Postterm Pregnancy: Introductionfamilymed.uthscsa.edu/residency08/resources/Williams OB Text...The definition of postterm pregnancy as one that persists for 42 weeks or more from the

Print: Chapter 37. Postterm Pregnancy

Print Close Window

Note: Large images and tables on this page may necessitate printing in landscape mode.

Copyright © The McGraw-Hill Companies. All rights reserved. Williams Obstetrics > Section VII. Obstetrical Complications > Chapter 37. Postterm Pregnancy >

Postterm Pregnancy: Introduction

The terms postterm, prolonged, postdates, and postmature are often loosely used interchangeably to signify pregnancies that have

exceeded a duration considered to be the upper limit of normal. Postmature should be used to describe the infant with recognizable

clinical features indicating a pathologically prolonged pregnancy. Postdates probably should be abandoned, because the real issue in

many postterm pregnancies is "post-what dates?" Therefore, postterm or prolonged pregnancy is the preferred expression for an

extended pregnancy, and "postmature" is reserved for a specific clinical fetal syndrome. Because few infants from prolonged pregnancies

have stigmata of the postmaturity syndrome, use of this term can falsely imply a pathologically prolonged pregnancy.

The standard internationally recommended definition of prolonged pregnancy, endorsed by the American College of Obstetricians and

Gynecologists (1997), is 42 completed weeks (294 days) or more from the first day of the last menstrual period. It is important to

emphasize the phrase "42 completed weeks." Pregnancies between 41 weeks 1 day and 41 weeks 6 days, although in the 42nd week, do

not complete 42 weeks until the seventh day has elapsed. Thus, technically speaking, prolonged pregnancy could begin either on day 294

or on day 295 following the onset of the last menses. Which is it? Day 294 or 295? We cannot resolve this question, and emphasize this

dilemma only to ensure that litigators and others understand that some imprecision is inevitable when attempting to define prolonged

pregnancy. Amersi and Grimes (1998) have cautioned against use of ordinal numbers such as "42nd week" because of their imprecision.

For example, "42nd week" refers to 41 weeks and 1 through 6 days, whereas the cardinal number "42 weeks" refers to precisely 42

completed weeks.

http://www.accessmedicine.com/popup.aspx?aID=732449&print=yes_chapter (1 of 28)9/10/2008 11:12:53 AM


Estimated Gestational Age Using Menstrual Dates

The definition of postterm pregnancy as one that persists for 42 weeks or more from the onset of a menstrual period assumes that the

last menses was followed by ovulation 2 weeks later. This said, some pregnancies may not actually be postterm, but rather are the result

of an error in estimation of gestational age because of faulty recall of the dates of menstruation or delayed ovulation. Thus, there are two

categories of pregnancies that reach 42 completed weeks:

1. Those truly 40 weeks past conception.

2. Those of less advanced gestation due to inaccurate estimate of gestational age.

Munster and associates (1992) described a high incidence of large variations in menstrual cycles in normal women. Boyce and associates

(1976) studied 317 French women with conceptional basal body temperature profiles and found that 70 percent who completed 42

postmenstrual weeks had less advanced gestations based on ovulation dates. Blondel and colleagues (2002) analyzed postterm

pregnancy rates according to six algorithms for gestational age estimates based on either the last menstrual period, ultrasound at 16 to

18 weeks, or both. This Canadian study included 44,623 women giving birth between 1978 and 1996 at the Royal Victoria Hospital in

Montreal. The proportion of births at 42 weeks or longer was 6.4 percent when based on the last menstrual period alone and 1.9 percent

when based on ultrasound alone. This raises the possibility that the menstrual dates are frequently inaccurate in predicting postterm

pregnancy. The recent study of Bennett and associates (2004) confirmed this. Because a few women ovulate earlier than expected, it is

possible that 40 completed postconceptional weeks could be achieved after 41 weeks of amenorrhea.

Therefore, most pregnancies that are reliably 42 completed weeks beyond the last menses probably are not biologically prolonged.

Conversely, a few that are not yet 42 weeks might be postterm. These variations in menstrual cycle likely explain, at least partially, why

a relatively small proportion of fetuses delivered postterm have evidence of postmaturity. Because there is no method to identify

pregnancies that are truly prolonged, all pregnancies judged to be 42 completed weeks should be managed as if abnormally prolonged.



Incidence

As shown in Figure 37–1, approximately 7 percent of 4 million infants born in the United States during 2001 were estimated to have been

delivered at 42 weeks or more. In comparison, 12 percent of live births were preterm, defined as 36 weeks or less.

Figure 37–1.

Gestational age at delivery of 4 million live births in the United States during 2001. (Adapted from Martin and colleagues, 2002.)

Contradictory results have been found concerning the significance of a variety of maternal demographic factors, such as parity, prior

postterm birth, socioeconomic class, and age. One interesting feature—the tendency for some mothers to have repeated postterm births—

suggests that some prolonged pregnancies are biologically determined. In an analysis of 27,677 births to Norwegian women, the

incidence of a subsequent postterm birth increased from 10 to 27 percent if the first birth was postterm. This was increased to 39 percent

if there had been two previous, successive postterm deliveries (Bakketeig and Bergsjø, 1991). Mogren and colleagues (1999) reported



that prolonged pregnancy also recurred across generations in Swedish women. When mother and daughter had had a prolonged

pregnancy, the risk for a daughter's subsequent postterm pregnancy was increased two- to threefold. In another Swedish study, Laursen

and associates (2004) found that maternal, but not paternal, genes influenced prolonged pregnancy. Fetal–placental factors that have

been reported as predisposing to postterm pregnancy include anencephaly, adrenal hypoplasia, and X-linked placental sulfatase

deficiency (MacDonald and Siiteri, 1965; Naeye, 1978; Rabe and colleagues, 1983). These cause a lack of the usually high estrogen levels

of normal pregnancy (see Chap. 3, Placental Estrogen Production). Finally, reduced cervical nitric oxide release may be a factor (Vaisanen-

Tommiska and co-workers, 2004).

