A post-term or prolonged pregnancy is one that reaches 42 weeks' gestation; approximately 5 to 10 percent of pregnancies are post-term. Studies have shown a reduction in the number of pregnancies considered post-term when early ultrasound dating is performed. Maternal and fetal risks increase with gestational age, but the management of otherwise low-risk prolonged pregnancies is controversial. Antenatal surveillance with fetal kick counts, nonstress testing, amniotic fluid index measurement, and biophysical profiles is used, although no data show that monitoring improves outcomes. Studies show a reduction in the rate of cesarean deliveries and possibly in neonatal mortality with a policy of routine labor induction at 41 weeks' gestation. (Am Fam Physician 2005;71:1935-41, 1942. Copyright[C] 2005 American Academy of Family Physicians.)
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Approximately have lowered the 5 to 10 percent of all pregnancies continue to at least 42 weeks' gestation. (1,2) Advances in obstetric and neonatal care absolute mortality risk; however, retrospective studies (1,3) of these so-called post-term pregnancies have found an increased risk to the mother and fetus. The perinatal mortality rate (i.e., stillbirths plus neonatal deaths) of two to three deaths per 1,000 deliveries at 40 weeks' gestation approximately doubles by 42 weeks and is four to six times greater at 44 weeks. (4,5)
Risks of Prolonged Gestation
Post-term pregnancies are associated with numerous adverse outcomes (Table 1). (1) In a recent Danish birth-registry study, (1) increased rates of multiple maternal and perinatal complications were found in singleton pregnancies of at least 42 weeks' gestation. The risks were not limited to deliveries of large infants; the underlying causes of the prolonged pregnancies also may have been responsible. The occurrence of complications was particularly high in low-birth-weight infants and likely resulted from the cause of fetal growth restriction. A Norwegian birth cohort study (3) found that maternal complications usually are associated with larger fetal size, and fetal complications are associated with smaller size. When these factors are considered, the impact of post-term pregnancies is minor by comparison.
Review of the Evidence
The management of pregnancy beyond 40 weeks' gestation relies on an accurate assessment of the gestational age. A Cochrane review (6) found that, compared with selective ultrasonography, routine prenatal ultrasonography before 24 weeks' gestation provides better gestational age assessment and earlier detection of multiple pregnancies and fetal malformations. In a retrospective study (7) of more than 34,000 pregnant women with "certain" menstrual dates (i.e., patient stated that she was sure of her dates, that she had regular menstrual cycles, and that she had not taken oral contraceptive pills in the preceding three months), ultrasound dating during the estimated gestational age range of 13 to 24 weeks gave a more accurate prediction of the delivery date than estimates based on the last menstrual period alone or in combination with ultrasonography. Early ultrasound dating also resulted in a 70 per-cent reduction in the number of pregnancies that were considered post-term.
Elective labor induction before 42 weeks' gestation has been proposed to reduce rates of adverse fetal and maternal complications. The Canadian Multicenter Post-term Pregnancy Trial (CMPPT) (8) is the largest individual randomized controlled trial (RCT) to date comparing labor induction at 41 weeks with expectant management. In this study, 3,407 women with pregnancies of at least 41 weeks' gestation were randomized to immediate induction or expectant management with fetal monitoring. Monitoring consisted of daily kick counts, nonstress tests (NSTs) three times per week (Table 2), (9) and ultrasound amniotic-fluid-volume assessments two or three times per week (Table 3). (10-12) Expectant management continued until 44 weeks' gestation or until there was an obstetric indication for labor induction. The expectant management group had a significantly higher rate of cesarean deliveries than the induction group (odds ratio [OR], 1.22; 95 percent confidence interval [CI], 1.02 to 1.45; number needed to induce to prevent one excess cesarean delivery, 30). The expectant management group had a significantly higher rate of cesarean deliveries resulting from fetal distress, but there was no difference between groups in the rate of cesarean deliveries resulting from dystocia. No difference was found in perinatal mortality rates, although the study was too under-powered to detect this outcome. No differences were found in neonatal morbidity outcomes.
A Cochrane review (13) of 19 RCTs found that routine labor induction at 41 weeks' gestation resulted in lower perinatal mortality rates but similar cesarean delivery rates. Approximately 500 women needed to be induced to prevent one perinatal death, and the number may be higher in current-day practice. Meconium-stained amniotic fluid was more common in the expectant management group, but rates of meconium aspiration syndrome and other neonatal morbidities were not significantly different between groups. This review also found that routine ultrasonography in early pregnancy-- even in low-risk women--reduced the number of patients who required labor induction for apparent post-term pregnancies.
In a more recent meta-analysis (2) of 16 RCTs comparing induction at 41 weeks versus expectant management, the induction group had lower cesarean delivery rates (OR, 0.88; 95 percent CI, 0.78 to 0.99; number needed to induce, 53). A nonsignificant reduction in perinatal mortality rates also was found in the induction group (OR, 0.41; 95 per-cent CI, 0.14 to 1.18). Approximately 6,600 women were included in this meta-analysis; approximately 16,000 were needed to detect a 50 percent reduction in mortality. No significant difference was found in neonatal intensive care unit admissions, meconium aspiration, meconium below the vocal cords, or low Apgar scores.
Neither review (2,13) found any adverse perinatal complications from labor induction, and both noted that the results were consistent regardless of the Bishop score at the time of induction. In neither review did an "unfavorable" cervix at the time of induction result in increased rates of cesarean delivery or other adverse outcomes. In both reviews, the largest single RCT was the CMPPT (3,407 patients); the next largest individual RCT included 440 women. The primary difference between the meta-analyses was in the studies chosen for inclusion. Trials included in the Cochrane review (13) fell into two main groups: those in which women were recruited at or before 40 weeks' gestation and those in which recruitment occurred at 41 or 42 weeks' gestation. The benefit of induction was found in studies in which induction occurred after 41 weeks. In the other review, (2) the inclusion criteria were limited strictly to studies in which induction was initiated at 41 weeks' gestation or later. This study also included RCTs published after the most recent amendment to the Cochrane review.
The Society of Obstetricians and Gynecologists of Canada (SOGC) issued guidelines (14) in 1997 encouraging the routine induction of labor at 41 weeks' gestation. In a 1989 technical bulletin, (15) the American College of Obstetricians and Gynecologists (ACOG) recommended labor induction in low-risk pregnancies in the 43rd week of gestation. Interestingly, an updated ACOG evidence-based guideline (16) released in 1997 does not describe an upper gestational age limit for allowing expectant management. It does, however, recommend that fetal health assessments begin by 42 weeks' gestation.
Not all authorities agree with routine intervention in prolonged pregnancies. A com-mentary (17) based on a reanalysis of CMPPT data argues strongly against the SOGC guidelines, stating that the risks of post-term pregnancies are very small and that the benefit of a policy of routine labor induction was overestimated because of cesarean deliveries resulting from fetal distress.
Fetal Monitoring
