Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 37. Postterm Pregnancy

The terms postterm, prolonged, postdates, and postmature are often loosely interchanged to signify pregnancies that have exceeded a normal duration. Postmature should be used to describe the infant with recognizable clinical features indicating a pathologically prolonged pregnancy. Postterm or prolonged pregnancy should be the preferred expressions for extended pregnancies, and the term postdates should probably be abandoned as few infants from prolonged pregnancies are truly postmature.

The standard internationally recommended definition of prolonged pregnancy, endorsed by the American College of Obstetricians and Gynecologists (Management of postterm pregnancy. Practice Bulletin No. 55, September 2004), is 42 completed weeks (294 days) or more from the first day of the last menstrual period. This definition assumes that the onset of a menstrual period was followed by ovulation 2 weeks later. The use of menstrual dating results in approximately 10 percent of all pregnancies being classified as postterm and is most likely an overestimation of the incidence of prolonged pregnancy due to the large variation in menstrual cycles. As there is no exact method to identify pregnancies that are truly prolonged, all pregnancies judged to be 42 completed weeks should be managed as if abnormally prolonged. Intrapartum perinatal risk is increased in prolonged pregnancies, particularly when meconium is present.

PERINATAL MORTALITY AND MORBIDITY

The principal reason for increased risk to the postterm fetus is intrapartum fetal distress that is a consequence of cord compression associated with oligohydramnios. Such fetal distress is typically manifest as prolonged and variable fetal heart rate decelerations. Another reason for increased risk to the fetus undelivered at 42 weeks is preexisting but recognized fetal growth restriction and stillbirth.

Postmaturity Syndrome

The postmature newborn infant has a characteristic appearance that includes wrinkled, patchy, peeling skin, and a long, thin wasting body suggesting advanced maturity. Some of these infants are seriously ill due to birth asphyxia and meconium aspiration. The belief persists that postmaturity syndrome is secondary to placental senescence. However, other investigators have not been able to demonstrate histological degeneration of the placenta, and to date no morphological or quantitative changes in the postterm placenta have been found. In fact, the postterm fetus continues to gain weight in most cases, although at a slower rate than at earlier gestational ages and is at risk for macrosomia. This continued growth suggests placental function is not compromised in the majority of postterm pregnancies.

MANAGEMENT

It is generally accepted that antepartum interventions are indicated in the management of prolonged pregnancies. The types of intervention include elective induction of labor and antepartum fetal testing (see Chapter 12). The exact timing and type of intervention, however, is uncertain and the source of controversy.

Induction versus Fetal Testing

Recognizing the risks associated with postterm pregnancy, termination of pregnancy has been recommended by many authorities before morbidity or mortality occurs. The fear of increased cesarean rates due to induction of labor has led to the use of antenatal testing while pursuing expectant management. Randomized trials have compared routine induction with expectant management and fetal testing, and provide support for either management approach.

Intervention at 41 versus 42 Weeks

Should intervention (induction or fetal testing) be used at 41 or 42 weeks’ gestation? Evidence to substantiate when to intervene is limited. No randomized study to date has shown that intervention employed before 42 weeks is beneficial. In fact, some evidence exists that intervention prior to 42 weeks may cause some harm through increased cesareans without improvement in neonatal outcome.

Oligohydramnios

Fetal jeopardy is more common in postterm pregnancies complicated by oligohydramnios. Although there is no doubt the fetus is at risk in the presence of oligohydramnios, the standard to use for diagnosis of oligohydramnios is not universally agreed upon. Criteria proposed include measuring with ultrasound the largest vertical pocket less than 1 or 2 cm, a four-quadrant amnionic fluid index (AFI) of less than 5 or 6 cm or an AFI less than the 5th percentile. It has been suggested that normal amnionic fluid should not provide false reassurance of fetal well-being as amnionic fluid volume can decrease suddenly.

Recommendations

FIGURE 37-1 shows the American College of Obstetricians and Gynecologists (2004) recommendations for evaluation and management of prolonged pregnancies. Figure 37-1 summarizes the management scheme of postterm pregnancy that has been successfully used at Parkland Hospital for the past 20 years.

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FIGURE 37-1 Management of postterm pregnancy—summary of recommendations of the American College of Obstetricians and Gynecologists (2004). aSee text for options; bprostaglandins may be used of cervical ripening or induction. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Labor is a particularly dangerous time for the postterm fetus. It is therefore important for postterm women to come to the hospital as soon as they suspect labor. Upon arrival, the fetal heart rate and uterine contractions should be electronically monitored. Identification of thick meconium in the amnionic fluid is particularly worrisome in the postterm fetus. This is minimized with effective suctioning of the pharynx as soon as the head is delivered. The trachea should be suctioned as soon as possible after delivery. At times, the continued growth seen in the postterm pregnancy will result in a large-for-gestational age infant, and shoulder dystocia may develop. Therefore, an experienced obstetrician should be available to help manage this condition.


For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 37, “Postterm Pregnancy.”