Freeman (1975) and Lee and colleagues (1975) introduced the nonstress test to describe fetal heart rate acceleration in response to fetal movement as a sign of fetal health. This test involved the use of Doppler-detected fetal heart rate acceleration coincident with fetal movements perceived by the mother. By the end of the 1970s, the nonstress test had become the primary method of testing fetal health. The nonstress test was easier to perform, and normal results were used to further discriminate false-positive contraction stress tests. Simplistically, the nonstress test is primarily a test of fetal condition, and it differs from the contraction stress test, which is considered a test of uteroplacental function. Currently, nonstress testing is the most widely used primary testing method for assessment of fetal well-being. It has also been incorporated into the biophysical profile testing system, subsequently discussed.
Fetal Heart Rate Acceleration
Autonomic influences are mediated by sympathetic or parasympathetic impulses from brainstem centers to normally raise or slow the fetal heart rate. Beat-to-beat variability is also under the control of the autonomic nervous system (Matsuura, 1996). Consequently, pathological loss of fetal heart rate acceleration may be seen in conjunction with significantly decreased beat-to-beat variability (Chap. 24, Cardiac Arrhythmia). Loss of such reactivity, however, is most commonly associated with sleep cycles. It also may be caused by central depression from medications or cigarette smoking (Jansson, 2005).
The nonstress test is based on the hypothesis that the heart rate of a fetus that is not acidemic as a result of hypoxia or neurological depression will temporarily accelerate in response to fetal movement. Fetal movements during testing are identified by maternal perception and recorded. As hypoxia develops, these fetal heart rate accelerations diminish (Smith, 1988).
Gestational age influences acceleration or reactivity of the fetal heart rate. Pillai and James (1990b) studied the development of fetal heart rate acceleration patterns during normal pregnancy. The percentage of body movements that is accompanied by accelerations and the amplitude of these accelerations both increase with gestational age (Fig. 17-5). Guinn and colleagues (1998) studied nonstress test results between 25 and 28 weeks’ gestation in 188 normal fetuses. Only 70 percent of these normal fetuses demonstrated the required 15 beats per minute (bpm) or more of heart rate acceleration. Lesser degrees of acceleration, that is, 10 bpm, occurred in 90 percent of the fetuses.
Percentage of fetuses with at least one acceleration of 15 bpm sustained for 15 seconds concurrent with fetal movement. (Redrawn from Pillai M, James D: The development of fetal heart rate patterns during normal pregnancy, Obstet Gynecol. 1990 Nov;76(5 Pt 1):812–816.)
The National Institute of Child Health and Human Development Fetal Monitoring Workshop defined normal acceleration based on gestational age (Macones, 2008). In fetuses at or beyond 32 weeks’ gestation, the acceleration acme is 15 bpm or more above the baseline rate, and the acceleration lasts 15 seconds or longer but less than 2 minutes. Before 32 weeks, normal accelerations are defined as having an acme that is 10 bpm or more above baseline for 10 seconds or longer. Cousins and associates (2012) compared the Workshop criteria recommended before 32 weeks, that is, 10 bpm/10 seconds, with standard 15 bpm/15 seconds criteria in a randomized trial of 143 women. They found no differences in perinatal outcomes.
Criteria to define normal nonstress test results differ. They vary regarding the number, amplitude, and duration of accelerations and the test duration. The definition recommended by the American College of Obstetricians and Gynecologists (2016) requires two or more accelerations peaking at 15 bpm or more above baseline, each lasting 15 seconds or more, and all occurring within 20 minutes of beginning the test (Fig. 17-6). It is also recommended that accelerations with or without fetal movements be accepted, and that a 40-minute or longer tracing—to account for fetal sleep cycles—should be performed before concluding that fetal reactivity is insufficient. Miller and coworkers (1996b) reviewed outcomes in fetuses with nonstress tests considered as nonreactive because there was only one acceleration. They concluded that one acceleration was just as reliable as two in predicting healthy fetal status.
Reactive nonstress test. In the upper panel, notice the increase of fetal heart rate by more than 15 beats/min for longer than 15 seconds following fetal movements, which are indicated by the vertical marks (lower panel).
Although a normal number and amplitude of accelerations seems to reflect fetal well-being, their absence does not invariably predict fetal compromise. Indeed, some investigators have reported 90-percent or higher false-positive rates (Devoe, 1986). Because healthy fetuses may not move for periods of up to 75 minutes, some have considered that a longer duration of nonstress testing might increase the positive-predictive value of an abnormal, that is, nonreactive, test (Brown, 1981). In this scheme, either the test became reactive during a period up to 80 minutes or the test remained nonreactive for 120 minutes, which indicated that the fetus was very ill.
Not only do definitions of normal nonstress test results differ, but the reproducibility of interpretations is problematic (Hage, 1985). Thus, although nonstress testing is popular, the reliability of test interpretation needs improvement.
