The Committee on Practice Bulletins—Obstetrics of the American College of Obstetricians and Gynecologists has released a new guideline entitled, “Management of Postterm Pregnancy.” The guideline appears in the September 2004 issue of Obstetrics and Gynecology and is available online at http://www.greenjournal.org/cgi/reprint/104/3/639.

Postterm pregnancy, by definition, refers to a pregnancy that has extended to or beyond 42 weeks of gestation. Accurate pregnancy dating is critical to the diagnosis. The term “postdate” is poorly defined and should be avoided. Although some cases of postterm pregnancy likely result from an inability to accurately define the estimated date of delivery, many cases result from a true prolongation of gestation. The reported frequency of postterm pregnancy is approximately 7 percent.

When postterm pregnancy truly exists, the cause usually is unknown. Primiparity and prior postterm pregnancy are the most common identifiable risk factors for prolongation of pregnancy. Rarely, postterm pregnancy may be associated with placental sulfatase deficiency or fetal anencephaly.

Postterm pregnancy is associated with significant risks to the infant. The perinatal mortality rate at more than 42 weeks of gestation is twice that at term and increases sixfold and higher at 43 weeks of gestation and beyond. Uteroplacental insufficiency, meconium aspiration, and intrauterine infection contribute to the increased rate of perinatal mortality. Postterm pregnancy also is an independent risk factor for low umbilical artery pH levels at delivery and low five-minute Apgar scores. For these reasons, the trend has been toward delivery by 41 completed weeks of gestation.

Although postterm infants are larger than term infants and have a higher incidence of fetal macrosomia, no evidence supports inducing labor as a preventive measure in such cases, according to the guideline. Complications associated with fetal macrosomia include prolonged labor, cephalopelvic disproportion, and shoulder dystocia with resultant risks of orthopedic or neurologic injury.

Approximately 20 percent of postterm infants have dysmaturity syndrome, which refers to infants with characteristics resembling chronic intrauterine growth restriction from uteroplacental insufficiency. These pregnancies are at increased risk for umbilical cord compression from oligohydramnios, meconium aspiration, and short-term neonatal complications, and have an increased incidence of nonreassuring fetal testing. Infants born postterm also are at increased risk of death within the first year of life, although some of these infant deaths clearly result from peripartum complications (such as meconium aspiration syndrome).

There are significant risks to the pregnant woman as well, including an increase in labor dystocia, an increase in severe perineal injury related to macrosomia, and a doubling in the rate of cesarean delivery. Cesarean delivery is associated with higher risks of complications, such as endometritis, hemorrhage, and thromboembolic disease. Finally, postterm pregnancy can be a source of substantial anxiety for the pregnant woman.