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Postterm pregnancy

Postterm pregnancy
Literature review current through: Jan 2024.
This topic last updated: Mar 25, 2022.

INTRODUCTION — The timely onset of labor is an important determinant of perinatal outcome. Although it has long been known that there is a small absolute increase in perinatal mortality as pregnancies extend beyond the estimated due date, the optimum gestational age for beginning formal fetal monitoring (eg, nonstress test, biophysical profile) and the optimum gestational age for giving birth rather than continued expectant management and formal fetal monitoring has been more controversial.

This topic will discuss maternal and fetal issues related to postterm pregnancies. Issues related to the postterm infant are reviewed separately. (See "Postterm infant".)

CLASSIFICATION OF TERM GESTATIONS — The following definitions describe the subgroups of term pregnancy [1,2]:

Postterm – ≥42+0 weeks of gestation (ie, ≥294 days from the first day of the last menstrual period and ≥14 days from the estimated day of delivery).

Late term – 41+0 to 41+6 weeks of gestation.

Full term – 39+0 to 40+6 weeks of gestation.

Early term – 37+0 to 38+6 weeks of gestation.

PREVALENCE — In the United States, birth certificate data indicate that 0.25 percent of pregnancies deliver at ≥42 weeks and 4.88 percent deliver at or after 41 weeks [3]. A study of birth rates ≥42 weeks in 13 European countries observed a wide range across the continent: from 0.4 and 0.6 percent in Austria and Belgium to 7.5 and 8.1 percent in Sweden and Denmark [4]. Variations in prevalence are likely due to differences among populations in the factors discussed below.

Role of early ultrasound – One of the most important factors impacting prevalence of postterm pregnancy is whether early sonographic assessment of gestational age is performed routinely since it provides more accurate dating compared with menstrual dating alone and results in fewer postterm pregnancies. In a meta-analysis of randomized trials of early (<24 weeks) ultrasound examination, routine rather than selective examination reduced the chances of induction for postterm pregnancy by approximately 50 percent (1.4 versus 2.8 percent; RR 0.48, 95% CI 0.31-0.73) [5].

Other factors affecting prevalence – Other factors that affect the prevalence of postterm pregnancy in a population include the prevalence of nulliparous patients, which shifts the curve for gestational age at birth to the right since they are more likely to give birth postterm. Factors that shift the curve to the left include a high prevalence of spontaneous preterm birth and/or patients with pregnancy complications leading to indicated preterm birth, and local practice patterns such as a high rate of planned cesarean birth and/or labor induction at ≥39 weeks.

ETIOLOGY

The majority of postterm pregnancies have no known etiology.

Up to one-third to one-half of the variation in postterm birth in a population can be attributed to maternal or fetal genetic influence on the initiation of parturition [6,7]. Epigenetic changes may also play a role.

In rare cases, postterm pregnancy has been attributed to defects in fetal production of hormones involved in parturition [8] (see "Physiology of parturition at term"). For example, fetal disorders associated with placental sulfatase deficiency (eg, X-linked ichthyosis) result in extremely low estriol levels and other hormonal changes compared with normal pregnancies. The mean duration of pregnancies with placental sulfatase deficiency appears to be one week longer than pregnancies without this disorder, although most such pregnancies still deliver before 42 weeks of gestation [9,10].

Anencephaly, which results in absence or hypoplasia of the hypothalamus and pituitary and adrenal hypoplasia, often results in postterm pregnancy even when polyhydramnios is absent (mean gestational age at delivery: 311 days with no polyhydramnios versus 253 days with polyhydramnios [11]). Since pregnancies with anencephaly are now routinely detected antepartum and terminated or induced, postterm duration is no longer observed.

RISK FACTORS — Patients at highest risk (relative risk ≥2) of postterm pregnancy are those with a previous postterm pregnancy. (See 'Recurrence risk' below.)

Additional, more modest risk factors (relative risk <2) include [6,12-19]:

Nulliparity

Male fetus

Obesity

Older maternal age

Maternal (and to a lesser extent paternal) personal history of postterm birth

Maternal race/ethnicity (non-Hispanic White individuals are at higher risk than non-Hispanic Black, Hispanic, and Asian individuals)

DIAGNOSIS — The diagnosis of pregnancy ≥42+0 weeks of gestation is based on the clinician's most accurate estimate of the patient's delivery date (EDD).

