Postterm Pregnancy
BS. Nguyễn Hồng Anh
Te adjectives postterm, prolonged, postdates, and postmature are used interchangeably to describe pregnancies that have exceeded a duration considered to be the upper limit o normal. We eschew use o the term postdates because the real issue in many postterm pregnancies is uncertainty in the estimated date o delivery (EDD). Postmature is reserved or the uncommon clinical etal syndrome in which the newborn has recognizable eatures indicating a pathologically prolonged pregnancy.
Tus, postterm or prolonged pregnancy is our preerred term or an extended pregnancy. Te denition o postterm pregnancy is one that exceeds 420/7 weeks or is ≥294 days rom the rst day o the last menstrual period (LMP). Importantly, this is 42 “completed weeks.” Pregnancies between 411/7 and 416/7 weeks, although in the 42nd week, do not complete 42 weeks until the seventh day has elapsed. Tese are considered late term (American College o Obstetricians and Gynecologists, 2019e, 2020b).
ESTIMATED GESTATIONAL AGE
Te current denition o postterm pregnancy assumes that ovulation occurs 2 weeks ater the LMP. Tus, some pregnancies may not actually be postterm because o error in menstrual date recall or delayed ovulation. Even with exactly recalled menstrual dates, there still is imprecision, and rst-trimester sonography is the most accurate method to establish or conrm gestational age (American College o Obstetricians and Gynecologists, 2019b, 2020b). Several studies support this (Bennett, 2004; Joseph, 2007). I available, gestational ages calculated rom the LMP and rom the rst accurate ultrasound are reconciled as shown in able 14-1 (p. 248), and the EDD is recorded (American College o Obstetricians and Gynecologists, 2019c).
INCIDENCE
O the 3.75 million neonates born in the United States during 2019, 0.3 percent were delivered at ≥42 weeks (Martin, 2021). Tis rate has declined because o improved pregnancy dating accuracy and earlier intervention. o identiy predisposing actors or postterm pregnancy, one analysis o the Danish Birth Cohort ound only prepregnancy body mass index ≥25 and nulliparity to be signicantly associated (Olesen, 2006). Others reported similar associations (Arrowsmith, 2011; Mission, 2015). Nulliparas with a long midpregnancy cervical length (third or ourth quartile) are twice as likely to deliver ater 42 weeks (van der Ven, 2016). Last, the risk or adverse pregnancy outcomes in postterm pregnancies increases with advancing maternal age (Kortekaas, 2020).
Te tendency or some mothers to have repeated postterm births suggests that some prolonged pregnancies are biologically determined. In one study, i a mother and daughter had a prolonged pregnancy, the risk or the daughter to have a subsequent postterm pregnancy was signicantly increased (Oberg, 2013). Maternal, not paternal, genes inuence prolonged pregnancy (Laursen, 2004). For example, genes rom one locus o chromosome 2q13 are associated with gestational duration (Liu, 2019). As discussed in Chapter 5 (p. 102), rare etal-placental actors that predispose to postterm pregnancy include anencephaly, adrenal hypoplasia, and X-linked placental sulatase deciency (Ayyavoo, 2014; MacDonald, 1965).
PERINATAL MORTALITY AND MORBIDITY
Rates o stillbirth, neonatal death, and inant morbidity all rise ater the EDD. Data rom times beore widespread intervention or postterm pregnancies are most illustrative. In two large Swedish studies, the perinatal mortality rate reached a nadir at 39 to 40 weeks’ gestation and rose thereater (Fig. 46-1). Tis trend is also reported or the United States (MacDorman, 2009). Te major causes o morbidity in these studies include gestational hypertension, prolonged labor with cephalopelvic disproportion, birth injuries, and hypoxic-ischemic encephalopathy (Table 46-1). Similar outcomes were reported in 78,022 women with postterm pregnancies delivered beore routine labor induction was adopted in Denmark (Olesen, 2003). A slight increase in cerebral palsy rates and two-point lower intelligence quotient (IQ) scores have been reported in postterm births (Moster, 2010; Williams, 2020; Yang, 2010).