Perinatal Mortality

The historical basis for the concept of an upper limit of human pregnancy duration was the observation that perinatal mortality increased

after the expected due date was passed. This is best seen when perinatal mortality is analyzed from times before widespread use of

interventions for pregnancies exceeding 42 weeks. In two large Swedish studies shown in Figure 37–2, after reaching a nadir at 39 to 40

weeks, perinatal mortality increased as pregnancy exceeded 41 weeks. Lucas and co-workers (1965) compared perinatal outcomes in

6624 postterm pregnancies with those of almost 60,000 singleton pregnancies delivered between 38 and 41 weeks. All components of

perinatal mortality—antepartum, intrapartum, and neonatal deaths—were increased at 42 weeks and beyond. The most significant

increases occurred intrapartum. The major causes of death included pregnancy hypertension, prolonged labor with cephalopelvic

disproportion, "unexplained anoxia," and malformations. Similar outcomes were reported by Olesen and colleagues (2003) in their

analysis of 78,022 women with postterm pregnancies delivered before routine labor induction was adopted in Denmark.

Figure 37–2.



Perinatal mortality in late pregnancy according to gestational age in Sweden of all births during 1943–1952 compared with those during 1977–1978.

The logarithmic scale is used for convenience in depiction. (Adapted from Bakketeig and Bergsjø, 1991, and Lindell, 1956.)

Alexander and colleagues (2000a) reviewed 56,317 consecutive singleton pregnancies delivered at 40 or more weeks between 1988 and

1998 at Parkland Hospital. As shown in Table 37–1, labor was induced in 35 percent of pregnancies reaching 42 weeks. The rate of

cesarean delivery for dystocia and fetal distress was significantly increased at 42 weeks compared with that of earlier deliveries. More

infants were admitted to intensive care in postterm pregnancies. The incidence of neonatal seizures and deaths doubled at 42 weeks.

Caughey and Musci (2004) reported similar outcomes in 45,673 pregnancies.

Table 37–1. Pregnancy Outcomes in 56,317 Consecutive Singleton Pregnancies Delivered at or Beyond

40 Weeks from 1988 through 1998 at Parkland Hospital



Weeks' Gestation

Outcome 40 (n = 29,136) 41 (n = 16,386) 42 (n = 10,795) P Value

Maternal outcomes (percent)

Labor induction 2 7 35 <.001

Cesarean delivery

Dystocia 7 6 9 <.001

Fetal distress 2 3 4 <.001

Perinatal outcomes (per 1000)

Neonatal intensive care 4 5 6 <.001

Neonatal seizures 1 1 2 .12

Stillbirth 2 1 2 .84

Neonatal death 0.2 0.2 0.6 .17

Adapted from Alexander and colleagues (2000a).

Smith (2001) has challenged analyses such as these because the population at risk for perinatal mortality in a given week consists of all

ongoing pregnancies rather than just the births in a given week. Figure 37–3 shows perinatal mortality rates calculated using only births

in a given week of gestation from 37 to 43 completed weeks compared with the cumulative probability (perinatal index) of death when all

ongoing pregnancies are included in the denominator. Smith found that delivery at 38 weeks was associated with the lowest risk of

perinatal death.

Figure 37–3.



Perinatal risk index (o) and perinatal mortality rate ( ) related to births between 37 and 43 weeks of gestation in Scotland, 1985 through 1996,

expressed as deaths per 1000 births. The perinatal risk index is the cumulative probability of perinatal death multiplied by 1000. The perinatal

mortality rate is the number of perinatal deaths with delivery in a given gestational week divided by the total number of births in that week

multiplied by 1000. (Modified from Smith, 2001, with permission.)

Pathophysiology

Clifford (1954) described a recognizable clinical syndrome that did much to dispel the prevailing obstetrical opinion that prolonged human

pregnancy did not exist (Calkins, 1948). Infants, either live or stillborn, demonstrating these clinical characteristics, are now diagnosed to

be pathologically postmature. Many of the postmature infants described by Clifford died, and many were seriously ill due to birth asphyxia

and meconium aspiration. Several survivors were brain damaged. Interestingly, Ballantyne (1902) reported this same postmature infant

more than 50 years before Clifford did.

Postmaturity Syndrome

The postmature infant presents a unique and characteristic appearance (Fig. 37–4). Features include wrinkled, patchy, peeling skin; a

long, thin body suggesting wasting; and advanced maturity because the infant is open-eyed, unusually alert, and appears old and worried-



looking. Skin wrinkling can be particularly prominent on the palms and soles. The nails are typically quite long. Most such postmature

infants are not growth restricted because their birthweight seldom falls below the 10th percentile for gestational age. Severe growth

restriction, however, which logically must have preceded completion of 42 weeks, may occur.

Figure 37–4.



Postmature infant delivered at 43 weeks of gestation. Thick, viscous meconium coated the desquamating skin. Note the long, thin appearance and

wrinkling of the palms of the hands.

The incidence of postmaturity syndrome in infants at 41, 42, or 43 weeks, respectively, has not been conclusively determined. In one of

the rare contemporary reports that chronicle postmaturity, Shime and colleagues (1984) found this syndrome in about 10 percent of

pregnancies between 41 and 43 weeks. The incidence increased to 33 percent at 44 weeks. Associated oligohydramnios substantially

increases the likelihood of postmaturity. Trimmer and colleagues (1990) diagnosed oligohydramnios when the ultrasonic maximum

vertical amnionic fluid pocket measured 1 cm or less at 42 weeks and 88 percent of the infants were postmature.