Based on the foregoing, an abnormal nonstress test is not always ominous and can be seen with a sleeping fetus. Also, an abnormal test can revert to normal as the fetal condition changes, such as the example shown in Figure 17-7. Importantly, a normal nonstress test can become abnormal if the fetal condition deteriorates.
Two antepartum fetal heart rate (FHR) tracings in a 28-week pregnant woman with diabetic ketoacidosis. A. FHR tracing (upper panel) and accompanying contraction tracing (second panel). Tracing, obtained during maternal and fetal acidemia, shows absence of accelerations, diminished variability, and late decelerations with weak spontaneous contractions. B. Fetal heart rate tracing shows return of normal accelerations and variability of the fetal heart rate following correction of maternal acidemia.
There are abnormal patterns that reliably forecast severe fetal jeopardy (Fig. 17-8). Devoe and coworkers (1985) concluded that nonstress tests that were nonreactive for 90 minutes were almost invariably—93 percent—associated with significant perinatal pathology. Hammacher and coworkers (1968) described tracings with what they termed a silent oscillatory pattern that he considered dangerous. This pattern consisted of a fetal heart rate baseline that oscillated less than 5 bpm and presumably indicated absent acceleration and beat-to-beat variability.
Nonreactive nonstress test (left side of tracing) followed by contraction stress test showing mild, late decelerations (right side of tracing). Cesarean delivery was performed, and the severely acidemic fetus could not be resuscitated.
Visser and associates (1980) described a terminal cardiotocogram, which included: (1) baseline oscillation of less than 5 bpm, (2) absent accelerations, and (3) late decelerations with spontaneous uterine contractions. These results were similar to experiences from Parkland Hospital in which absence of accelerations during an 80-minute recording period in 27 fetuses was associated consistently with evidence of uteroplacental pathology (Leveno, 1983). The latter included fetal-growth restriction in 75 percent, oligohydramnios in 80 percent, fetal acidemia in 40 percent, meconium in 30 percent, and placental infarction in 93 percent.
Set originally rather arbitrarily at 7 days, the interval between tests appears to have been shortened as experience evolved with nonstress testing. According to the American College of Obstetricians and Gynecologists (2016), more frequent testing is advocated by some investigators for women with postterm pregnancy, multifetal gestation, pregestational diabetes, fetal-growth restriction, or pregnancy hypertension. In these circumstances, some investigators perform twice-weekly tests, with additional testing completed for maternal or fetal deterioration regardless of the time elapsed since the last test. Others perform nonstress tests daily or even more frequently, such as with severe preeclampsia remote from term.
Decelerations During Nonstress Testing
Fetal movements commonly produce heart rate decelerations. Timor-Tritsch and associates (1978) reported this during nonstress testing in half to two thirds of tracings, depending on the vigor of the fetal motion. This high incidence of decelerations inevitably makes interpretation of their significance problematic. Indeed, Meis and coworkers (1986) reported that variable fetal heart rate decelerations during nonstress tests were not a sign of fetal compromise. The American College of Obstetricians and Gynecologists (2016) has concluded that variable decelerations, if nonrepetitive and brief—less than 30 seconds—do not indicate fetal compromise or the need for obstetrical intervention. In contrast, repetitive variable decelerations—at least three in 20 minutes—even if mild, have been associated with a greater risk of cesarean delivery for fetal distress. Decelerations lasting 1 minute or longer have been reported to have an even worse prognosis (Bourgeois, 1984; Druzin, 1981; Pazos, 1982).
Hoskins and associates (1991) attempted to refine interpretation of testing that shows variable decelerations by adding sonographic estimation of amnionic fluid volume. The incidence of cesarean delivery for intrapartum fetal distress progressively rose concurrently with the severity of variable decelerations and decline of amnionic fluid volume. Severe variable decelerations during a nonstress test plus an amnionic fluid index (AFI) ≤5 cm resulted in a 75-percent cesarean delivery rate. Fetal distress in labor, however, also frequently developed in those pregnancies with variable decelerations but with normal amounts of amnionic fluid. Similar results were reported by Grubb and Paul (1992).
False-Normal Nonstress Tests
Smith and associates (1987) performed a detailed analysis of the causes of fetal death within 7 days of normal nonstress tests. The most common indication for testing was postterm pregnancy. The mean interval between testing and death was 4 days, with a range of 1 to 7 days. The single most common autopsy finding was meconium aspiration, often associated with some type of umbilical cord abnormality. They concluded that an acute asphyxial insult had provoked fetal gasping. They also concluded that nonstress testing was inadequate to preclude such an acute asphyxial event and that other biophysical characteristics might be beneficial. Importantly, assessment of amnionic fluid volume was considered valuable. Other ascribed frequent causes of fetal death included intrauterine infection, abnormal cord position, malformations, and placental abruption.