Conversely, data do not associate autism with postterm birth (Gardener, 2011). Alexander and colleagues (2000a) reviewed 56,317 singleton pregnancies delivered at ≥40 weeks between 1988 and 1998 at Parkland Hospital. Labor was induced in 35 percent o pregnancies completing 42 weeks. Te rate o cesarean delivery or dystocia or etal distress was signicantly greater at 42 weeks compared with earlier gestations. More newborns o postterm pregnancies were admitted to intensive care units. Notably, the incidence o neonatal seizures and deaths was doubled at 42 weeks. Smith (2001) has challenged analyses such as these because the population at risk or perinatal mortality in a given week consists o all ongoing pregnancies rather than just the births in a given week. He calculated perinatal mortality rates using only births in a given week o gestation rom 37 to 43 completed weeks compared with the cumulative probability—the perinatal index—o death when all ongoing pregnancies are included in the denominator. Using this computation, delivery at 38 weeks’ gestation had the lowest risk for perinatal death. In current practice, perinatal death is balanced against morbidity associated with immaturity. Tis shapes the rationale or delaying delivery until 39 weeks’ gestation unless a comorbidity warrants earlier intervention (American College o Obstetricians and Gynecologists, 2019d).
PATHOPHYSIOLOGY
■ Postmaturity Syndrome
Te postmature newborn is unique, and eatures include wrinkled, patchy, peeling skin; a long, thin body that suggests wasting; and advanced maturity in that the neonate is open-eyed, unusually alert, and appears old and worried (Fig. 46-2). Skin wrinkling can be prominent on the palms and soles, and nails are typically long. Most postmature neonates are not technically growth restricted because their birthweight seldom alls below the 10th percentile or gestational age (Chap. 47, p. 825).
However, severe growth restriction—which logically must have preceded completion o 42 weeks—may be present. Te incidence o postmaturity syndrome in newborns at 41, 42, or 43 weeks, respectively, has not been conclusively determined, but the syndrome complicates 10 to 20 percent o pregnancies at 42 completed weeks (American College o Obstetricians and Gynecologists, 2020b). Associated oligohydramnios substantially raises the likelihood o postmaturity. rimmer and associates (1990) reported that 88 percent o etuses were postmature i there was oligohydramnios, dened by a sonographic maximal vertical amnionic uid pocket that measured ≤1 cm at 42 weeks
■ Placental Dysfunction
Many believe that postterm pregnancy is an abnormal state. Redman and Sta (2015) suggest that limited placental capacity, characterized by dysunctional syncytiotrophoblast, explains the greater risks o the postmaturity syndrome. A murine model o stillborn etuses supports this theory (Rahman, 2017). Cliord (1954) proposed that the associated skin changes ollow loss o the protective eects o vernix caseosa. He also attributed the postmaturity syndrome to placental senescence, although he did not nd placental degeneration histologically. Still, the concept that postmaturity stems rom placental insu- ciency has persisted despite an absence o morphological or signicant quantitative ndings (Larsen, 1995; Redman, 2015; Rushton, 1991). However, the rate o placental apoptosis— programmed cell death—is signicantly greater at 41 to 42 completed weeks compared with that at 36 to 39 weeks (Smith, 1999). Several proapoptotic genes such as kisspeptin are upregulated in postterm placental explants compared with the same genes in term ones (orricelli, 2012). Te clinical signicance o such apoptosis is unclear. Jazayeri and coworkers (1998) investigated cord blood erythropoietin levels in 124 appropriately grown newborns delivered rom 37 to 43 weeks. Te only known stimulator o erythropoietin is decreased partial oxygen pressure. Tus, they sought to assess whether etal oxygenation was compromised due to placental aging in postterm pregnancies. All women had an uncomplicated labor and delivery. Cord blood erythropoietin levels were signicantly elevated in pregnancies reaching 41 weeks or more. Although Apgar scores and acid-base studies were normal, these researchers concluded that etal oxygenation was decreased in some postterm gestations.
However, the postterm etus typically continues to gain weight and thus be unusually large at birth. Tis suggests that placental unction is not severely compromised. Indeed, continued etal growth is the norm—albeit at a slower rate beginning at 37 completed weeks (Fig. 46-3). Nahum and associates (1995) conrmed that etal growth continues until at least 42 weeks. However, umbilical blood ow does not increase concomitantly (Link, 2007).
