Management of Late-Term and Postterm Pregnancy

Breanna Gawrys; Diana Trang; Whay Cheng

BREANNA GAWRYS, DO, DIANA TRANG, DO, AND WHAY CHENG, DO

Am Fam Physician. 2024;110(4):378-384

This clinical content conforms to AAFP criteria for CME.

Author disclosure: No relevant financial relationships.

Postterm pregnancy is defined as a pregnancy that has reached 42 weeks' gestation and late-term pregnancy includes 41 weeks' and 0 days' to 41 weeks' and 6 days' gestation. Accurate first-trimester dating is essential for determining or verifying gestational age. Ideal management of late-term and postterm pregnancy involves shared decision-making on timing of planned delivery based on risks and preferences. Starting at 42 weeks' gestation, the risks of fetal mortality, admission to the neonatal intensive care unit, and stillbirth increase exponentially. Induction of labor at 41 weeks' gestation reduces perinatal mortality and stillbirth compared with expectant management or induction starting at 42 weeks' gestation. Recent studies have shown a decrease in cesarean deliveries and hypertensive disorders of pregnancy with induction of labor in nulliparous, low-risk pregnancies beginning at 39 weeks' gestation. Induction of labor before 42 weeks' gestation decreases the risk of stillbirth, perinatal mortality, and cesarean delivery compared with expectant management. The American College of Obstetricians and Gynecologists suggests considering an elective induction of labor in low-risk, nulliparous patients starting at 39 weeks' and 0 days' gestation and recommends induction of labor in all patients by 42 weeks' gestation. The American College of Obstetricians and Gynecologists recommends antepartum monitoring of pregnancies beginning at 41 weeks' gestation to mitigate the risks of perinatal morbidity and mortality.

Postterm pregnancy starts at 42 weeks' and 0 days' gestation and late-term pregnancy includes 41 weeks' and 0 days' to 41 weeks' and 6 days' gestation.¹ Gestational age is established using the first day of the last menstrual period (LMP) and confirmed by first-trimester ultrasonography or by the established gestational age from assisted reproductive technology.^1–3 Recent data show that 0.25% of all pregnancies continue to at least 42 weeks' gestation, a notable decrease from 5.5% reported in 2014.^1,4 Table 1 lists risk factors for prolonged gestation.^1,2

Clinical recommendation	Evidence rating	Comments
Labor induction should be considered beginning at 41 weeks' gestation based on the reduction of perinatal mortality and stillbirth.^{1,9–11,16–18}	B	Limited evidence from multiple randomized controlled trials, expert guidelines, and Cochrane reviews
Labor induction at 42 weeks' gestation is recommended for reduction of perinatal morbidity and mortality compared with expectant management.^1,8,10,16,18	A	Consistent evidence from expert guidelines and systematic reviews
Elective induction of labor may be offered to low-risk, nulliparous patients at 39 weeks' gestation and has been shown to reduce the rate of cesarean deliveries (NNT = 28), without an increased risk of maternal or neonatal harm.^20–22	B	Evidence of improved patient outcomes in a large randomized controlled trial; ACOG practice guideline and Society for Maternal-Fetal Medicine committee recommendation
Antepartum fetal surveillance performed once or twice weekly is recommended at 41 weeks' gestation despite limited supporting evidence.^1,23	C	ACOG expert committee statement
Membrane sweeping at 38 weeks' gestation or later may be considered to encourage spontaneous labor, with an NNT of 8, to decrease the risk of late-term and postterm pregnancies.^1,26,27	B	Cochrane review of lower-quality studies, systematic reviews, expert guidelines

In the United States, risks of late-term and postterm pregnancies contribute to increased infant mortality. As of 2021, the overall infant mortality rate was 5.48 infant deaths per 1,000 live births, with the highest rates in infants of Black patients (10.52), followed by infants of American Indian and Alaska Native (7.69), Hispanic (4.84), White (4.42), and Asian (3.40) patients.⁵

