FIGURE 43-4 A. Prolonged fetal heart rate deceleration before emergency cesarean delivery in a postterm pregnancy with oligohydramnios. B. Severe-less than 70 bpm for 60 seconds or longer-variable decelerations in a postterm pregnancy with oligohydramnios. C. Saltatory baseline fetal heart rate showing oscillations exceeding 20 bpm and associated with oligohydramnios in a postterm pregnancy. (Reproduced with permission from Leveno J, Quirk JG, Cunningham FG, et al: Prolonged pregnancy, I. Observations concerning the causes of fetal distress, Am J Obstet Gynecol. 1984 Nov 1 ;1o50(5 Pt 1 ):465-473.) decelerations such as shown in of emergency cesarean deliveries for nonreassuring fetal heart rate tracings. In all but two cases, there were also variable decelerations. Another common fetal heart rate pattern, although not ominous by itself, was the saltatory baseline. As described in Chapter 24 (p. 468), these indings are consistent with cord occlusion as the proximate cause of the nonreassuring tracings. Other correlates included oligohydramnios and viscous meconium. Schafer and colleagues (2005) implicated a nuchal cord in abnormal intrapartum fetal heart rate patterns, meconium, and compromised newborn condition in prolonged pregnancies.
he volume of amnionic luid normally continues to decline ater 38 weeks and may become problematic. Moreover, meconium release into an already reduced amnionic luid volume results in thick, viscous meconium that may cause meconium aspiration syndrome (Chap. 33, p. 620).
Trimmer and coworkers (1990) sonographically measured hourly fetal urine production using sequential bladder volume measurements in 38 postterm pregnancies. Diminished urine production was found to be associated with oligohydramnios. They hypothesized that decreased fetal urine flow was likely the result of preexisting oligohydramnios that limited fetal swallowing. Oz and associates (2002), using Doppler waveforms, concluded that fetal renal blood low is reduced in those postterm pregnancies complicated by oligohydramnios. s a possible cause, again, the study by Link and associates (2007) showed that umbilical blood flow did not increase past term.
In the late 19905, the clinical significance of fetal-growth restriction in the otherwise uncomplicated pregnancy became more fully appreciated. Divon (1998) and Clausson (1999) and their coworkers analyzed births between 1991 and 1995 in the National Swedish Medical Birth Registry. Stillbirths were more common among growth-restricted newborns who were delivered after 42 weeks. Indeed, a third of postterm stillborn neonates were growth restricted. During this time in Sweden, labor induction and antenatal fetal testing usually commenced at 42 weeks. In a study from Parkland Hospital, Alexander and colleagues (2000d) analyzed outcomes for 355 neonates from pregnancies ::42 weeks and whose birthweights were <3rd percentile. hey compared these with outcomes of 14,520 similarly aged newborns above the 3rd percentile and found that morbidity and mortality rates were significantly increased in the growth-restricted neonates. Notably, a fourth of all stillbirths associated with prolonged pregnancy were in this comparatively small number of growth-restricted fetuses.
In the event of a medical or other obstetrical complication, it is generally not recommended that a pregnancy be allowed to continue past 42 weeks. Indeed, in many such instances, earlier delivery is indicated. Common examples include gestational hypertensive disorders, prior cesarean delivery, and diabetes. Other clinically important factors include amnionic luid volume and potential fetal macrosomia.