■ Fetal Distress and Oligohydramnios
Te principal reasons or increased risks to postterm etuses were described by Leveno and colleagues (1984). Both antepartum etal jeopardy and intrapartum nonreassuring etal status were ound to be the consequence o cord compression associated with oligohydramnios. In their analysis o 727 postterm pregnancies, intrapartum nonreassuring etal status detected with electronic monitoring was not associated with late decelerations characteristic o uteroplacental insufciency. Instead, one or more prolonged decelerations such as shown in Figure 46-4 preceded three ourths o emergency cesarean deliveries or nonreassuring etal heart rate tracings. In all but two cases, there were also variable decelerations. Another common etal heart rate pattern, although not ominous by itsel, was the saltatory baseline. As described in Chapter 24 (p. 453), these ndings are consistent with cord occlusion as the proximate cause o the nonreassuring tracings.
Other correlates included oligohydramnios and viscous meconium. Schaer and coworkers (2005) implicated a nuchal cord in abnormal intrapartum etal heart rate patterns, meconium, and compromised newborn condition in prolonged pregnancies. Normally, the volume o amnionic uid continues to decline ater 38 weeks, and logically oligohydramnios may become problematic. Moreover, meconium release into an already reduced amnionic uid volume results in thick, viscous uid that may cause meconium aspiration syndrome (Chap. 33, p. 600). Te risk or this increases to 5 percent at 42 weeks (Ward, 2020).
In the late 1990s, the clinical signi- cance o etal-growth restriction in the otherwise uncomplicated pregnancy became more ully appreciated. Data rom the National Swedish Medical Birth Registry showed that stillbirths were more common among growthrestricted newborns who were delivered ater 42 weeks (Clausson, 1999; Divon, 1998). Indeed, a third o postterm stillborn neonates were growth restricted. During this time in Sweden, labor induction and antenatal etal testing usually commenced at 42 weeks. In a study rom Parkland Hospital, Alexander and colleagues (2000d) analyzed outcomes or 355 neonates rom gestations ≥42 weeks and with birthweights <3rd percentile. Tese outcomes were compared with those o 14,520 similarly aged newborns above the 3rd percentile. Morbidity and mortality rates were signicantly higher in the growth-restricted neonates. Moreover, a ourth o all stillbirths associated with prolonged pregnancy were in this comparatively small number o growth-restricted etuses.
COMPLICATIONS
■ Oligohydramnios
Recognized postterm complications include oligohydramnios and etal macrosomia. In 38 postterm pregnancies, rimmer and colleagues (1990) sonographically measured hourly etal urine production using sequential bladder volume measurements. Diminished urine production was ound to be associated with oligohydramnios. Using Doppler waveorms, Oz and associates (2002) concluded that etal renal blood ow is reduced in postterm pregnancies complicated by oligohydramnios. Te lack o increasing umbilical blood ow postterm may be a possible cause (Link, 2007).
Diminished amnionic uid volume at any gestational age signies increased etal risk. Unortunately, an exact method to dene this decreased volume is lacking. Currently, sonographic measurement o the deepest vertical pocket or calculation o the amnionic uid index (AFI) are options (Chap. 14, p. 257). One study o postterm pregnancies attempted to determine which amnionic uid volume criteria best predicted normal and abnormal outcomes (Fischer, 1993). Te smaller the amnionic uid pocket, the greater the likelihood o clinically signicant oligohydramnios. However, normal amnionic uid volume did not preclude abnormal outcomes. One randomized trial assigned 500 women with postterm pregnancies to assessment using either AFI or the deepest vertical pocket (Alrevic, 1997). Te authors concluded that the AFI overestimated the number o abnormal outcomes in postterm pregnancies.
Regardless o the criteria used to diagnose oligohydramnios in postterm pregnancies, most investigators have ound a higher incidence o some measure o “etal compromise” during labor. Tus, oligohydramnios by most denitions is a clinically meaningul nding. Conversely, reassurance o continued etal well-being in the presence o “normal” amnionic uid volume is tenuous. Tis may be related to how quickly pathological oligohydramnios can develop.
■ Macrosomia
Te velocity o etal weight gain peaks at approximately 37 weeks (see Fig. 46-3). Although growth velocity slows at that time, most etuses continue to gain weight. According to Duryea and associates (2014), the 95th percentile at 42 weeks is 4475 g. Even so, in one study, brachial plexus injury was not related to postterm gestation (Walsh, 2011). Intuitively, it seems that both maternal and etal morbidity associated with macrosomia would be mitigated with timely induction to preempt urther growth. Tis, however, does not appear to be the case. Te American College o Obstetricians and Gynecologists (2020a) concludes that current evidence does not support such a practice in women at term with suspected etal macrosomia. Moreover, the College notes that in the absence o diabetes, vaginal delivery is not contraindicated or women with an estimated etal weight up to 5000 g (Chap. 27, p. 501). Obvious problems with all such recommendations are substantive variations in etal weight estimation.