ESTABLISHING ACCURATE DATING

Determining an accurate estimated gestational age is essential in recognizing and mitigating the risks of late-term and post-term pregnancies.^1,3,6 Studies have found that using the LMP alone to establish gestational age is imprecise and leads to more pregnancies that are incorrectly classified as late-term or post-term.¹ Measurement in the first trimester (up to and including 13 weeks' and 6 days' gestation) with ultrasonography is the most accurate method for establishing or confirming gestational age. Assuming the patient has a 28-day menstrual cycle, those who are certain of the first day of their LMP can compare the estimated gestational age that was calculated using their LMP with the estimated gestational age calculated by the mean crown-rump length on ultrasonography in the first trimester.^3,6 Criteria for redating estimated gestational age based on ultrasonography are outlined in Table 2.³

Gestational age range based on LMP	Method of measurement	Discrepancy between ultrasound dating and LMP dating that supports redating
≤ 13 6/7 weeks	Crown-rump length
≤ 8 6/7 weeks		More than 5 days
9 0/7 weeks to 13 6/7 weeks		More than 7 days
14 0/7 weeks to 15 6/7 weeks	Biparietal diameter, head circumference, abdominal circumference, femur length	More than 7 days
16 0/7 weeks to 21 6/7 weeks	Biparietal diameter, head circumference, abdominal circumference, femur length	More than 10 days
22 0/7 weeks to 27 6/7 weeks	Biparietal diameter, head circumference, abdominal circumference, femur length	More than 14 days
28 0/7 weeks and beyond	Biparietal diameter, head circumference, abdominal circumference, femur length	More than 21 days

Although accurate dating during the first trimester is preferred, many pregnant patients have limited access to health care during their first trimester.⁷ The accuracy of dating with ultrasonography significantly decreases in the second and third trimesters. Pregnancies without confirmatory dating with ultrasonography before 22 weeks' gestation are considered suboptimally dated.^3,6 It is important to note that patients who receive first-trimester ultrasonography should not adjust their dates based on subsequent second-or third-trimester ultrasonography because any discrepancies likely will be due to fetal growth.³

RISKS OF PROLONGED GESTATION

Late-term and postterm pregnancies increase the risk of adverse fetal and maternal outcomes (Table 3^1,2,8–11), including perinatal death, stillbirth, meconium aspiration, oligohydramnios, and admissions to the neonatal intensive care unit.^1,2,12 A 2019 systematic review and meta-analysis found a nearly 30-fold increased risk of stillbirth with increased gestational age across term gestation from 0.11 per 1,000 births at 37 weeks' gestation (95% CI, 0.07 to 0.15) to 3.18 per 1,000 births at 42 weeks' gestation (95% CI, 1.84 to 4.35).⁸ For postterm pregnancies, stillbirth rates continue to increase, reaching 11.5 per 1,000 births at 43 weeks' gestation.^8,10 Postterm pregnancies also carry a higher risk of admission to the neonatal intensive care unit (odds ratio = 2.05; 95% CI, 1.35 to 3.12) and fetal mortality (odds ratio = 2.9 at 43 weeks' gestation and 1.8 at 42 weeks' gestation) compared with 40 weeks' gestation. Taking these risks into consideration, the American College of Obstetricians and Gynecologists (ACOG) recommends induction of labor in all pregnant patients by 42 weeks' gestation.¹

MITIGATING RISKS OF PROLONGED GESTATION

Induction of labor before 42 weeks' gestation is the primary means of preventing postterm pregnancy. Several studies have examined maternal and neonatal outcomes following induction of labor at different gestational ages.^10,13–15

Most studies evaluated delivery before reaching late term. Moderate-quality evidence showed that delivery at 41 weeks' gestation reduces perinatal mortality and stillbirth compared with delivery at 42 weeks' gestation.^{1,9–11,16–18} A 2018 systematic review demonstrated that there was a number needed to treat (NNT) of 501 to prevent one perinatal death with induction at 41 weeks' gestation compared with 42 weeks' gestation.¹⁹ A 2018 Cochrane review that included 20 randomized controlled trials (RCTs) with 9,960 patients found a similar NNT of 500 for perinatal death and an NNT of 1,000 for stillbirth with induction before 42 weeks' gestation compared with expectant management.^1,8,10,16,18 Patients who underwent induction of labor before 42 weeks' gestation were less likely to require cesarean delivery compared with expectant management (relative risk [RR] = 0.92; 95% CI, 0.85 to 0.99; 27 RCTs).¹¹ A large, multicenter trial of low-risk pregnancies that was conducted after these reviews found a more significant reduction of perinatal mortality, with an NNT of 230 for induction at 41 weeks' compared with 42 weeks' gestation. This study ended early because of the difference in perinatal mortality.¹⁷