The EDD is based on an ultrasound examination performed before 22+0 weeks of gestation if the ultrasound-based EDD differs from that calculated from menstrual dating (LMP) by more than five to seven days, as described in the table (table 1). If the ultrasound-based EDD is within five to seven days of the LMP-based EDD, then LMP is used to determine EDD. Exceptions to this approach include pregnancies with known dates of conception/implantation, such as pregnancies conceived by in vitro fertilization. (See "Prenatal assessment of gestational age, date of delivery, and fetal weight".)

MORBIDITY AND MORTALITY — Postterm pregnancy is associated with maternal, fetal, and neonatal complications. Many of the complications are sequelae of either excessive fetal growth or uteroplacental insufficiency.

Macrosomia — Because of the longer duration of intrauterine growth, postterm fetuses tend to be larger than term fetuses and have a higher incidence of macrosomia (≥4500 grams: 2.5 to 10 percent postterm versus 0.8 to 1 percent at term) [20-22].

Macrosomia increases the risks for several adverse sequelae, including abnormal labor progression, cesarean birth, assisted (vacuum or forceps) vaginal birth, shoulder dystocia, maternal/fetal/newborn birth injury, postpartum hemorrhage, and neonatal metabolic problems. (See "Fetal macrosomia", section on 'Significance'.)

Dysmaturity — Not all postterm fetuses continue to grow along a normal growth trajectory. Up to 20 percent develop "fetal dysmaturity (postmaturity) syndrome," a term used to describe fetuses with characteristics of chronic intrauterine malnutrition [23-25]. These fetuses are at increased risk of umbilical cord compression due to oligohydramnios and abnormal antepartum or intrapartum fetal heart rate patterns due to uteroplacental insufficiency or cord compression. Meconium passage is common and may be related to physiologic maturation of the gut, fetal hypoxia, or both. (See "Meconium aspiration syndrome: Pathophysiology, clinical manifestations, and diagnosis".)

Dysmature neonates have a long thin body, long nails, and are small for gestational age. Their skin is dry (vernix caseosa is decreased or absent), meconium-stained, parchment-like, and peeling; it appears loose, especially over the thighs and buttocks, and has prominent creases; lanugo hair is sparse or absent, while scalp hair is increased. These neonates are at risk for morbidities associated with growth restriction, including hypoglycemia, polycythemia, perinatal asphyxia, meconium aspiration, and persistent pulmonary hypertension. They are also at risk for neurodevelopmental complications (eg, seizures, cerebral palsy). (See "Postterm infant".)

Perinatal mortality

Rate – The perinatal mortality rate at ≥42 weeks of gestation is twice the rate at term, increasing fourfold at 43 weeks and five- to sevenfold at 44 weeks [26-30]. Neonates born at ≥41 weeks of gestation experience a one-third greater risk of neonatal mortality than term neonates born at 38 to 40 weeks of gestation [31,32].

However, the absolute rate of fetal or neonatal death is low. In large studies from the United Kingdom, the rate of antepartum fetal death was [33]:

40 to 41 weeks – 0.86 to 1.08 per 1000 ongoing pregnancies

41 to 42 weeks – 1.2 to 1.27 per 1000 ongoing pregnancies

42 to 43 weeks – 1.3 to 1.9 per 1000 ongoing pregnancies

>43 weeks – 1.58 to 6.3 per 1000 ongoing pregnancies

Causes – Perinatal mortality increases as pregnancy extends beyond full term, particularly after 41 weeks, due to increases in both nonanomalous stillbirths and early neonatal deaths [31,34-37]. Intrauterine infection, uteroplacental insufficiency due to placental aging, and cord compression leading to fetal hypoxia, asphyxia, and meconium aspiration are thought to contribute to the excess perinatal deaths [29,38,39].

It is unclear whether, or to what degree, perinatal mortality is increased in postterm pregnancies without fetal malformations, signs of placental insufficiency (growth restriction, oligohydramnios), or maternal disorders associated with adverse pregnancy outcome (eg, diabetes, hypertension) [40-43]. In a pooled analysis, two perinatal deaths occurred among 3914 such pregnancies after 42 weeks of gestation (0.05 percent), and data were available for only 238 such pregnancies over 43 weeks [44]. (See "Postterm infant", section on 'Perinatal mortality'.)