ANTEPARTUM MANAGEMENT
Ater completing 42 weeks, labor induction is recommended to help avoid the just-described morbidity and mortality. For late-term gestations, some intervention is indicated, but the method and timing are not unanimous. Te decision ocuses on whether labor induction is warranted or i expectant management with etal surveillance is best. In a survey done more than 15 years ago, 73 percent o members o the American College o Obstetricians and Gynecologists reported routinely induced women at 41 weeks (Cleary-Goldman, 2006). Most o the remainder perormed twice-weekly etal antepartum testing until 42 completed weeks.
■ Induction Factors
A cervix that is sot, dilated, and eaced improves labor induction success. However, investigators have used diering criteria in studies o prolonged pregnancy. Harris and coworkers (1983) dened an unavorable cervix by a Bishop score <7 and reported this in 92 percent o women at 42 weeks (Chap. 26, p. 488). Others ound that 40 percent o women with a 41-week gestation had an “undilated cervix” (Hannah, 1992). In a study o 800 women undergoing induction or postterm pregnancy at Parkland Hospital, women without cervical dilation had a twoold higher cesarean delivery rate or “dystocia” (Alexander, 2000b). Instead, a cervical length ≤3 cm measured with transvaginal sonography predicted successul induction (Yang, 2004). In a similar study, cervical length ≤25 mm positively predicted spontaneous labor or successul induction (Vankayalapati, 2008).
Several investigators have evaluated prostaglandin E2 (PGE2) and E1 (PGE1) or induction in women with an unavorable cervix and postterm pregnancy. A study by the Maternal–Fetal Medicine Units Network (1994) ound that PGE2 gel was not more eective than placebo. Alexander and associates (2000c) treated 393 women with a postterm pregnancy with PGE2, regardless o cervical “avorability,” and reported that almost hal o the 84 women with cervical dilation o 2 to 4 cm entered labor with PGE2 use alone. Prostaglandins and other agents used or cervical ripening are discussed in Chapter 26 (p. 488). Sweeping or stripping of the membranes to induce labor and thereby prevent postterm pregnancy has been evaluated.
A metaanalysis o 44 studies ound that membrane stripping slightly increased spontaneous labor and lowered induction rates (Finucane, 2020). However, this practice did not lower the cesarean delivery rate. Other trials have ound that sweeping membranes did not reduce the need to induce labor (Hill, 2008; Kashanian 2006; Wong, 2002). Drawbacks o membrane stripping included pain, vaginal bleeding, and irregular contractions without labor.
Te station o the etal head within the pelvis is another predictor o successul postterm pregnancy induction. Shin and colleagues (2004) studied 484 nulliparas who underwent induction ater 41 weeks’ gestation. Te cesarean delivery rate was directly related to station. Te rate was 6 percent i the vertex beore 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
Te less than optimal benets rom induction with an unavorable cervix lead some clinicians to preer etal antepartum testing beginning at 41 completed weeks. In a Canadian study, 3407 women were randomly assigned at ≥41 weeks to induction or to etal testing (Hannah, 1992). Labor induction resulted in a small but signicant reduction in the cesarean delivery rate compared with etal testing—21 versus 24 percent, respectively. Tere were only two stillbirths in the etal testing group.
Te Maternal–Fetal Medicine Network then perormed a randomized trial o induction versus etal testing beginning at 41 weeks’ gestation (Gardner, 1996). Fetal surveillance included nonstress testing and sonographic estimation o amnionic uid volume perormed twice weekly in 175 women. Perinatal outcomes were compared with those o 265 women also at 41 weeks but randomly assigned to induction with or without cervical ripening. Tere were no perinatal deaths, and the cesarean delivery rate did not dier between groups. Te results o this study support the validity o either management scheme.