Another multicenter trial found that labor induction of low-risk, nulliparous patients at 39 weeks' gestation showed no significant difference in composite adverse perinatal outcomes but significantly lowered the frequency of cesarean deliveries (NNT = 28) and hypertensive disorders of pregnancy (NNT = 20).²⁰ Based on these results, ACOG and the Society for Maternal-Fetal Medicine recommend elective induction of labor beginning at 39 weeks' gestation for low-risk, nulliparous patients based on patient preferences, available resources, and the implementation setting.^13,21,22

For patients with a history of one or two cesarean deliveries desiring a trial of labor, patients should be counseled that the likelihood of a successful vaginal delivery after cesarean decreases by up to 35% after 41 weeks' gestation.^1,19

A 2020 Cochrane review involving 34 RCTs and more than 21,000 patients that compared induction of labor between 37 and 42 weeks' gestation with expectant management found high-certainty evidence of fewer perinatal deaths (RR = 0.31; 95% CI, 0.15 to 0.64; NNT = 544), stillbirths (RR = 0.30; 95% CI, 0.12 to 0.75; NNT = 1,000), and cesarean deliveries (RR = 0.90; 95% CI, 0.85 to 0.95). Notably, a subgroup analysis found that most perinatal deaths occurred after 41 weeks' gestation (two of four in the induction group and 19 of 24 in the expectant management group).¹⁰

FETAL MONITORING

For uncomplicated late-term and postterm pregnancies, antenatal fetal surveillance beginning at 41 weeks' and 0 days' gestation is recommended because of the risk of stillbirth. Antepartum fetal surveillance can be performed with a nonstress test (NST), a biophysical profile, or a modified biophysical profile that includes NST and amniotic fluid assessment^1,23 (Table 4^1,8,19,23). Despite a lack of evidence regarding optimal fetal monitoring, ACOG recommends performing a modified biophysical profile once or twice per week or a biophysical profile once per week starting at 41 weeks' gestation.²³ A few small studies suggest that surveillance twice per week may improve outcomes, and high-risk conditions may require more frequent testing.^1,23

Method	Criteria
NST	Fetal heart tone assessment: reactive is ≥ two accelerations over a 20-minute period (≥ 32 weeks' gestation defined by 15 beats per minute for 15 seconds above the baseline heart rate; < 32 weeks' gestation defined by 10 beats per minute for 10 seconds above the baseline heart rate) Consider vibroacoustic stimulation and/or an additional 20 minutes of assessment if fetal sleep-wake cycle varies Nonreactive (abnormal) stress test results are < two accelerations over a 40-minute period
Biophysical profile	Consists of five components, totaling 10 points (normal score: 8 to 10; abnormal score: ≤ 6): (1) NST: reactive (2 points) vs. nonreactive (0 points) (2) Fetal breathing: one or more episodes of rhythmic fetal breathing movement of ≥ 30 seconds within 30 minutes (2 points) vs. no episodes of breathing movements (0 points) (3) Fetal movement: ≥ three discrete body or limb movements in 30 minutes (2 points) vs. < three movements (0 points) (4) Fetal tone: one or more episodes of an extremity showing extension-to-flexion or opening or closing of a hand (2 points) vs. no movements (0 points) (5) Amniotic fluid volume: single deepest vertical pocket^* > 2 cm (2 points) vs. ≤ 2 cm (0 points) Despite a normal composite score, oligohydramnios (single deepest vertical pocket^* ≤ 2 cm) should prompt further evaluation
Modified biophysical profile	Amniotic fluid volume assessment and NST Single deepest vertical pocket^* > 2 cm and NST is reactive Abnormal: fluid volume ≤ 2 cm or NST is nonreactive
Amniotic fluid index (part of the biophysical profile and modified biophysical profile)	Single deepest vertical pocket^* Oligohydramnios: 0 to 2 cm Normal: 2.1 to 8 cm Polyhydramnios: > 8 cm Amniotic fluid index Oligohydramnios: 0 to 5 cm Normal: 5.1 to 25 cm Polyhydramnios: > 25 cm
Contraction stress test†	Fetal heart tone assessment with relation to uterine contractions (adequate contractions are defined as at least three contractions that last 40 seconds each in a 10-minute period); uterine stimulation can be induced with nipple stimulation or intravenous oxytocin Negative: no late or significant variable decelerations Positive: late deceleration after 50% or more of contractions Equivocal-suspicious: intermittent late decelerations, significant variable decelerations, or decelerations that occur more frequently than every 2 minutes or lasting longer than 90 seconds Unsatisfactory: fewer than three uterine contractions in 10 minutes or uninterpretable tracing

