Although some intervention is indicated for prolonged pregnancies, the method and timing of this are not unanimous. The decision focuses on whether labor induction is warranted or if expectant management with fetal surveillance is best. In a survey done more than 10 years ago, Cleary-Goldman and associates (2006) reported that 73 percent of members of the American College of Obstetricians and Gynecologists routinely induced women at 41 weeks. Most of the remainder performed twice weekly fetal testing until 42 weeks.
Although all obstetricians know what an “unfavorable cervix” is, the term unfortunately defies precise objective definition. Thus, investigators have used differing criteria for studies of prolonged pregnancies. Harris and coworkers (1983) defined an unfavorable cervix by a Bishop score <7 and reported this in 92 percent of women at 42 weeks (Chap. 26, Preinduction Cervical Ripening). Hannah and colleagues (1992) found that 40 percent of 3407 women with a 41-week pregnancy had an “undilated cervix.” In a study of 800 women undergoing induction for postterm pregnancy at Parkland Hospital, Alexander and associates (2000b) reported that women in whom there was no cervical dilation had a twofold higher cesarean delivery rate for “dystocia.” Yang and coworkers (2004) found that cervical length ≤3 cm measured with transvaginal sonography was predictive of successful induction. In a similar study, Vankayalapati and associates (2008) found that cervical length ≤25 mm was predictive of spontaneous labor or successful induction.
Several investigators have evaluated prostaglandin E2 (PGE2) and E1 (PGE1) for induction in women with an unfavorable cervix and prolonged pregnancies. A study by the Maternal–Fetal Medicine Units Network (1994) found that PGE2 gel was not more effective than placebo. Alexander and associates (2000c) treated 393 women with a postterm pregnancy with PGE2, regardless of cervical “favorability,” and reported that almost half of the 84 women with cervical dilatation of 2 to 4 cm entered labor with PGE2 use alone. In another study, mifepristone was reported to increase uterine activity without uterotonic agents in women beyond 41 weeks (Fasset, 2008). Prostaglandins and other agents used for cervical ripening are discussed in Chapter 26 (Pharmacological Techniques).
Sweeping or stripping of the membranes to induce labor and thereby prevent postterm pregnancy was studied in 15 randomized trials during the 1990s. Boulvain and coworkers (2005) performed a metaanalysis of these and found that membrane stripping at 38 to 40 weeks lowered the frequency of postterm pregnancy. Although maternal and neonatal infection rates were not increased, this practice did not modify the cesarean delivery rate. Since then, randomized trials by Wong (2002), Kashanian (2006), Hill (2008), and their coworkers found that sweeping membranes did not reduce the need to induce labor. Drawbacks of membrane stripping included pain, vaginal bleeding, and irregular contractions without labor.
The station of the fetal head within the pelvis is another predictor of successful postterm pregnancy induction. Shin and colleagues (2004) studied 484 nulliparas who underwent induction after 41 weeks. The cesarean delivery rate was directly related to station. The rate was 6 percent if the vertex before induction was at –1 station; 20 percent at –2 station; 43 percent at –3 station; and 77 percent at –4 station.
Induction versus Fetal Testing
Because of the marginal benefits from induction with an unfavorable cervix, as just discussed, some clinicians prefer instead to implement a strategy of fetal testing beginning at 41 completed weeks. For example, in a Canadian study, 3407 women were randomly assigned at 41 or more weeks to induction or to fetal testing (Hannah, 1992). In the surveillance group, evaluation included: (1) counting fetal movements during a 2-hour period each day, (2) nonstress testing three times weekly, and (3) amnionic fluid volume assessment two to three times weekly, with pockets <3 cm considered abnormal. Labor induction resulted in a small but significant reduction in the cesarean delivery rate compared with fetal testing—21 versus 24 percent, respectively. This difference was due to fewer procedures for fetal distress. There were only two stillbirths in the fetal testing group.
The Maternal–Fetal Medicine Network performed a randomized trial of induction versus fetal testing beginning at 41 weeks (Gardner, 1996). Fetal surveillance included nonstress testing and sonographic estimation of amnionic fluid volume performed twice weekly in 175 women. Perinatal outcomes were compared with those of 265 women also at 41 weeks randomly assigned to induction with or without cervical ripening. There were no perinatal deaths, and the cesarean delivery rate was not different between management groups. The results of this study could be used to support the validity of either management scheme.
