of labor in women with premature rupture of membranes appears to be safe and effective (SOR: A). The use of misoprostol in women with prior cesarean delivery or major uterine surgery has been associated with an increase in uterine rupture and therefore should be avoided in the third trimester (SOR: A).

Misoprostol (50 ^g every 6 hours) to induce labor may be appropriate in some situations, although higher doses are associated with an increased risk of complications, including uterine tachysystole with fetal heart rate decelerations (SOR: B).

Data from ACOG, 2009.

Table 21-13 Bishop's Pelvic Scoring for Elective Induction of Labor

Score

Dilation

Effacement Station

Position Consistency

0

Closed

0-30%

-3

1

1-2 cm

40%-50%

-2

Posterior Firm

2

3-4 cm

60%-70%

-1, 0

Midposition Moderately firm

3

5+ cm

80+%

+1, +2

Anterior Soft

The Bishop's score generally follows this scale:

Modifiers

A point is added to the score for each of the following:

Preeclampsia

Each prior vaginal delivery

A point is subtracted from the score for:

Postdates pregnancy

Nulliparity

Premature or prolonged rupture of membranes

Interpretation

Indications for Cervical Ripening with Prostaglandins

1. Bishop's score <5

2. Membranes intact

3. No regular contractions

Indications for Labor Induction with Pitocin

1. Bishop's score >5

2. Rupture of membranes

Cesarean Rates

First-Time Mothers

Women with Past Vaginal Deliveries

Scores of 0 to 3

45%

7.7%

Scores of 4 to 6

10%

3.9%

Scores of 7 to 10

1.4%

0.9%

Points for each parameter are added. A score of 9 or greater is favorable for induction.

Modified from Bishop EH. Pelvic scoring for elective induction. Obstet Gynecol 1964;24:266.

Oxytocin has been available for labor induction for 40 years. Administered appropriately, it is as safe as spontaneous labor. Many protocols are available for IV oxytocin infusion for induction of labor, from low-dose constant infusion (Mercer et al., 1990) to higher-dose incremental infusion (Muller et al., 1992; Satin et al., 1994). Steady-state levels after IV infusion occur about 40 minutes after the start of infusion. Starting doses range from 0.5 to 2 mU/min with incremental increases of 1 to 2 mU/min every 30 to 60 minutes (ACOG, 1995b). The goal is to achieve two to four uterine contractions in 10 minutes. All protocols share certain precautions. Continuous fetal heart rate and uterine contraction monitoring is mandatory. Hyperstimulation can occur even after a stable infusion rate has been established. Often, contraction intensity and frequency will increase spontaneously after the pregnant woman reaches the active phase or if the amniotic membranes rupture, necessitating a decrease in the oxytocin infusion rate. Hyperstimulation associated with fetal heart rate decelerations requires discontinuing or lowering of the infusion until recovery. Prolonged use of high doses of oxytocin can cause water intoxication because of its biochemical similarity to antidiuretic hormone.

Amniotomy alone or in conjunction with oxytocin has been shown to decrease the length of labor. In some cases, amniotomy alone can stimulate normal labor, omitting the need for oxytocin. Risks of early amniotomy is cord prolapse and chorioamnionitis. In clinical practice, the decision between induction and expectant management should include favorability of the cervix, maternal parity, and complicating medical or obstetric issues, as well as patient or physician convenience and preferences.

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