In this chapter, the nonstress test, contraction stress test, and components of the biophysical profile are reviewed. Doppler velocimetry is discussed in Chapter 9. The goal of antepartum fetal surveillance is to prevent fetal death. In most cases, a normal test result is highly reassuring, because fetal deaths within 1 week of a normal test result are rare. There is no overall agreement regarding the best test to evaluate fetal well-being. All three testing systems—contraction stress test, nonstress test, and biophysical profile—have different end points that are considered depending on the clinical situation. Important considerations in deciding when to begin testing are the prognosis for neonatal survival and the severity of maternal disease. For the majority of high-risk pregnancies, most recommend that testing begin by 32 to 34 weeks, though pregnancies with severe complications might require testing as early as 26 to 28 weeks. The frequency for repeating tests has been arbitrarily set as 7 days, but more frequent testing is often done. Importantly, the widespread use of antepartum fetal surveillance is primarily based on circumstantial evidence, because there have been no definitive randomized clinical trials.
The nonstress test is the most widely used primary testing method for assessment of fetal well-being. It is based on the hypothesis that the heart rate of a nonacidotic fetus will temporarily accelerate in response to fetal movement. The nonstress test involves Doppler-detected acceleration of the heart rate. A normal or reactive nonstress test generally contains two or more accelerations of 15 beats per minute or more, each lasting 15 seconds or more, within 20 minutes of beginning the test (see Figure 12-1). Accelerations need not be associated with fetal movement. If the tracing is nonreactive (does not contain at least two accelerations as described), the testing period may be extended for 40 minutes or longer to account for fetal sleep cycles. The amplitude of accelerations increases with gestational age, and it is recommended that prior to 32 weeks of gestation an acceleration be defined as a fetal heart rate increase of at least 10 bpm lasting 10 seconds or longer.
FIGURE 12-1 Reactive nonstress test. In the upper panel, notice increase of fetal heart rate to more than 15 beats/min for longer than 15 s following fetal movements, indicated by the vertical marks (lower panel). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
It is important to note that whereas a reactive fetal heart rate tracing appears to predict fetal well-being, “insufficient acceleration” does not invariably predict fetal compromise. As many as 90 percent of nonreactive tests are in fact false-positive results (i.e., associated with good pregnancy outcomes). Despite widespread use of this testing method, there is remarkable difference of opinion in the interpretation of abnormal test results. However, there are abnormal nonstress test patterns that reliably forecast severe fetal jeopardy. An example of such a situation is absence of accelerations during an 80-minute recording period, particularly if seen in association with decreased baseline oscillation of the fetal heart rate (variability), or the presence of late decelerations following spontaneous uterine contractions (Figure 12-2A). This pattern has been consistently associated with uteroplacental insufficiency.
FIGURE 12-2 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. FHR tracing shows return of normal accelerations and variability of the fetal heart rate following correction of maternal acidemia. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
CONTRACTION STRESS TESTING
The contraction stress test, also known as the oxytocin challenge test, is based on the principle that uterine contractions may cause an increase in myometrial and amnionic pressure great enough to collapse myometrial vessels. Brief periods of impaired oxygen exchange result, and if uteroplacental pathology is present, contractions may elicit late fetal heart rate decelerations. This test is considered positive if there are repetitive late fetal heart rate decelerations after induced or spontaneous contractions. (see Figure 12-2). The contraction stress test may be viewed as a test of uteroplacental function, whereas the nonstress test is primarily a test of fetal condition.
To perform the test, uterine contractions and fetal heart rate are recorded. If three or more spontaneous contractions lasting at least 40 seconds are present in a 10-minute period, no uterine stimulation is necessary. Otherwise, a dilute intravenous infusion of oxytocin is initiated at a rate of 0.5 mU/min and doubled every 20 minutes until a satisfactory contraction pattern is established. Criteria for interpretation of the test are listed in Table 12-1.
TABLE 12-1. Criteria for Interpretation of the Contraction Stress Test
One disadvantage of the contraction stress test is that it requires an average of 90 minutes to complete. A similar test, the nipple stimulation test, involves the woman rubbing one nipple through her clothing for 2 minutes or until a contraction begins. She is instructed to restart after 5 minutes if the first nipple stimulation did not induce three contractions in 10 minutes. Reported advantages include reduced cost and shortened testing times.
The biophysical profile is a combination of five biophysical variables: (1) the nonstress test, (2) fetal movements, (3) fetal breathing, (4) fetal tone, and (5) amnionic fluid volume (Table 12-2). These components are each given a score of 2 if normal and 0 if abnormal, such that the highest possible score is 10. A biophysical score of 0 is invariably associated with significant fetal acidemia, whereas normal scores of 8 or 10 are associated with normal pH. A suggested protocol for interpretation and management of the biophysical profile is outlined in Table 12-3. The false-negative rate, defined as the antepartum death of a structurally normal fetus, is approximately 1 per 1000, and more than 97 percent of fetuses tested have normal results.
TABLE 12-2. Components and Their Scores for the Biophysical Profile
TABLE 12-3. Biophysical Profile Score, Interpretation, and Pregnancy Management
Unstimulated fetal activity commences as early as 7 weeks. Between 20 and 30 weeks, general body movements become organized, and the fetus also starts to have evidence of sleep–wake cycles. “Sleep cyclicity” has been described to vary from about 20 minutes to as much as 75 minutes. For the biophysical profile, at least 3 discrete fetal movements must be seen with real-time ultrasound over a 30-minute period.
Fetal breathing movements differ from those of newborns and adults in two main ways: they are discontinuous, and they are paradoxical, in that the chest wall paradoxically collapses during inspiration, and the abdomen protrudes (Figure 12-3). Although the physiological basis for the breathing reflex is not completely understood, such exchange of amnionic fluid appears to be essential for normal lung development. Of note, several variables in addition to hypoxia can affect fetal respiratory movements. These include labor—during which it is normal for respiration to cease—hypoglycemia, sound stimuli, cigarette smoking, amniocentesis, impending preterm labor, gestational age, and the fetal heart rate itself.
FIGURE 12-3 Paradoxical chest movement with fetal respiration. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. Adapted from Johnson T, Besigner R, Thomas R: New clues to fetal behavior and well-being. Contemp Ob Gyn, May 1988.)
Amnionic Fluid Volume
Assessment of amnionic fluid has become an integral component in the antepartum assessment of pregnancies at risk for fetal death. This is based on the rationale that decreased uteroplacental perfusion may lead to diminished fetal renal blood flow, decreased micturition, and ultimately, oligohydramnios.
Modified Biophysical Profile
The biophysical profile is labor intensive and usually requires 30 to 60 minutes or more to complete. A modified biophysical profile combines a nonstress test with ultrasound assessment of amnionic fluid. It may require less time and has also been found to be an excellent method of fetal surveillance. Typically, an amnionic fluid index (see Chapter 10) below 5 cm is considered abnormal. False-negative and false-positive rates are comparable to those of the standard 5-component biophysical profile.
HISTORY OF DECREASED FETAL MOVEMENT
Various methods have been described to quantify fetal movement, in an effort to prognosticate well-being. Most investigators have reported excellent correlation between maternally perceived fetal motion and movements documented by instrumentation. Neither the optimal number of movements nor the ideal duration for counting them has been defined. In one method, perception of 10 fetal movements in up to 2 hours is considered normal. A particularly bothersome clinical situation occurs when women present in the third trimester with a chief complaint of subjectively reduced fetal movement. Typically, the fetal heart rate monitoring tests described earlier are employed.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 15, “Antepartum Assessment.”