Antenatal testing is best performed with a biophysical profile or modified biophysical profile, which has a false-negative rate of 0.8 per 1,000 patients compared with 1.9 per 1,000 patients for NST.²³ The primary reason for the superior false-negative rate is the detection of oligohydramnios, a low amniotic fluid level that is most often caused by uteroplacental insufficiency. Amniotic fluid level is measured by the single deepest vertical pocket of fluid with ultrasonography during a biophysical profile or modified biophysical profile. Use of the deepest vertical pocket is more specific and recommended instead of an amniotic fluid index to diagnose oligohydramnios.^1,23

OTHER CONSIDERATIONS BEYOND 40 WEEKS' GESTATION

In an updated 2022 committee opinion, ACOG recommends repeating group B streptococcus screening if it has been 5 weeks since an initial negative screening because screening cultures are only valid for 5 weeks.²⁴ A suggested approach for the management of low-risk pregnancy beyond 40 weeks' gestation is outlined in Figure 1.²⁵

OUTPATIENT METHODS TO PROMOTE SPONTANEOUS LABOR

Some patients prefer to delay or avoid hospital induction and choose to use outpatient methods to promote spontaneous labor. Low-quality evidence suggests that membrane sweeping, nipple stimulation, and exercise can increase the likelihood of spontaneous labor. It is important to maintain respect for patient autonomy regarding birth preferences and discuss the risks and benefits of induction vs. spontaneous labor for uncomplicated late-term and postterm pregnancies. A recent article in American Family Physician discusses induction of labor, including outpatient methods.²⁶

In-Office Methods

A Cochrane review found low-certainty evidence that membrane sweeping may be considered to promote spontaneous labor and decrease the risk of late-term and postterm pregnancies (NNT = 8; 95% CI, 5 to 21), with most studies evaluating pregnancies at 38 weeks' gestation or later.^1,26,27

At-Home Methods

Exercise may encourage spontaneous labor. A small RCT (n = 160) studying the effects of exercise in low-risk pregnant patients at 38 weeks' gestation and beyond found that walking for 30 minutes three times per week at 4 km (2.5 miles) per hour is safe, increases the likelihood of spontaneous onset of labor, and reduces operative vaginal delivery rates.²⁸

Breast massage and nipple stimulation for 15 to 20 minutes three times per day starting at 38 weeks' gestation aid cervical ripening and increase the chances of spontaneous vaginal delivery (n = 100; NNT = 8).^29,30 According to a 2019 systematic review, sexual intercourse does not significantly increase the incidence of spontaneous onset of labor.³¹

Available evidence does not support the use of herbal supplements, acupuncture, castor oil, hot baths, enemas, or homeopathy to promote spontaneous onset of labor.¹⁶

This article updates previous articles on this topic by Wang and Fontaine² and Briscoe, et al.²⁵

Data Sources: A PubMed search was completed in Clinical Queries using the following terms: late-term pregnancy, post-term pregnancy, ARRIVE trial, induction of labor, antepartum fetal monitoring, elective induction and expectant management late-term pregnancies. The search included meta-analyses, randomized controlled trials, clinical trials, clinical practice guidelines, and reviews. The Agency for Healthcare Research and Quality Effective Healthcare Reports, the Cochrane database, DynaMed, and Essential Evidence Plus were also searched. Whenever possible, if studies used race and/or gender as patient categories but did not define how these categories were assigned, they were not included in our final review. If studies that used these categories were determined to be essential and therefore included, limitations were explicitly stated in the manuscript. Search dates: August 31 to October 20, 2023, and August 26, 2024.

The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the U.S. Air Force, U.S. Department of Defense, or U.S. government.

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