MANAGEMENT — The following discussion refers to the singleton, cephalic fetus of an otherwise uncomplicated pregnancy that reaches 41+0 weeks of gestation. Multiple gestations, noncephalic presentations, and pregnancies with medical or obstetric complications are generally delivered before 41 weeks (refer to individual topic reviews on each subject).

Overview — Average-risk pregnancies that reach 41+0 weeks of gestation can be induced or managed expectantly with ongoing fetal assessment and intervention if spontaneous labor does not occur by a predefined gestational age or fetal assessment is not reassuring. In the low-risk postterm patient, both of these approaches are associated with low absolute rates of adverse outcome [45]. Patients should be counseled about what these approaches involve (labor induction with/without cervical ripening agents, type and frequency of antepartum fetal and maternal monitoring) and the maternal and fetal benefits and risks of both approaches.

Recent studies and consensus opinions state that induction at an earlier gestational age (≥39 weeks) may have similar benefits and no increased risk. (See "Induction of labor with oxytocin", section on 'Risk-reducing induction'.)

Our approach: Induction at 41+0 weeks — For pregnancies that are ongoing after the EDD, we favor induction at or shortly after 41+0 weeks of gestation irrespective of cervical status, given that intervention at this time reduces perinatal mortality without increasing perinatal morbidity, and reduces cesarean birth rates. (See 'Evidence' below.)

We generally administer a cervical ripening agent (eg, prostaglandin, balloon catheter) to patients with unfavorable cervixes. A large randomized trial comparing a balloon catheter with vaginal dinoprostone for cervical ripening in pregnancies ≥41+0 weeks found the rates of cesarean birth for nonreassuring fetal status, maternal morbidity, and neonatal morbidity were similar for the two approaches [46]. (See "Induction of labor: Techniques for preinduction cervical ripening".)

This approach is in general agreement with professional guidelines that suggest routine induction between 41+0 and 42+0 weeks of gestation, with the exact timing during this week determined by clinician and patient preferences and local circumstances. The American College of Obstetricians and Gynecologists recommends induction of labor after 42+0 weeks and by 42+6 weeks, but considers induction at 41+0 to 42+0 weeks reasonable [47].

For patients who want to avoid standard cervical ripening agents and/or induction, membrane stripping (also called sweeping) may reduce the proportion of patients who remain undelivered at 42 weeks [48]. The optimum time to initiate membrane stripping/sweeping and the frequency (one time versus on multiple days) has not been studied in randomized trials, but beginning any time after 39+0 weeks of gestation is reasonable. (See "Induction of labor with oxytocin", section on 'Membrane stripping'.)

Evidence — The following meta-analyses support our approach, which reduces perinatal mortality and meconium aspiration syndrome, reduces the cesarean birth rate, and is satisfactory to most patients.

In a 2020 meta-analysis of randomized trials comparing labor induction with expectant management of pregnancies ≥37 weeks of gestation (34 trials; over 21,000 participants), routine induction resulted in [45]:

70 percent reduction in perinatal mortality (0.4 versus 3.0 per 1000, relative risk [RR] 0.31, 95% CI 0.15-0.64, high-quality evidence). Subgroup analysis showed a similar reduction in perinatal mortality when only pregnancies ≥41 weeks were considered (RR 0.26, 95% CI 0.11-0.64).

70 percent reduction in stillbirth (1 versus 2 per 1000, RR 0.30, 95% CI 0.12-0.75, high-quality evidence). Subgroup analysis showed a similar reduction in stillbirth when only pregnancies ≥41 weeks were considered (RR 0.24, 95% CI 0.08-0.78).

10 percent reduction in cesarean birth (167 versus 186 per 1000, RR 0.92, 95% CI 0.85-0.99, moderate-quality evidence). Subgroup analysis showed a similar reduction in cesarean birth when only pregnancies ≥41 weeks were considered (RR 0.90, 95% CI 0.83-0.97).