Placental Dysfunction

Clifford (1954) proposed that the skin changes of postmaturity were due to loss of the protective effects of vernix caseosa. He also

attributed the postmaturity syndrome to placental senescence, although he did not find placental degeneration histologically. Still, the

concept that postmaturity is due to placental insufficiency has persisted despite an absence of morphological or significant quantitative

findings (Larsen and co-workers, 1995; Rushton, 1991). Of interest, Smith and Baker (1999) reported that placental apoptosis—

programmed cell death—was significantly increased at 41 to 42 completed weeks compared with that at 36 to 39 weeks. The clinical

significance of such apoptosis is unclear at this time.

Jazayeri and co-workers (1998) investigated cord erythropoietin levels in 124 appropriately grown newborns delivered from 37 to 43

weeks. They sought to assess whether fetal oxygenation was compromised due to placental aging in postterm pregnancies. Decreased

partial oxygen pressure is the only known stimulator of erythropoietin. Each woman studied had an uncomplicated labor and delivery.

Cord erythropoietin levels were significantly increased in pregnancies reaching 41 weeks or more. Although Apgar scores and umbilical

cord blood gases were normal in these infants, the investigators concluded that there was decreased fetal oxygenation in some postterm

gestations.

The postterm fetus may continue to gain weight and thus be an unusually large infant at birth. This at least suggests that placental

function is not compromised. Indeed, continued fetal growth, although at a slower rate, is characteristic between 38 and 42 weeks (Fig.

37–5). Nahum and colleagues (1995) confirmed that fetal growth continues until at least 42 weeks.



Figure 37–5.

Mean daily fetal growth during previous week of gestation. (From Jazayeri and co-workers, 1998, with permission.)

Fetal Distress and Oligohydramnios

The principal reasons for increased risks to postterm fetuses were described by Leveno and associates (1984). They reported that both

antepartum fetal jeopardy and intrapartum fetal distress were the consequence of cord compression associated with oligohydramnios. In

their analysis of 727 postterm pregnancies, intrapartum fetal distress detected with electronic monitoring was not associated with late

decelerations characteristic of uteroplacental insufficiency. Instead, one or more prolonged decelerations such as shown in Figure 37–6

preceded three fourths of emergency cesarean deliveries for fetal jeopardy. In all but two cases, there were also variable decelerations

(Fig. 37–7). Another common fetal heart rate pattern, although not ominous by itself, was the saltatory baseline shown in Figure 37–8.

These findings are consistent with cord occlusion as the proximate cause of fetal distress. Other correlates found were oligohydramnios

and viscous meconium.



Figure 37–6.

Prolonged fetal heart rate deceleration prior to emergency cesarean delivery in a postterm pregnancy with oligohydramnios. (From Leveno and co-

workers, 1984.)

Figure 37–7.



Severe—less than 70 bpm for 60 seconds or longer—variable decelerations in a postterm pregnancy with oligohydramnios and cesarean delivery for

fetal jeopardy. (From Leveno and co-workers, 1984.)

Figure 37–8.



Saltatory baseline fetal heart rate showing oscillations exceeding 20 bpm and associated with oligohydramnios in a postterm pregnancy. (From

Leveno and co-workers, 1984.)

Decreased amnionic fluid volume commonly develops as pregnancy advances beyond 42 weeks (Fig. 37–9). It is also likely that

meconium release into an already reduced amnionic fluid volume causes thick, viscous meconium implicated in meconium aspiration

syndrome (see Chap. 29, Meconium Aspiration Syndrome).



Figure 37–9.

Volume of amnionic fluid during the last weeks of pregnancy. (Adapted from Elliott and Inman, 1961.)

Trimmer and co-workers (1990) measured hourly fetal urine production using sequential ultrasonic bladder volume measurements in 38

pregnancies of 42 weeks or more. Diminished urine production was found to be associated with oligohydramnios. They hypothesized,

however, that decreased fetal urine flow was likely the result of preexisting oligohydramnios that limited fetal swallowing. Oz and co-

workers (2002), using Doppler waveforms, reported that fetal renal blood flow is reduced in postterm pregnancies with oligohydramnios.

Fetal Growth Restriction

Until recently, the clinical significance of fetal growth restriction in the otherwise uncomplicated pregnancy has received little attention.


Print: Chapter 37. Postterm PregnancyDivon and co-authors (1998) and Clausson and co-workers (1999) analyzed births of almost 700,000 women between 1991 and 1995

using the National Swedish Medical Birth registry. As shown in Table 37–2, stillbirths were more common among growth-restricted infants

who were delivered at 42 weeks or beyond. Indeed, one third of the postterm stillbirths were growth restricted. During the years of these

births in Sweden, labor induction and antenatal fetal testing usually commenced at 42 weeks. Alexander and colleagues (2000d) studied

infant outcomes for 355 postterm infants 42 weeks or greater whose birthweights were at or below the third percentile. They compared

their outcomes with those of 14,520 infants above the third percentile delivered at Parkland Hospital and found that morbidity and

mortality were significantly increased in the growth-restricted infants. Notably, one fourth of all stillbirths associated with prolonged

pregnancy were in this comparatively small number of growth-restricted infants.

Table 37–2. Effects of Fetal Growth Restriction on Stillbirth Rates in Pregnancies Reaching 42 Weeks or

More Compared with Those at 37 to 41 Weeks in Sweden (1991–1995)

Pregnancy Duration

Outcome 37–41 Weeks 42 Weeks

Births 469,056 40,973

Fetal growth restrictiona (%)

10,312 (2) 1558 (4)

Stillbirths (per 1000)

Appropriate growth 650 (1.4) 69 (1.8)

Fetal growth restriction 116 (11) 23 (15) aDefined as birthweight below two standard deviations from the mean birthweight for fetal gender and gestational age.

From Clausson and colleagues (1999), with permission.