In an analysis of 22 trials, Gulmezoglu and colleagues (2012) found that induction after 41 weeks rather than surveillance was associated with significantly fewer perinatal deaths and meconium aspiration syndrome cases and a lower cesarean delivery rate. In a review of two metaanalyses and a randomized study, similar conclusions were reached (Mozurkewich, 2009).
In most studies, labor induction at 420/7 weeks has a higher cesarean delivery rate compared with spontaneous labor. From Parkland Hospital, Alexander and coworkers (2001) evaluated pregnancy outcomes in 638 such women in whom labor was induced and compared them with outcomes of 687 women with postterm pregnancies who had spontaneous labor. Cesarean delivery rates were significantly increased—19 versus 14 percent—in the induced group because of failure to progress. When these investigators corrected for risk factors, however, they concluded that intrinsic maternal factors, rather than the induction itself, led to the higher rate. These factors included nulliparity, an unfavorable cervix, and epidural analgesia.
A large study from Denmark by Zizzo and associates (2017) is also instructive. In 2011, the Danish national guidelines were changed from labor induction at 420/7 weeks with no fetal surveillance to labor induction at 412/7 to 416/7 weeks with fetal surveillance beginning at 410/7 weeks. They compared two 3-year epochs—one before and one after 2011—and the results are shown in Table 43-2. The rate of pregnancies that progressed past 420/7 weeks decreased from 2.85 to 0.62 percent. Concurrently, as expected, the induction rate rose significantly, and this was accompanied by a drop in the perinatal mortality rate—22 to 13 per 1000 births. The cesarean delivery rate was not changed. A similar before-and-after observational study reported that induction at ≥42 weeks was associated with a significantly lower cesarean delivery rate—15 versus 19.4 percent (Bleicher, 2017).
TABLE 43-2National Death Cohort Study of 102,167 Pregnancies that Reached 410/7 Weeks’ Gestation ||Download (.pdf) TABLE 43-2 National Death Cohort Study of 102,167 Pregnancies that Reached 410/7 Weeks’ Gestation
|Factor ||2008–2010a ||2012–2014 ||p value |
|EGA >420/7 ||2.85% ||0.62% || |
|Stillbirths ||9/1000 ||5/1000 ||0.018 |
|Neonatal deaths ||13/1000 ||8/1000 ||.033 |
|Cesarean delivery ||15% ||15% ||NS |
|Vacuum delivery ||11.3% ||10.2% ||<0.001 |
|Induction ||28% ||43% ||<0.001 |
From the foregoing, evidence to substantiate intervention—whether induction or fetal testing—commencing at 41 versus 42 weeks is limited. Most evidence used to justify intervention at 41 weeks is from the randomized Canadian and American investigations cited earlier. No randomized studies have specifically assessed intervention at 41 weeks versus an identical intervention used at 42 weeks. A large Swedish multicenter randomized trial of more than 10,000 women at 410/7 weeks has been designed to address the question (Elden, 2016).
The American College of Obstetricians and Gynecologists (2016a) defines postterm pregnancies as having completed 42 weeks, namely, beyond 420/7 weeks. There is insufficient evidence to mandate a management strategy between 40 and 42 completed weeks. Thus, although not considered mandatory, initiation of fetal surveillance at 41 weeks is a reasonable option. After completing 42 weeks, recommendations are for labor induction as summarized in Figure 43-6.
When gestational age is uncertain, the American College of Obstetricians and Gynecologists (2017b) recommends delivery at 41 weeks’ gestation using the best clinical estimate of gestational age. The College also recommends against amniocentesis for fetal lung maturity.
At Parkland Hospital, based on results from the trials just discussed, we consider 41-week pregnancies without other complications to be normal. Thus, no interventions are practiced solely based on fetal age until 42 completed weeks. With complications such as hypertension, decreased fetal movement, or oligohydramnios, labor induction is carried out. It is our view that large, randomized trials should be performed before otherwise uncomplicated 41-week gestations are routinely considered pathologically prolonged. In women in whom a certain gestational age is known, labor is induced at the completion of 42 weeks. Almost 90 percent of such women are induced successfully or enter labor within 2 days of induction. For those who do not deliver with the first induction, a second induction is performed within 3 days. Almost all women are delivered using this management plan, but in the unusual few who are not delivered, management decisions involve a third—or even more—induction versus cesarean delivery. Women classified as having uncertain postterm pregnancies are managed with weekly nonstress fetal testing and assessment of amnionic fluid volume. Women with an AFI ≤5 cm or with reports of diminished fetal movement undergo labor induction.