Perinatal Morbidity and Mortality
Although studies from the 1960s indicated a twofold increase in the perinatal mortality rate beginning at 42 weeks and a fourfold increase by 44 weeks’ gestation, more recent evidence suggests that the risk of fetal death in the postdates pregnancy monitored by serial nonstress tests (NSTs) is about 2 in 1000.4 Nevertheless, the postdates pregnancy is associated with an increased rate of postmaturity syndrome, fetal malformations, fetal macrosomia, and meconium aspiration syndrome.
Meta-analyses of randomized controlled trials demonstrate that elective induction at 41+ weeks’ gestation is associated with a reduced perinatal mortality rate that translates into one life saved for every 500 elective inductions.4-5 In these studies the control or expectant management groups generally underwent fetal surveillance using NSTs. The patients who underwent induction did not experience increased cesarean section (CS) rates, although the baseline CS rates in these institutions were about 20%. The effect on the incidence of cesarean intervention in institutions where baseline CS rates are much lower is unknown. It is also important to note that this reduction in perinatal mortality has not been demonstrated when elective inductions are carried out between 39 and 41 weeks’ gestation.5 Finally, the viewpoints of women regarding these management options have not been adequately studied.4
Expectant Management: Fetal Surveillance
As discussed above, the NST, performed at least weekly, has been the most commonly used form of antenatal surveillance, resulting in a perinatal mortality rate of 2 in 1000 postdate pregnancies. There is preliminary evidence that the combination of the NST plus measurement of the amniotic fluid index (AFI), known as a modified biophysical profile, yields clinical information comparable to the contraction stress test (increased sensitivity, or fewer false negatives) and may lower the perinatal mortality rate to 1 in 1000.6 Although Doppler flow studies have been shown to decrease perinatal mortality rates in selected high risk conditions, such as intrauterine growth retardation, there is no evidence at this time to support its use for surveillance of the postdates pregnancy.
The standard method of induction uses oxytocin, of which there are two accepted regimens endorsed by the American College of Obstetricians and Gynecologists (ACOG).7 Cervical ripening with prostaglandin E2 (PGE2) gel decreases prolonged induction, failed induction, and operative delivery rates.8
Intrapartum management of the postdates pregnancy takes into account an increased incidence of meconium passage, oligohydramnios, and macrosomia.9 The first two conditions, if clinically significant, may be treated with the use of amnioinfusion, described later in the chapter. It is one of only a few conditions where continuous electronic fetal monitoring (EFM) results in improved neonatal outcomes if fetal scalp sampling is used as an adjunct to evaluate the persistently nonreassuring fetal tracing. If intermittent fetal monitoring is employed, ACOG recommends its application every 15 minutes during stage 1 and every 5 minutes in stage 2.