Estimation of Fetal Well-Being

Indications for Surveillance

Intrauterine fetal death may result from a wide range of potential etiologies, including congenital structural malformations, genetic abnormalities, fetomaternal hemorrhage, infection (TORCH and chorioamnionitis), umbilical cord obstruction, placental abruption, and uteroplacental insufficiency. The methods commonly used for antenatal fetal surveillance rely on fetal biophysical parameters that are sensitive to hypoxemia and acidemia, such as heart rate and movement. Thus, it is primarily useful in a fetus at risk for hypoxemia specifically because of chronic uteroplacental insufficiency. Intrauterine demise from sudden catastrophic events, such as abruption, maternal trauma, or cord occlusion, is likely not predictable by antepartum monitoring.

The indications for antenatal testing are those that increase the risk of uteroplacental insufficiency, many of which are listed in Table 12.1 . The optimal antenatal testing strategy for each of these would be beyond the scope of a single chapter, and additionally in many circumstances the exact strategy is controversial, because there is often little or no prospective or randomized data from which to determine an optimal approach. Many conditions for which testing has been suggested are those for which epidemiologic studies have identified an increased risk of intrauterine demise. However, in some circumstances the risk of stillbirth, although achieving statistical significance in large studies, may remain small in actual magnitude. Additionally, an association between a particular risk factor and stillbirth alone does not necessarily demonstrate that there is a benefit from antenatal surveillance, because that would require a specific study of antepartum testing for the given risk factor. For example, maternal obesity is associated with an increased risk of stillbirth, but because there are few or no prospective interventional studies, exact recommendations for monitoring are not available from professional societies.

Of note, uterine contraction monitoring (tocometry) is performed simultaneously with electronic fetal cardiac monitoring as part of the non-stress and contraction stress test. This is primarily to allow for the interpretation of fetal heart rate decelerations relative to uterine contractile activity. Uterine contraction monitoring alone as a method of identifying patients at increased risk of preterm birth is of low clinical utility.

Physiologic Basis for Antenatal Surveillance

The application and interpretation of antepartum fetal monitoring necessitates an understanding of the progressive fetal changes that occur secondary to increasing placental insufficiency progressing to intrauterine demise. In experiments involving animal and human fetuses, hypoxemia and acidosis have been shown consistently to alter fetal biophysical parameters such as heart rate, movement, breathing, and tone. The fetal heart rate (FHR) is normally controlled by the fetal central nervous system (CNS) and mediated by sympathetic or parasympathetic nerve impulses originating in the fetal brainstem. The presence of intermittent FHR accelerations associated with fetal movement is believed to be an indicator of an intact fetal autonomic nervous system. In historic studies of fetal blood sampling of pregnancies resulting in healthy neonates, Weiner and colleagues established a range of normal fetal venous pH measurements. In this population, the lower 2.5 percentile of fetal venous pH was 7.37. Manning and colleagues showed that fetuses without heart rate accelerations had a mean umbilical vein pH of 7.28 (±0.11), and fetuses with abnormal movement had a mean pH of 7.16 (±0.08). These and similar observations were the basis for the development of antenatal fetal testing modalities that are currently in use.

Patient Assessment of Fetal Movement

The patient's own subjective assessment of the fetal activity is perhaps the simplest and most universal of antepartum surveillance methods, although its subjective nature leads to difficulty in quantification and empiric evaluation. As described above, fetal movement decreases with increasing hypoxia, which also serves as the physiologic basis of the biophysical profile as described below. As a result, a decrease in subjective fetal movement should usually be evaluated. Beyond this generalized recommendation, various formalized strategies of fetal monitoring (colloquially referred to as “ kick counts”) have been proposed. One of the more common is to determine the time it takes for the perception of 10 movements during a period of specific, restful evaluation and to contact one's provider if the duration is 2 hours or more. However, systematic reviews have identified neither an optimal strategy nor clear evidence that routine, quantified fetal movement assessment can prevent stillbirth.

Non-Stress Test

In most institutions, the first-line assessment tool for fetal surveillance is the non-stress test (NST). In the outpatient setting, the patient typically rests in a reclining chair with a lateral tilt or semi-Fowler. Ideally she should have not recently smoked. Although commonly provided in antepartum testing units, the maternal ingestion of juice or food has not been demonstrated to increase the probability of a reactive non-stress test. The FHR is monitored with an external transducer for up to 40 minutes and observed for the presence of accelerations above the baseline. A reactive test is one in which there are at least two accelerations that peak 15 beats/min above the baseline and last (not at the peak) for at least 15 seconds before returning to baseline ( Fig. 12.1 ), colloquially referred to as “15 × 15.” Most NSTs are reactive within the first 20 minutes. For tests that are not, possibly because of a fetal sleep cycle, an additional 20 minutes of monitoring may be needed. A nonreactive NST is one in which two such accelerations do not occur within 40 minutes.

