Assessment of Fetal Health

Fetal Heart Rate Changes During Development

As gestation advances, the fetal heart rate (FHR) is increasingly dominated by the parasympathetic system, resulting in a gradual decrease in heart rate, increase in variability, and increasing responsiveness to acute changes in fetal status, including accelerations and decelerations. The ability of the fetus to accelerate its heart rate in response to movement is related to fetal oxygenation and metabolic state; this provides the basis for nonstress testing.

Fetal Behavior

Fetal movements appear and change in complexity over time, with tone and movement observable in the first trimester and breathing becoming evident at 21 weeks’ gestation and beyond. By the end of pregnancy, defined kicking, hand movements, fetal breathing, and a variety of individual behaviors can be demonstrated. The presence of normal fetal muscle tone, gross body movements, and breathing have been reliably tied to the absence of fetal hypoxemia and acidemia, which is the basis for the BPP.

By the beginning of the third trimester, fetal behavioral states ^, analogous to the neonatal behavioral states originally described by Prechtl can be defined. Two patterns dominate: quiet sleep and active sleep. In quiet sleep, rapid eye movements and repetitive mouthing movements are present, but almost all other movements are absent. As term approaches, the time spent with this level of inactivity extends from a mean of about 220 seconds in midtrimester to as long as 110 minutes by 40 weeks’ gestation. During the state of active sleep, movements are grouped, providing efficient monitoring, because multiple activities overlap. In the active awake state, the “jogging fetus” illustrates a high level of voluntary activity and a sustained high heart rate, for which return to baseline may be difficult to differentiate from decelerations. As with its neonatal equivalent, the quiet awake state is unusual and short and is seldom observed before term. Understanding these behavioral states and particularly the patterns of time spent in particular states is useful for defining normal behavior, but these descriptions are rarely used clinically.

Near term, periods of inactivity that correspond to episodes spent in the quiet sleep state, as illustrated by the nonreactive nonstress test (NST) shown in Fig. 32.1 , can be confused with fetal compromise. Although the quiet sleep state and the nonreactive NST result usually resolve by 40 minutes, intervals of up to 2 hours are likely normal. The definition of these abnormal periods depends on the modality of testing. Real-time ultrasound observation of fetal activity often reveals frequent, small movements of hands, mouth, and trunk when nonstress testing has elicited concern regarding inactivity. This illustrates the importance of using multiple methods of fetal assessment to decrease the false-positive rate of any one test.

Fetal Heart Rate Monitoring

FHR monitoring is a valuable component of virtually all multivariable fetal assessment schemes. It relies on the unique coupling of fetal neurologic status to cardiovascular reflex responses. Because many studies have shown it to be the most sensitive short-term predictor of worsening hypoxemia or acidosis, ^, it has become part of fetal monitoring of labor and delivery. The range of FHR tracings that can be obtained for different fetuses and even for a single fetus over time is significant, which makes combination testing with ultrasound useful in many situations.

The combination of fetal movements and FHR acceleration provides the basis of the NST. The classic criteria for a reactive NST result are at least two FHR accelerations lasting at least 15 seconds and rising at least 15 beats/min above the established baseline heart rate. Most term fetuses have many of these accelerations in each 20- to 30-minute period of active sleep, and the term fetus seldom goes more than 60 minutes and certainly not more than 100 minutes without meeting these criteria. However, preterm fetuses, fetuses with growth restriction (FGR), or fetuses exposed to maternal medications such as narcotics or magnesium sulfate frequently have paired FHR acceleration and fetal movements that do not meet these criteria. Modification of these criteria based on gestational age (e.g., including accelerations of 10 beats/min lasting 10 seconds in a background of normal FHR variability for fetuses <32 weeks’ gestation) accepts the principle that younger fetuses have smaller accelerations but that they should always demonstrate some degree of FHR acceleration with documented/palpated fetal movements.

Falsely reassuring NST results (i.e., false-negative screening test results), as defined by fetal death within 1 week, occurred at a rate of 1.9:1000 fetuses in the largest study. Although fetal nonstress testing in isolation has frequently been the first-line test for postterm pregnancies, additional fetal assessment may improve sensitivity. In particular, incorporating an assessment of amniotic fluid volume may help to identify additional pregnancies at risk.

The nonreactive NST result is defined by an FHR monitoring interval that does not meet the criteria previously described. However, there is a large variation in the total duration allowed, ranging from a minimum of 10 minutes of monitoring to 40 or 60 minutes according to some investigators. In the context of the BPP, 30 minutes of monitoring is allowed for the NST to demonstrate reactivity. About 10% to 12% of fetuses in the third trimester do not meet these criteria at 30 minutes, but this number falls below 6% by 40 minutes. The choice of maximum duration for the NST is critical in determining the rate of false-positive screening test results, which is the major clinical drawback of using the NST. The most common explanation for a nonreactive NST result is a sleep cycle in a normal fetus that is longer than average. A nonreactive result, especially if FHR variability is preserved and there are no decelerations, should not be assumed to indicate fetal compromise. Ultrasound evaluation with a BPP should be available as the backup test.

Beyond the BPP, ultrasound provides additional fetal evaluation that may help to diagnose the reason for an apparently abnormal NST result. For example, a repeatedly nonreactive finding for a fetus with normal serial ultrasound assessments may lead to a diagnosis of central nervous system abnormalities, drug exposure, or prior fetal central nervous system injury. Late decelerations or variable decelerations may occur in the context of NST monitoring, with no clear demonstration of contractions. Either pattern should lead to evaluation by ultrasound to exclude FGR and oligohydramnios.