Management

It is generally accepted that antepartum interventions are indicated in management of prolonged pregnancies. The type(s) of

interventions and when to use them are somewhat controversial. One major issue is whether to intervene at 41 or 42 weeks. Another is

whether labor induction is warranted compared with expectant management using antepartum fetal testing. Roussis and colleagues

(1993) surveyed members of the Society for Maternal–Fetal Medicine in 1990 and found that about two thirds of respondents induced

labor at 41 weeks if the cervix was favorable. Antepartum fetal testing was advocated beginning at 41 weeks when the cervix was

unfavorable. At 42 weeks, virtually all respondents induced labor when the cervix was favorable, and 58 percent did so even when the

cervix was unfavorable. Others (42 percent) recommended antepartum testing when the cervix was unfavorable at 42 weeks. Clearly,

cervical favorability has considerable impact on management (see Chap. 22, Preinduction Cervical Ripening).

Unfavorable Cervix

It is difficult to precisely define an "unfavorable cervix" in prolonged pregnancies, because investigators have used different criteria. For

example, Harris and colleagues (1983) reported that 92 percent of women at 42 weeks had an unfavorable cervix defined by a Bishop

score of less than 7. Hannah and colleagues (1992) found that only 40 percent of 3407 women with 41-week pregnancies had an

undilated cervix. Alexander and associates (2000b) evaluated 800 women induced for postterm pregnancy at Parkland Hospital. They

reported that women in whom there was no cervical dilatation had a twofold increased cesarean delivery rate for "dystocia."Yang and co-

workers (2004) presented preliminary findings that cervical length of 3 cm or less determined by transvaginal ultrasonography was

predictive of successful induction.

A number of investigators have evaluated prostaglandin E2 for cervical ripening in women with prolonged pregnancies and an unfavorable

cervix. National Institute of Child Health and Development Network of Maternal–Fetal Medicine Units (1994) evaluated prostaglandin E2

gel and found that it was not more effective than placebo. Alexander and associates (2000c) studied 393 women with postterm

pregnancy who were given prostaglandin E2 for cervical ripening. These women were treated even if the cervix was "favorable." The

investigators reported that almost half of the 84 women with cervical dilatation of 2 to 4 cm entered labor with prostaglandin E2 use

alone. The American College of Obstetricians and Gynecologists (1997) has concluded that prostaglandin gel can be used safely in

postterm pregnancies. Use of prostaglandins for cervical ripening is discussed in Chapter 22 (see Prostaglandin E2).

Sweeping or stripping of the membranes, to induce labor and prevent postterm pregnancy has been studied in 15 randomized trials

during the 1990s. Boulvain and co-authors (1999) performed a meta-analysis of these and found that membrane stripping at 38 to 40

weeks decreased the frequency of postterm pregnancy. Such stripping, however, did not modify the risk for cesarean delivery, and

maternal and neonatal infections were not increased. The authors emphasized that membrane stripping could be painful and might


Print: Chapter 37. Postterm Pregnancyprovoke vaginal bleeding and irregular contractions. Wong and colleagues (2002), in a randomized trial evaluating 120 women, found

that sweeping the membranes did not reduce the need to induce labor (see Chap. 22, Membrane Stripping).

Station of the vertex is likewise important in predicting successful postterm induction. Shin and colleagues (2004) studied 484 nulliparas

who underwent induction after 41 weeks. The cesarean delivery rate was directly related to station. It was 6 percent if the vertex was –1,

20 percent at –2, 43 percent at –3, and 77 percent at –4.

Induction versus Fetal Testing

A logical plan for reducing perinatal mortality and morbidity associated with prolonged pregnancy is to terminate pregnancy before such

events occur. There have been doubts about the value of labor induction, mainly because it was feared that induction would result in

more operative intervention without preventing perinatal death. As a result, many clinicians prefer to use fetal testing to avoid inductions.

Major studies designed to resolve these questions have been done in both Canada and the United States.

Hannah and colleagues (1992) randomized 3407 Canadian women with pregnancies at 41 or more weeks to labor induction or fetal

testing. Women assigned to fetal testing were asked to count the number of times they felt the fetus move over a 2-hour period each

day, and they also underwent nonstress testing three times weekly. Amnionic fluid volume was assessed two to three times each week,

and pockets less than 3 cm were considered abnormal. Labor induction resulted in a significantly lower cesarean rate (21 percent)

compared with pregnancies managed with antepartum testing (24 percent). There were two stillbirths in the fetal testing group and none

in the induction group. The lower cesarean rate in the induction group was due to fewer procedures for fetal distress. A cost-effectiveness

analysis of the Canadian data was later reported by Goeree and colleagues (1995). The mean cost was $3132 per patient managed with

fetal testing compared with $2939 for those who underwent labor induction. Thus, labor induction for postterm pregnancy resulted in

more favorable outcomes than fetal testing and at a lower cost.

Menticoglou and Hall (2002) from Winnipeg have subsequently lamented that induction of labor at 41 weeks has become the standard of

care in Canada. They concluded that "ritual induction at 41 weeks" had become the current practice. They opined that this practice was

based on evidence that was seriously flawed, and that it constituted "an abuse of biological norms." They recommended that this

nonsensus consensus be discontinued because it caused interference that had the potential to do more harm than good and have

staggering resource implications.

The Maternal–Fetal Medicine Network trial of induction versus fetal testing was reported by Gardner and associates (1996). Nonstress

tests and ultrasonic estimations of amnionic fluid volume were performed twice weekly in 175 women whose pregnancies had reached 41

weeks or more. Perinatal outcomes in these women were compared with those of 265 women randomized to induction with or without

cervical ripening. There were no perinatal deaths in any study subgroup, and the rate of cesarean delivery was not different between

management groups. Thus, this study supported the validity of either management scheme.