In another study o women reaching 41 completed weeks’ gestation, those who entered spontaneous labor prior to 420/7 weeks had a lower cesarean delivery rate than those who did not spontaneously labor but who underwent induction ater 420/7 weeks (Alexander, 2001). Cesarean delivery rates were signi- cantly increased—19 versus 14 percent—in the induced group because o ailure to progress.
A study rom Denmark is also instructive (Zizzo, 2017). In 2011, the Danish national guidelines were changed rom labor induction at 420/7 weeks with no etal surveillance to labor induction at 412/7 to 416/7 weeks with etal surveillance beginning at 410/7 weeks. Tey compared two 3-year epochs—one beore and one ater 2011. Te rate o pregnancies that progressed past 420/7 weeks decreased rom 2.85 to 0.62 percent.
Concurrently, as expected, the induction rate rose signicantly, and this was accompanied by a drop in the perinatal mortality rate—22 to 13 per 1000 births. Te cesarean delivery rate was not changed. A similar beore-and-ater observational study reported that induction at ≥42 weeks’ gestation was associated with a signicantly lower cesarean delivery rate—15 versus 19.4 percent (Bleicher, 2017). o urther address the question, the SWEdish Post-term Induction Study—SWEPIS—was a randomized trial that included 2760 low-risk pregnancies at 41 weeks’ gestation. Te trial compared outcomes with induction o labor at 41 weeks against expectant management and induction at 42 weeks. As shown in Table 46-2, maternal and perinatal outcomes were not dierent with the exception o perinatal mortality and macrosomia (Wennerholm, 2019). Te study was stopped early because o a signicantly higher perinatal mortality rate in the expectantly management group. Te investigators concluded that these results should be interpreted with caution because the primary composite outcomes were not signicantly dierent.
■ Management Strategies
At this time, evidence is insufcient to mandate a management strategy between 40 and 42 completed weeks. Tus, induction o labor or initiation o etal surveillance at 41 weeks’ gestation is a reasonable option. Te American College o Obstetricians and Gynecologists and the Society or Maternal-Fetal Medicine (2021) suggest that etal surveillance should be done once or twice weekly beginning at 41 0/7 weeks. Ater completing 42 weeks, labor induction is recommended. A management algorithm by the American College o Obstetricians and Gynecologists (2020b) is summarized in Figure 46-5. In light o the recent studies that show an increased stillbirth rate associated with expectant management at 41 weeks, it is likely that routine induction will eventually become preerable in most circumstances.
When gestational age is uncertain, the American College o Obstetricians and Gynecologists (2019b) recommends delivery at 41 weeks’ gestation using the best clinical estimate o gestational age. Te College also recommends against amniocentesis or etal lung maturity.
INTRAPARTUM MANAGEMENT
Labor is a particularly dangerous time or the postterm etus. Tus, women whose pregnancies are known or suspected to be postterm ideally come to the hospital as soon as they suspect labor. While being evaluated or active labor, etal heart rate and uterine contractions are monitored electronically or variations consistent with etal compromise (Chap. 24, p. 457). During labor, the decision to perorm amniotomy is problematic. Further reduction in uid volume ollowing amniotomy can enhance the possibility o cord compression. Conversely, ater membrane rupture, a scalp electrode and an intrauterine pressure catheter can be placed. Tese usually provide more precise data concerning etal heart rate and uterine contractions.
Amniotomy also aids identication o thick meconium. Tick meconium in the amnionic uid is particularly worrisome. Te viscosity probably signies the lack o liquid and thus oligohydramnios. Aspiration o thick meconium may cause severe pulmonary dysunction and neonatal death (Chap. 33, p. 600). According to the American College o Obstetricians and Gynecologists (2019a), amnioinusion does not prevent meconium aspiration syndrome. However, it remains a reasonable treatment approach or repetitive variable decelerations (Chap, 24, p. 459). Suctioning the pharynx once the head is delivered is not recommended (American College o Obstetricians and Gynecologists, 2019a; Wycko, 2020). I the depressed newborn has meconiumstained uid, intubation is perormed.
Te likelihood o a successul vaginal delivery is reduced appreciably or the nullipara who is in early labor with thick, meconium-stained amnionic uid. Tus, i a woman is remote rom delivery, strong consideration should be given to prompt cesarean delivery, especially when cephalopelvic disproportion is suspected or either hypotonic or hypertonic dysunctional labor is evident. Some practitioners choose to avoid oxytocin use in these cases.
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