Biophysical Profile

The BPP relies on the premise that multiple parameters of well-being are better predictors of outcome than any single parameter. Fig. 32.2 shows a detailed evaluation of outcome variables performed during development. ^, The traditional BPP study includes five variables ( Table 32.1 ), with a total possible score of 10, but several variations have been proposed. The most frequently utilized is the modified BPP, which usually includes fetal heart rate monitoring and amniotic fluid evaluation. ^, These approaches emphasize the principle of multivariable fetal assessment.

Biophysical Profile Score Variables

Amniotic Fluid Measurement

The physiologic principle connecting decreased amniotic fluid volume to fetal compromise is the understanding that fetal oliguria in an anatomically normal fetus is a consequence of redistribution of fetal blood flow away from the kidneys and is frequently a reflection of uteroplacental insufficiency.

Many methods of assessing amniotic fluid volume have been suggested. The technique for determining the BPP requires assessment of a single adequate pocket of fluid. With the transducer vertical to the maternal abdomen, the maximum vertical depth of a clear amniotic fluid pocket is recorded. The transducer is then rotated 90 degrees in the same vertical axis, confirming that the measured pocket has true biplanar dimensions. The phrase 2 × 2 pocket does not mean that the pocket is 2 cm deep and 2 cm wide; it refers to the documentation that the pocket is 2 cm deep in at least two intersecting ultrasound planes, avoiding the possibility that a sliver of amniotic fluid is misconstrued as a true three-dimensional pocket. Amniotic fluid is measured in real time, and when there is doubt about a true pocket, it is confirmed by pulsed Doppler. Continuous color imaging may lead to the false impression of oligohydramnios ( Fig. 32.3 ). This method reflects the relative amount of amniotic fluid and was not meant for determining an absolute physiologic parameter.

Amniotic Fluid Volume

A deepest vertical pocket (DVP) that is less than 2 cm or more than 8 cm suggests oligohydramnios or polyhydramnios, respectively; in this setting, a detailed fetal evaluation is suggested to exclude anatomic and anomalous explanations. For moderately increased fluid (i.e., maximum vertical pocket depth of 8 to 12 cm), the most common explanations are idiopathic polyhydramnios, fetal macrosomia resulting from maternal diabetes, and structural abnormalities, and fetal testing is likely to reflect fetal neurologic and acid-base status accurately. For pockets deeper than 12 cm in singleton pregnancies, neurologic issues, structural defects, and chromosomal abnormalities are more likely (especially if associated with fetal growth restriction), and the BPP may not predict the neonatal outcome. Through the normal range (i.e., maximum vertical pocket depth of 3 to 8 cm), the maximum vertical depth method assigns normal status accurately, although it may not correlate precisely with absolute volumes.

Diagnosis of Oligohydramnios

The original criterion for the diagnosis of oligohydramnios was a maximum vertical pocket of only 1 cm. Although this finding highly correlated with FGR, it was so uncommon as to be clinically meaningless. A meta-analysis identified four high-quality randomized controlled trials (RCTs) that compared the amniotic fluid index (AFI) with the single DVP with respect to preventing adverse pregnancy outcome. The limits used were an AFI less than 5 cm and a DVP less than 2 × 1 cm. The trials included 3125 participants, with the primary outcome measure defined as admission to the neonatal intensive care unit. No difference was observed for the primary outcome (risk ratio [RR] = 1.04; confidence interval [CI], 0.85 to 1.26). When the AFI was used for fetal surveillance, however, the diagnosis of oligohydramnios was made more frequently (RR = 2.33; CI, 1.67 to 3.24); labor induction was used more frequently (RR = 2.1; CI, 1.6 to 2.76), and there was a higher rate of cesarean deliveries for lack of assurance of fetal well-being (RR = 1.45; CI, 1.07 to 1.97). There were no differences in Apgar scores, umbilical artery pH (<7.1), or nonreassuring FHR tracings. The study authors concluded that the DVP seemed to be superior to the AFI for fetal surveillance because it resulted in less intervention and resulted in a similar perinatal outcome.

Fetal Breathing Movements

Rhythmic fetal diaphragm contractions or hiccups lasting more than 30 seconds meet normal criteria for fetal breathing movements. This fetal behavior is the one most easily suppressed by hypoxemia, but it is also the most episodic in normal fetuses. Because the amplitude of fetal breathing depends on gestational age, maternal glucose levels, exposure to increased oxygen concentrations, and many medications (e.g., corticosteroids, magnesium), careful evaluation of all parameters and consideration of potential etiologies is necessary before intervention is undertaken. The unusual presence of continuous, monotonous fetal breathing or fetal gasping, with complete absence of all other behavior for an extended period, may indicate acidosis, especially in the fetus of a diabetic mother.

Fetal Movement and Tone

One of the pitfalls in the interpretation of the BPP is that some movement must be present to evaluate tone. Tone is not simply the flexed posture of a normal fetus. The evaluation of tone is subjective, but absent tone strongly correlates with fetal acidosis.

Fine motor movement of the face and hands and purposeful movement such as swallowing, facial expressions, sucking, yawning, large kicks, small kicks, and rolling motions may be included as movements. When a fetus does not move for a period of 30 minutes, extended testing is required, and completion of fetal heart rate testing can help to extend the time of continuous observation. Further intervention is dictated by the clinical situation and gestational age.