Crowley (1997) used the Oxford Database of Perinatal Trials to perform a meta-analysis of 18 studies in which management of postterm

pregnancy was assessed. Routine induction after 41 weeks resulted in reduced perinatal mortality without increased risk of cesarean or

instrumental delivery. Similar results were reported by Sanchez-Ramos and colleagues (2003) in their meta-analysis of 16 trials. In a

study of 540,116 births, Roberts and colleagues (1999) concluded that Crowley's meta-analysis prompted routine induction after 41

weeks to be widely adopted in New South Wales, Australia. Specifically, from 1990 to 1996, there was a significant decrease in births at

42 completed weeks or more—4.6 compared with 2.8 percent, respectively. Correspondingly, there was an increased induction rate at 41

weeks, and the cesarean delivery rate also increased. Because infant outcomes were not described in this report, any beneficial neonatal

effects are undetermined.

Alexander and colleagues (2001) examined the effect of labor induction at 42 weeks on rates of cesarean delivery at Parkland Hospital.

Pregnancy outcomes in 638 women in whom labor was induced were compared with those of 687 women with spontaneous labor at 42

weeks. Rates of cesarean delivery were significantly increased in the induced group because of failure to progress (19 versus 14 percent).

When corrected for risk factors using logistic regression, however, the researchers concluded that factors intrinsic to the patient, rather

than labor induction itself, led to the increased cesarean deliveries. These factors included nulliparity, an unfavorable cervix, and epidural

analgesia.

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. Importantly, a national

policy of intervention for prolonged pregnancy at 41 versus 42 weeks would mean that approximately 500,000 additional women would

be subjected to interventions that have not been conclusively proved necessary or harmless.

Usher and colleagues (1988) analyzed several outcomes in 7663 pregnancies with gestational ages determined to be 40, 41, or 42 weeks

confirmed by early ultrasound examinations. Perinatal death rates, corrected for malformations, were 1.5, 0.7, and 3.0 per 1000 for 40,

41, and 42 weeks, respectively. These results could be used to challenge the concept of routine intervention at 41 instead of 42 weeks.

Interestingly, Divon and colleagues (1996) found that, although adverse fetal outcomes increased at 41 compared with those at 40

weeks, these were associated with growth restriction. They suggested that management strategies should be focused on fetal growth

rather than simply gestational age in pregnancies reaching 41 weeks.

Based on results summarized in Table 37–1, 41-week pregnancies without other complications such as hypertension are considered

normal pregnancies at Parkland Hospital. No interventions are practiced solely on the basis of fetal age until 42 completed weeks. It is

our view that large, randomized studies should be performed before 41-week gestations are routinely considered pathologically prolonged.



Oligohydramnios

It has been suggested that the identification of diminished amnionic fluid determined by various ultrasonic methods may help identify a

postterm fetus in jeopardy. There is no doubt that when amnionic fluid is decreased in a postterm pregnancy—or for that matter in any

pregnancy—the fetus is at increased risk. Many different criteria for ultrasonic diagnosis of oligohydramnios have been proposed. Fischer

and colleagues (1993) attempted to determine which criteria were most predictive of normal versus abnormal outcomes in postterm

pregnancies. As shown in Figure 37–10, the smaller the amnionic fluid pocket, the greater the likelihood that there was clinically

significant oligohydramnios. Normal amnionic fluid volume, however, did not preclude abnormal outcomes. Alfirevic and co-authors

(1997) randomized 500 women with postterm pregnancies—defined as 290 days or longer gestation—to assessment of amnionic fluid

volume using either the amnionic fluid index (AFI) or the deepest vertical pocket. They concluded that the AFI overestimated the number

of abnormal outcomes in postterm pregnancies.

Figure 37–10.



Comparison of the diagnostic value of various ultrasonic estimates of amnionic fluid volume in prolonged pregnancies. Abnormal outcomes include

cesarean or operative vaginal delivery for fetal jeopardy, 5-minute Apgar score of 6 or less, umbilical arterial blood pH less than 7.1, or admission to

the neonatal intensive care unit. (Adapted from Fischer and associates, 1993.)

Regardless of the criteria used to diagnose oligohydramnios in postterm pregnancies, most investigators have found an increased

incidence of "fetal distress" during labor. Thus, oligohydramnios by most definitions is a clinically meaningful finding. Conversely,

reassurance of continued fetal well-being in the presence of "normal" amnionic fluid volume is tenuous because it is unknown how quickly

pathological oligohydramnios develops. Clement and co-workers (1987) reported six postterm pregnancies in which amnionic fluid volume

diminished abruptly over 24 hours and one fetus died.

Macrosomia

The incidence of macrosomia (defined as birthweight greater than 4500 g) increases from 1.4 percent at 37 to 41 weeks to 2.2 percent at

42 weeks or more (Martin and colleagues, 2002). This raises the possibility that both the maternal and fetal morbidity associated with http://www.accessmedicine.com/popup.aspx?aID=732449&print=yes_chapter (20 of 28)9/10/2008 11:12:53 AM


macrosomia might be avoidable with timely labor induction to preempt further growth. The American College of Obstetricians and

Gynecologists (2000) has reviewed such interventions. It concluded that current evidence does not support a policy of early labor

induction in women at term who have suspected fetal macrosomia. Moreover, in the absence of maternal diabetes, vaginal delivery is not

contraindicated for women with estimated fetal weights up to 5000 g. Cesarean delivery was recommended for estimated fetal weights

greater than 4500 g in the presence of a prolonged second-stage labor or a second-stage arrest of descent.

Recommendations of the American College of Obstetricians and Gynecologists

These recommendations are summarized in Table 37–3. Although providing flexibility in the evaluation and management of pregnancies

completing 42 weeks, the recommendations are that either antenatal testing or labor induction should be commenced at 42 weeks. There

was insufficient evidence to recommend a management strategy between 40 and 42 completed weeks.

Table 37–3. Evaluation and Management of Postterm Pregnancy

1. Postterm pregnancy is defined as a pregnancy that has extended to or beyond 42 completed weeks.

2. Women with a postterm gestation who have an unfavorable cervix can either undergo labor induction or be managed expectantly.