12 percent trend in reduction in neonatal intensive care unit (NICU) admission (83 versus 95 per 1000, RR 0.88, 95% CI 0.80-0.96, high-quality evidence). Subgroup analysis showed a similar reduction in NICU admission when only pregnancies ≥41 weeks were considered (RR 0.85, 95% CI 0.74-0.97).

28 percent reduction in the rate of macrosomia (>4000 g: 23 versus 74 per 300 infants, average RR 0.72, 95% CI 0.54-0.96).

25 percent reduction in meconium aspiration syndrome (RR 0.75, 95% CI 0.62-0.92).

No clear reduction in neonatal birth trauma (7 per 1000 in both groups, RR 0.97, 95% CI 0.63-1.49, moderate-quality evidence) or neonatal encephalopathy (3 versus 5 per 1000, RR 0.69, 95% CI 0.37-1.31, low-quality evidence).

In addition, an individual participant data meta-analysis of randomized trials specifically comparing labor induction at 41+0 to 41+2 weeks with expectant management until 42+0 to 42+1 weeks in uncomplicated pregnancies (two trials, 4561 pregnancies) found that induction reduced the primary composite outcome of mortality and severe neonatal morbidity (0.4 versus 1.0 percent, RR 0.43, 95% CI 0.21-0.91) [49]. Cesarean birth rates were similar for both groups.

Alternative approach: Expectant management with fetal monitoring — Expectant management is the alternative to induction. Expectantly managed pregnancies typically undergo twice-weekly fetal assessment beginning at 41+0 weeks (or shortly thereafter), with intervention if spontaneous labor does not begin by a predefined gestational age or fetal surveillance testing is abnormal [50,51]. Either a nonstress test plus assessment of amniotic fluid volume or the biophysical profile can be used for fetal surveillance; there is no convincing evidence that one method is superior to the other [52].

Expectant management with fetal monitoring is not unreasonable for several reasons [45]:

The absolute rate of perinatal death is relatively low at ≥41 weeks and not much higher in absolute terms with expectant management than with induction: approximately 3.0 perinatal deaths/1000 pregnancies with expectant management versus 0.4 perinatal deaths/1000 with labor induction.

A relatively high number of inductions (over 400) would need to be performed to prevent one perinatal death.

Some pregnant people place a high value on going into labor spontaneously.

Fetal surveillance — Case-control studies support initiating twice weekly fetal surveillance between 41+0 and 42+0 weeks of gestation [50,51]. Twice weekly amniotic fluid volume assessment is important because amniotic fluid can become severely reduced quickly, within 24 to 48 hours [53]. (See "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations" and "Overview of antepartum fetal assessment", section on 'Fetal assessment techniques'.)

Postterm pregnancy is a universally accepted indication for fetal surveillance because of the increased risk of fetal demise with advancing gestational age. However, the efficacy of fetal surveillance for preventing unexplained fetal demise in postterm pregnancies has not been validated by appropriately sized and placebo-controlled randomized trials, and probably never will be evaluated in this way because of ethical and medicolegal concerns of assigning some participants to an unmonitored group. The optimal type and frequency of fetal testing, and the gestational age for beginning surveillance, have not been determined.

It is clear that monitoring the postterm fetus with Doppler ultrasonography of the umbilical artery has no proven benefit [54-56]. Evaluation of pulsatility indices of the uterine arteries, middle cerebral artery, descending aorta, ductus venosus, or inferior vena cava is also not useful [56].

Timing of delivery in expectantly managed pregnancies — We agree with the American College of Obstetricians and Gynecologists recommendation to induce labor by 42+6 weeks of gestation in all pregnancies [47]. Earlier induction is indicated for development of any of the usual obstetric indications, including oligohydramnios [57,58]. Adverse pregnancy outcomes (abnormal fetal heart rate tracing, NICU admission, low Apgar score) are more likely when oligohydramnios is present [57-62]. (See "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations".)

Because so few pregnancies reach 43 weeks, there is no strong evidence on which to base a recommendation for the maximum gestational age at which an otherwise uncomplicated pregnancy should be delivered in the absence of standard maternal or fetal indications or spontaneous labor.

RECURRENCE RISK — After one postterm pregnancy, the risk of a second postterm birth is increased two- to fourfold; the risk of recurrence is even higher after two prior postterm pregnancies [12,13,63,64].

