The American College of Obstetricians and Gynecologists (ACOG) recently issued guidelines for the clinical management of post-term pregnancy. The guidelines appeared in the September 2004 issue of Obstetrics and Gynecology.

Post-term pregnancy is defined as a pregnancy that has extended to or beyond 42 weeks of gestation (294 days, or estimated date of delivery [EDD] plus 14 days). The reported frequency of post-term pregnancy is approximately 7 percent. Most cases of post-term pregnancy result from a prolongation of gestation. Other cases result from an inability to accurately define EDD. The risk of adverse sequelae may be reduced by making an accurate assessment of gestational age and diagnosis of post-term gestation, as well as recognition and management of risk factors. Two strategies that may decrease the risk of an adverse fetal outcome include antenatal surveillance and induction of labor.

Risk factors for post-term pregnancy may include primiparity and previous post-term pregnancy. Placental sulfatase deficiency, fetal anencephaly, and male sex have been associated with prolongation of pregnancy, and genetic predisposition also may play a role.

The EDD is most reliably and accurately determined early in the pregnancy and may be based on the last known menstrual period in women with regular, normal menstrual cycles. Because of normal variations in the size of infants during the third trimester, dating the pregnancy during this period is less reliable. If the estimated gestational age by a patient’s last menstrual period is different from the estimate obtained via assessment with ultrasonography, the ultrasound estimate should be used.

Post-term pregnancy is associated with risks to the fetus, including increased perinatal mortality rate, low umbilical artery pH levels at delivery, low 5-minute Apgar scores, dysmaturity syndrome, and increased risk of death within the first year of life. Although post-term infants are larger than term infants and have an increased incidence of fetal macrosomia, there is no evidence to support induction of labor as a preventive measure in these cases.

Risks of post-term pregnancy to the pregnant woman include an increase in labor dystocia, an increase in severe perineal injury related to macrosomia, and a doubling in the rate of cesarean delivery. Also, post-term pregnancy can cause anxiety for the pregnant woman.

Clinical Considerations and Recommendations

Are there interventions that decrease the rate of post-term pregnancy? Obtaining an accurate EDD using ultrasonography early in the pregnancy can reduce the incidence of pregnancies diagnosed as post-term and minimize unnecessary interventions. However, routine early ultrasonography has not been recommended as standard care in the United States. There is no evidence to show that stimulation of the breasts and nipples affects the incidence of post-term pregnancy. There is conflicting evidence as to the effectiveness of sweeping the membranes at term in reducing post-term pregnancy.

When should antepartum fetal testing begin? Although evidence shows that antenatal fetal surveillance for post-term pregnancies does not decrease perinatal mortality, it has become a common, universally accepted practice. Antenatal fetal surveillance also is often performed between 40 and 42 weeks of gestation, despite there being no randomized controlled trial demonstrating that it results in an improvement in perinatal outcome. There also is insufficient evidence to indicate whether routine antenatal surveillance of low-risk patients between 40 and 42 weeks’ gestation improves perinatal outcome. The authors add that, because of ethical and medicolegal issues, no studies have included post-term patients who were not monitored.

What form of antenatal surveillance should be performed, and how frequently should a post-term patient be reevaluated? Options for evaluating fetal well-being include, nonstress testing, biophysical profile (BPP) or modified BPP (nonstress test plus amniotic fluid volume estimation), contraction stress testing, and a combination of these modalities. None of these methods has been shown to be superior. Assessment of amniotic fluid volume appears to be important; however, a consistent definition of low amniotic fluid in the post-term pregnancy has not been established. There is no proven benefit to monitoring the post-term fetus with Doppler velocimetry. The authors state that no recommendation can be made regarding the frequency of antenatal surveillance; however, many practitioners use twice-weekly testing.

For a post-term patient with a favorable cervix, does the evidence support labor induction or expectant management? Factors to consider in the management of low-risk post-term pregnancy include the following: gestational age; the condition of the cervix; results of antepartum fetal testing; and maternal preference after discussion of the risks, benefits, and alternatives to expectant management with antepartum monitoring versus labor induction. There is insufficient data to make a recommendation for labor induction or expectant management in women who are experiencing a post-term pregnancy and have a favorable cervix. Labor usually is induced in post-term pregnancies in which the cervix is favorable because the risk of failed induction and subsequent cesarean delivery is low.

For a post-term patient with an unfavorable cervix, does the evidence support labor induction or expectant management? In low-risk post-term women with unfavorable cervices, both labor induction and expectant management are associated with low complication rates and good perinatal outcomes. There does appear to be a slight advantage to labor induction using cervical-ripening agents.

What is the role of prostaglandin preparations in managing a post-term pregnancy? Prostaglandin is a valuable tool for improving cervical ripeness and inducing labor; however, no standardized dose or dosing interval has been established. Lower doses of prostaglandins are preferred, because higher doses have been associated with an increased risk of uterine tachysystole and hyperstimulation leading to nonreassuring fetal testing results. When prostaglandin is used, routine fetal heart monitoring should be performed because of the risk of uterine hyperstimulation.

Is there a role for vaginal birth after cesarean delivery in the management of post-term pregnancy? Because of limited evidence on the safety or efficacy of vaginal birth after cesarean delivery after 42 weeks of gestation, the authors make no recommendation for its use as an alternative to elective repeat cesarean deliveries for some women.

Summary of Recommendations

The following recommendations are based on good and consistent scientific evidence (Level A):

• Women with post-term gestations who have unfavorable cervices can either undergo labor induction or be managed expectantly.

• Prostaglandin can be used in post-term pregnancies to promote cervical ripening and induce labor.

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

The following recommendations are based primarily on consensus and expert opinion (Level C):

• Despite a lack of evidence that monitoring improves perinatal outcome, it is reasonable to initiate antenatal surveillance of post-term pregnancies between 41 weeks (287 days; EDD plus seven days) and 42 weeks (294 days; EDD plus 14 days) of gestation because of evidence that perinatal morbidity and mortality increase as gestational age advances.

• Many practitioners use twice-weekly testing with some evaluation of amniotic fluid volume beginning at 41 weeks of gestation. A nonstress test and amniotic fluid volume assessment (a modified BPP) should be adequate.

• Many authorities recommend prompt delivery in a post-term patient with a favorable cervix and no other complications.