3. Prostaglandin can be used for cervical ripening and labor induction.

4. Delivery should be effected if there is evidence of fetal compromise or oligohydramnios.

5. It is reasonable to initiate antenatal surveillance between 41 and 42 weeks despite lack of evidence that monitoring improves

outcomes.

6. A nonstress test and amnionic fluid volume assessment should be adequate, although no single method has been shown to be

superior.

7. Many recommend prompt delivery in a woman with a postterm pregnancy, a favorable cervix, and no other complications.

From the American College of Obstetricians and Gynecologists (2004).

Postterm pregnancy has been identified by the American College of Obstetricians and Gynecologists (1999) as a high-risk condition in

which twice-weekly antepartum fetal testing may be indicated. Doppler velocimetry was not recommended. Oligohydramnios detected

using ultrasound, defined as no vertical pocket of amnionic fluid greater than 2 cm or an AFI of 5 cm or less, is considered an indication

for either delivery or close fetal surveillance.

Management at Parkland Hospital

In women with a certain gestational age, labor is induced at the completion of 42 weeks. Almost 90 percent of such women are induced


Print: Chapter 37. Postterm Pregnancysuccessfully 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 plan of management, but in the unusual few who are not delivered,

management decisions involve a third (or more) induction versus cesarean delivery.

Women classified as having uncertain postterm pregnancies are followed on a weekly basis and without intervention unless fetal jeopardy

is suspected. The latter is based on clinical or sonographic perception of decreased amnionic fluid volume. Equally worrisome is

diminished fetal movement reported by the mother. If fetal jeopardy is suspected by either method, labor induction is carried out as

described previously for the woman with a certain postterm gestation.

Medical or Obstetrical Complications

In the event of a medical or other obstetrical complication, it is generally unwise to allow a pregnancy to continue past 42 weeks. Indeed,

in many such instances early delivery is indicated. Timing of delivery depends on the individual complication. Common examples include

hypertensive disorders due to pregnancy, prior cesarean delivery, and diabetes.

Intrapartum Management

Labor is a particularly dangerous time for the postterm fetus. Therefore, it is important that women whose pregnancies are known or

suspected to be postterm come to the hospital as soon as they suspect they are in labor. On arrival, while being observed for possible

labor, we recommend that fetal heart rate and uterine contractions be monitored electronically for variations consistent with fetal distress

(American College of Obstetricians and Gynecologists, 1995).

When to perform amniotomy is problematic. Further reduction in fluid volume following amniotomy can certainly enhance the possibility

of cord compression. Conversely, amniotomy aids diagnosis of thick meconium, which may be dangerous to the fetus if aspirated.

Moreover, once the membranes are ruptured, a scalp electrode and intrauterine pressure catheter can be placed, which usually provide

more precise data concerning fetal heart rate and uterine contractions.

Identification of thick meconium in the amnionic fluid is particularly worrisome. The viscosity probably signifies the lack of liquid and thus

oligohydramnios. Aspiration of thick meconium may cause severe pulmonary dysfunction and neonatal death (see Chap. 29, Meconium

Aspiration Syndrome). Wenstrom and Parsons (1989) proposed amnioinfusion during labor as a way of diluting meconium to decrease the

incidence of meconium aspiration syndrome. As discussed in Chapter 18 (see Amnioinfusion), the benefits of amnioinfusion remain

controversial. In a recent randomized trial by Rathore and colleagues (2002), amnioinfusion was associated with fewer cesarean

deliveries for fetal distress and fewer neonatal intensive care unit admissions for neonates with moderate to thick meconium-stained

amnionic fluid. In contrast, Yoder and colleagues (2002) found that increased use of amnioinfusion—0 to 36 percent of women with

moderate to thick amnionic fluid meconium—had no impact on the incidence of meconium aspiration syndrome.



The likelihood of a successful vaginal delivery is reduced appreciably for the nulliparous woman who is in early labor with thick,

meconium-stained amnionic fluid. Therefore, when the woman is remote from delivery, strong consideration should be given to prompt

cesarean delivery, especially when cephalopelvic disproportion is suspected or either hypotonic or hypertonic dysfunctional labor is

evident. Some practitioners choose to avoid oxytocin use in these cases.

Aspiration of meconium may be minimized but not eliminated by effective suctioning of the pharynx as soon as the head is delivered but

before the thorax is delivered. If meconium is identified, the trachea should be aspirated as soon as possible after delivery. Immediately

thereafter, the infant should be ventilated as needed.

References

Alexander JM, McIntire DD, Leveno KJ: Forty weeks and beyond: Pregnancy outcomes by week of gestation. Obstet Gynecol 96:291,

2000a

Alexander JM, McIntire DD, Leveno KJ: Postterm pregnancy: Does induction increase cesarean rates? J Soc Gynecol Invest 7:79A, 2000b

Alexander JM, McIntire DD, Leveno KJ: Postterm pregnancy: Is cervical "ripening" being used in the right patients? J Soc Gynecol Invest

7:247A, 2000c

Alexander JM, McIntire DD, Leveno KJ: Prolonged pregnancy: Induction of labor and cesarean births. Obstet Gynecol 97:911, 2001

[PMID: 11384695] [Full Text]

Alexander JM, McIntire DD, Leveno KJ: The effect of fetal growth restriction on neonatal outcome in postterm pregnancy. Abstract No.