In the Netherlands Perinatal Registry database of over 230,000 patients with singleton pregnancies who delivered their first pregnancy at 37+0 to 42+6 weeks of gestation and had a subsequent singleton pregnancy, postterm births occurred in 7.7 percent of first pregnancies [64]. In second pregnancies, the postterm birth rate in those with a previous postterm birth was 15 percent but just 4 percent in those who had a previous term birth.

PREVENTION — As discussed above, early routine ultrasound examination reduces the rate of intervention for postterm pregnancy by approximately 50 percent [5]. (See 'Prevalence' above.)

Membrane sweeping at term can initiate labor and thus prevent postterm pregnancy. (See 'Our approach: Induction at 41+0 weeks' above and "Induction of labor with oxytocin", section on 'Membrane stripping'.)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topics (see "Patient education: When your baby is overdue (The Basics)")

Beyond the Basics topics (see "Patient education: Postterm pregnancy (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

Diagnosis – Postterm pregnancy refers to a pregnancy that is ≥42+0 weeks of gestation or 294 days from the first day of the last menstrual period (LMP). (See 'Classification of term gestations' above.)

The diagnosis is based on the clinician's most accurate estimate of the patient's delivery date, which is based on LMP for some patients and on ultrasound dating for others (table 1). (See 'Diagnosis' above.)

Prevalence – In pregnancies dated by first-trimester ultrasound examination, the prevalence of postterm pregnancy is approximately 2 percent and fewer inductions are performed for postterm pregnancy than in pregnancies dated by LMP. (See 'Prevalence' above.)

Etiology – The majority of postterm pregnancies have no known etiology. (See 'Etiology' above.)

Risk factors – Patients at highest risk of postterm pregnancy are those with a previous postterm pregnancy. The risk of a second postterm pregnancy is increased two- to fourfold and is even higher after two prior postterm pregnancies. (See 'Risk factors' above and 'Recurrence risk' above.)

Outcome – Many of the maternal, fetal, and neonatal complications of postterm pregnancy are sequelae of either excessive fetal growth (macrosomia) or placental insufficiency (fetal/neonatal dysmaturity, growth restriction, oligohydramnios). (See 'Macrosomia' above and 'Dysmaturity' above.)

Perinatal mortality increases with increasing gestational age after 40 weeks of gestation, but the absolute risk of fetal death is low:

40 to 41 weeks – 0.86 to 1.08 per 1000 ongoing pregnancies

41 to 42 weeks – 1.2 to 1.27 per 1000 ongoing pregnancies

42 to 43 weeks – 1.3 to 1.9 per 1000 ongoing pregnancies

>43 weeks – 1.58 to 6.3 per 1000 ongoing pregnancies

Intrauterine infection, placental insufficiency or cord compression leading to fetal hypoxia and asphyxia, and meconium aspiration are thought to contribute to the excess perinatal deaths. (See 'Perinatal mortality' above.)

In postterm pregnancies without fetal malformations, signs of placental insufficiency, or maternal disorders associated with adverse pregnancy outcome (eg, diabetes, hypertension), it is unclear whether or to what degree perinatal mortality is increased. (See 'Perinatal mortality' above.)

Management

For pregnancies that reach 41+0 weeks of gestation, we suggest induction rather than expectant management (Grade 2B). Induction is associated with lower perinatal mortality than expectant management and does not increase the risk of cesarean delivery. The absolute benefits of routine induction are modest, however, and depending on personal values and preferences, some women may choose to be managed expectantly. (See 'Our approach: Induction at 41+0 weeks' above.)

For patients at 41+0 weeks who choose expectant management, we monitor fetal well-being by nonstress testing with amniotic fluid volume assessment or by the biophysical profile, twice weekly beginning at 41+0 weeks or shortly thereafter. (See 'Alternative approach: Expectant management with fetal monitoring' above.)

In expectantly managed pregnancies, we agree with the American College of Obstetricians and Gynecologists recommendation to induce labor by 42+6 weeks of gestation. Earlier induction is indicated for development of any of the usual obstetric indications. (See 'Timing of delivery in expectantly managed pregnancies' above.)

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Topic 6749 Version 42.0

References

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