The optimal gestational age at which to begin antenatal surveillance depends on the clinical condition. In making this decision, the physician must weigh the risk of intervention at a premature gestational age against the risk of intrauterine fetal death. Initiation at 32-34 weeks’ gestation is reasonable for most at-risk patients, with the acknowledgment that some situations may warrant testing earlier at 26-28 weeks of gestation. FHR variability and reactivity vary with gestational age, with an increasing frequency of tracings that lack “15 × 15” accelerations with decreasing gestational age under 32 weeks. Thus, prior to 32 weeks, non-stress tests are considered reactive if there are two accelerations that peak 10 beats/min above baseline and last for at least 10 seconds (“10 × 10”). Of note, the magnitude of accelerations in fetuses less than 32 weeks can vary normally over time, thus a fetus at less than 32 weeks is reactive by 10 × 10 criteria even if it had previously demonstrated 15 × 15 accelerations.

Although the NST is noninvasive and easy to perform, it is limited by a high false-positive rate. Normal fetuses often have periods of nonreactivity because of benign variations such as sleep cycles. Vibroacoustic stimulation may be used safely in the setting of a nonreactive NST to elicit FHR accelerations without compromising the sensitivity of the NST. In this situation, the operator places an artificial larynx on the maternal abdomen and activates the device for 1-3 seconds. This technique is often useful in situations in which the FHR has normal beat-to-beat variability and no decelerations but does not show any accelerations. If the test remains nonreactive, further evaluation with a biophysical profile or contraction stress test (CST) is warranted as long as the FHR is otherwise reassuring. However, if the tracing is overtly concerning (see Category III later in this chapter), additional testing may need to be deferred in favor of delivery depending upon the exact gestational age and clinical circumstance.

Contraction Stress Test

The CST is designed to evaluate FHR response to maternal uterine contractions. The principles that are applied to the evaluation of intrapartum FHR monitoring (see FHR Monitoring ) are used here. In response to the stress of the contraction, a hypoxemic fetus shows FHR patterns of concern, such as late decelerations.

Similar to the NST, for the CST the patient is placed in a recumbent tilted position, and FHR is monitored with an external fetal monitor. The FHR pattern is evaluated while the patient experiences at least three contractions lasting 40 seconds within a 10-minute period. If the patient is not contracting spontaneously, contractions may be induced with nipple stimulation or intravenous oxytocin. Nipple stimulation can be self-administered by the patient or a breast pump can be used. If no late or significant variable decelerations are noted on FHR tracing, CST is considered to be negative. If there are late decelerations after at least 50% of the contractions, CST is positive. If late decelerations are present less than 50% of the time, or if significant variable decelerations are present, the test is considered to be equivocal. Contraindications to the performance of CST include clinical situations in which labor would be undesirable (e.g., placenta previa or previous classic cesarean section).

Biophysical Profile

The biophysical profile is based upon the general principle that a progressively hypoxic fetus will be less likely to engage in elective activities such as breathing or body motions, while an active fetus is likely also healthy. As originally described, it combines NST with four components evaluated by ultrasound ( Box 12.1 ). In a 30-minute period, either 2 or 0 points are assigned depending upon if the criteria are fulfilled or unfulfilled. The final score will thus range from 0-10 with no odd numbers (either 0 or 2; 1 point is never awarded for each criteria). A combined score of 8 or 10 indicates fetal well-being. A score of 6 is considered to be equivocal; it usually merits delivery if the pregnancy is at term or additional or repeat (24 hours) testing if the pregnancy is preterm. A score of 4 or less is considered to be abnormal, and in the absence of reversible causes, consideration would need to be given to delivery except in uncommon extenuating circumstances. Of note, one of the five criteria in the BPP is the amniotic fluid volume. This can complicate interpretation of the testing in situations where the fluid volume is low for reasons that are independent of placental function, such as membrane rupture or congenital anomalies of the urinary tract.

Of note, fetal breathing is not usually present during active labor, to the extent that the absence of fetal breathing on ultrasound had previously been evaluated as an assessment tool for the presence of “true” preterm labor. Thus, biophysical profiles are not performed in patients who are actively laboring.