463. Am J Obstet Gynecol 182:S148, 2000d

Alfirevic Z, Luckas M, Walkinshaw SA, et al: A randomized comparison between amniotic fluid index and maximum pool depth in the

monitoring of postterm pregnancy. Br J Obstet Gynaecol 104:207, 1997 [PMID: 9070140] [Full Text]

American College of Obstetricians and Gynecologists: Management of postterm pregnancy. Practice Bulletin No. 55, September 2004

American College of Obstetricians and Gynecologists: Fetal macrosomia. Practice Bulletin No. 22, November 2000

American College of Obstetricians and Gynecologists: Antepartum fetal surveillance. Technical Bulletin No. 9, October 1999

American College of Obstetrics and Gynecologists: Fetal heart rate patterns: Monitoring, interpretation, and management. Technical

Bulletin No. 207, July 1995



Amersi S, Grimes DA: The case against using ordinal numbers for gestational age. Obstet Gynecol 91:623, 1998 [PMID: 9540954] [Full

Text]

Bakketeig LS, Bergsjø P: Post-term pregnancy: Magnitude of the problem. In Chalmers I, Enkin M, Keirse M (eds): Effective Care in

Pregnancy and Childbirth. Oxford, Oxford University Press, 1991, p 765

Ballantyne JW: The problem of the postmature infant. J Obstet Gynaecol Br Emp 2:522, 1902

Bennett KA, Crane JM, O'Shea P, et al: First trimester ultrasound screening is effective in reducing postterm labor induction rates: A

randomized controlled trial. Am J Obstet Gynecol 190:1077, 2004 [PMID: 15118645] [Full Text]

Blondel B, Morin I, Platt RW, et al: Algorithms for combining menstrual and ultrasound estimates of gestational age: Consequences for

rates of preterm and postterm birth. Br J Obstet Gynaecol 109:718, 2002 [PMID: 12118654] [Full Text]

Boulvain M, Irion O, Marcoux S, et al: Sweeping of the membranes to prevent post-term pregnancy and to induce labour: A systematic

review. Br J Obstet Gynaecol 106:481, 1999 [PMID: 10430199] [Full Text]

Boyce A, Magaux MJ, Schwartz D: Classical and "true" gestational post maturity. Am J Obstet Gynecol 125:911, 1976 [PMID: 941944]

[Full Text]

Calkins LA: Postmaturity. Am J Obstet Gynecol 56:167, 1948

Caughey AB, Musci TJ: Complications of term pregnancies beyond 37 weeks of gestation. Obstet Gynecol 103:57, 2004 [PMID:

14704245] [Full Text]

Clausson B, Cnattingus S, Axelsson O: Outcomes of postterm births: The role of fetal growth restriction and malformations. Obstet

Gynecol 94:758, 1999 [PMID: 10546724] [Full Text]

Clement D, Schifrin BS, Kates RB: Acute oligohydramnios in postdate pregnancy. Am J Obstet Gynecol 157:884, 1987. The Cochrane

Library. The Cochrane Collaboration; Issue 4, Oxford: Update Software, 1997

Clifford SH: Postmaturity with placental dysfunction. Clinical syndromes and pathologic findings. J Pediatr 44:1, 1954 [PMID: 13131191]

[Full Text]



Crowley P: Interventions to prevent or improve outcome for deliveries at or beyond term. In Neilson JP, Crowther CA, Hodnett ED,

Hofmeyr GJ (eds): Pregnancy and Childbirth Module of the Cochrane Database of Systematic Reviews [updated September 1997]. The

Cochrane Library. The Cochrane Collaboration; Issue 4, Oxford, Update Software, 1997

Crowley P: Post-term pregnancy: Induction or surveillance? In Chalmers I, Enkin M, Keirse M (eds): Effective Care in Pregnancy and

Childbirth. Oxford, Oxford University Press, 1991, p 776

Divon MY, Haglund B, Nisell H, et al: Fetal and neonatal mortality in the postterm pregnancy: The impact of gestational age and fetal

growth restriction. Am J Obstet Gynecol 178:726, 1998 [PMID: 9579434] [Full Text]

Divon MY, Haglund B, Nisell H, et al: Perinatal outcome in a large cohort of postdate pregnancies. Am J Obstet Gynecol 174:351, 1996

Elliott PM, Inman WH: Volume of liquor amnii in normal and abnormal pregnancy. Lancet 2:835, 1961 [PMID: 13889956] [Full Text]

Fischer RL, McDonnell M, Bianculli KW, et al: Amniotic fluid volume estimation in the postdate pregnancy: A comparison of techniques.

Obstet Gynecol 81:698, 1993 [PMID: 8469457] [Full Text]

Gardner M, Rouse D, Goldenberg R, et al: Cost comparison of induction of labor at 41 weeks versus expectant management in the

postterm pregnancy. Am J Obstet Gynecol 174:351, 1996

Goeree R, Hannah M, Hewson S: Cost-effectiveness of induction of labour versus serial antenatal monitoring in the Canadian Multicentre

Postterm Pregnancy Trial. Can Med Assoc J 152:1445, 1995 [PMID: 7728693] [Full Text]

Hannah ME, Hannah WJ, Hellman J, et al: Induction of labor as compared with serial antenatal monitoring in post-term pregnancy. N Engl

J Med 326:1587, 1992 [PMID: 1584259] [Full Text]

Harris BA Jr, Huddleston JF, Sutliff G, et al: The unfavorable cervix in prolonged pregnancy. Obstet Gynecol 62:171, 1983 [PMID:

6866359] [Full Text]

Jazayeri A, Tsibris JC, Spellacy WN: Elevated umbilical cord plasma erythropoietin levels in prolonged pregnancies. Obstet Gynecol 92:61,

1998 [PMID: 9649094] [Full Text]

Larsen LG, Clausen HV, Andersen B, et al: A stereologic study of postmature placentas fixed by dual perfusion. Am J Obstet Gynecol

172:500, 1995 [PMID: 7856676] [Full Text]



Laursen M, Bille C, Olesen AW, et al: Genetic influence on prolonged gestation: A population-based Danish twin study. Am J Obstet


Leveno KJ, Quirk JG, Cunningham FG, et al: Prolonged pregnancy, I. Observations concerning the causes of fetal distress. Am J Obstet


Lindell A: Prolonged pregnancy. Acta Obstet Gynecol Scand 35:136, 1956 [PMID: 13354315] [Full Text]

Lucas WE, Anefil AO, Callagan DA: The problem of postterm pregnancy. Am J Obstet Gynecol 91:241, 1965 [PMID: 14258027] [Full Text]

MacDonald PC, Siiteri PK: Origin of estrogen in women pregnant with an anencephalic fetus. J Clin Invest 44:465, 1965 [PMID:

14271306] [Full Text]

Martin JA, Hamilton BE, Ventura SJ, et al: Births: Final data for 2001. National Vital Statistics Reports, Vol 51, No. 2. Hyattsville, Md:

National Center for Health Statistics, 2002

Menticoglou SM, Hall PF: Routine induction of labour at 41 weeks' gestation: Nonsensus consensus. Br J Obstet Gynaecol 109:485, 2002


Mogren I, Stenlund H, Högberg U: Recurrence of prolonged pregnancy. Int J Epidemiol 28:253, 1999 [PMID: 10342687] [Full Text]

Munster K, Schmidt L, Helm P: Length and variation in the menstrual cycle—a cross-sectional study from a Danish county. Br J Obstet

Gynaecol 99:422, 1992 [PMID: 1622917] [Full Text]

Naeye RL: Causes of perinatal mortality excess in prolonged gestations. Am J Epidemiol 108:429, 1978 [PMID: 727213] [Full Text]

Nahum GG, Stanislaw H, Huffaker BJ: Fetal weight gain at term: Linear with minimal dependence on maternal obesity. Am J Obstet


National Institute of Child Health and Human Development Network of Maternal–Fetal Medicine Units: A clinical trial of induction of labor

versus expectant management in postterm pregnancy. Am J Obstet Gynecol 170:716, 1994

Olesen AW, Westergaard JG, Olsen J: Perinatal and maternal complications related to postterm delivery: A national register-based study,

1978–1993. Am J Obstet Gynecol 189:227, 2003

Oz AU, Holub B, Mendilcioglu I, et al: Renal artery Doppler investigation of the etiology of oligohydramnios in postterm pregnancy. Obstet




Rabe T, Hosch R, Runnebaum B: Sulfatase deficiency in the human placenta: Clinical findings. Biol Res Pregnancy Perinatol 4:95, 1983


Rathore AM, Singh R, Ramji S, et al: Randomised trial amnioinfusion during labor with meconium stained amniotic fluid. Br J Obstet

Gynaecol 109:17, 2002

Roberts CL, Taylor L, Henderson-Smart D: Trends in births at and beyond term: Evidence of a change? Br J Obstet Gynaecol 106:937,

1999 [PMID: 10492105] [Full Text]

Roussis P, Cox SM, Campbell BA, et al: Survey on the management of postdate pregnancy. J Matern Fetal Neonatal Med 2:155, 1993

Rushton DI: Pathology of placenta. In Wigglesworth JS, Singer DB (eds): Textbook of Fetal and Perinatal Pathology. Boston, Blackwell,

1991, p 171

Sanchez-Ramos L, Olivier F, Delke I, Kaunitz AW: Labor induction versus expectant management for postterm pregnancies: A systematic

review with meta-analysis. Obstet Gynecol 101:1312, 2003 [PMID: 12798542] [Full Text]

Shime J, Gare DJ, Andrews J, et al: Prolonged pregnancy: Surveillance of the fetus and the neonate and the course of labor and delivery.

Am J Obstet Gynecol 148:547, 1984 [PMID: 6702916] [Full Text]

Shin KS, Brubaker KL, Ackerson LM: Risk of cesarean delivery in nulliparous women at greater than 41 weeks' gestational age with an

unengaged vertex. Am J Obstet Gynecol 190:129, 2004 [PMID: 14749648] [Full Text]

Smith SC, Baker PN: Placental apoptosis is increased in postterm pregnancies. Br J Obstet Gynaecol 106:861, 1999 [PMID: 10453839]

[Full Text]

Smith GC: Life-table analysis of the risk of perinatal death at term and post term in singleton pregnancies. Am J Obstet Gynecol 184:489,

2001 [PMID: 11228508] [Full Text]

Trimmer KJ, Leveno KJ, Peters MT, et al: Observation on the cause of oligohydramnios in prolonged pregnancy. Am J Obstet Gynecol

163:1900, 1990 [PMID: 2256502] [Full Text]

Usher RH, Boyd ME, McLean FH, et al: Assessment of fetal risk in postdate pregnancies. Am J Obstet Gynecol 158:259, 1988 [PMID:




Vaisanen-Tommiska M, Nuutila M, Ylikorkala O: Cervical nitric oxide release in women postterrm. Obstet Gynecol 103:657, 2004 [PMID:

15051555] [Full Text]

Wenstrom KD, Parsons MT: The prevention of meconium aspiration in labor using amnioinfusion. Obstet Gynecol 73:647, 1989 [PMID:


Wong SF, Hui SK, Choi H, et al: Does sweeping of membranes beyond 40 weeks reduce the need for formal induction of labour? Br J

Obstet Gynaecol 109:632, 2002 [PMID: 12118640] [Full Text]

Yang SH, Roh CR, Kim JH: Transvaginal ultrasonography for cervical assessment before induction of labor. Obstet Gynecol Surv 59:577,

2004

Yoder BA, Kirsch EA, Barth WH, et al: Changing obstetric practices associated with decreasing incidence of meconium aspiration

syndrome. Obstet Gynecol 99:731, 2002 [PMID: 11978280] [Full Text]

Copyright © The McGraw-Hill Companies. All rights reserved.

Privacy Notice. Any use is subject to the Terms of Use and Notice.


http://www.accessmedicine.com/public/privacy.aspx

http://www.accessmedicine.com/public/termsofuse.aspx

http://www.accessmedicine.com/public/notice.aspx

http://www.mheducation.com/

http://www.silverchair.com/

http://www.mcgrawhill.com/

Documents

Postterm Pregnancy: Introductionfamilymed.uthscsa.edu/residency08/resources/Williams OB Text...The definition of postterm pregnancy as one that persists for 42 weeks or more from the