Antepartum and Intrapartum Surveillance of the Fetus and the Amniotic Fluid



Antepartum and Intrapartum Surveillance of the Fetus and the Amniotic Fluid


Maria Andrikopoulou

Michael G. Ross

Anthony M. Vintzileos



Introduction

The challenge of assessing the intrauterine fetus as a patient lies in the inability to perform a truly direct examination. Instead, one must rely on indirect modalities such as dynamic, high-resolution ultrasonography, electronic fetal heart rate (FHR) monitoring, and fetal Doppler velocimetry. Fortunately, technological and scientific advances over the years have permitted more specific examinations of the fetus and its behavior. One of the most important goals of perinatal medicine is to recognize fetal disease states, optimize fetal outcomes, and prevent perinatal morbidity and mortality by interventions and/or timing of delivery. Because fetal jeopardy has a diverse etiology, forms of testing must also be able to survey both acute and chronic disease states. Therefore, surveillance of the fetus during the antepartum and intrapartum periods is an important component of this process. In the past, both the method and the frequency of antepartum testing were arbitrary and generalized. However, it is apparent that tailoring testing to each disease-specific state is not only practical, but also more appropriate. By utilizing the information from fetal surveillance, conservative management may be possible and may allow continued maturation in utero, while reducing the potential of neonatal prematurity complications. This chapter will discuss the current techniques available to survey the fetus and amniotic fluid during both antepartum and intrapartum periods and present older and newer data and studies on antepartum and intrapartum fetal surveillance.


Antepartum Surveillance Techniques (Fetus and Amniotic Fluid)

There are multiple maternal and fetal indications to perform antepartum surveillance (Table 21.1), although these are not absolute. The common basis for selecting these patients are those who are at increased risk of perinatal mortality due to a variety of pathophysiologic processes, including uteroplacental insufficiency, intra-amniotic infection, fetal anemia, fetal heart failure, metabolic causes, and umbilical cord compression, to name a few. Many surveillance methods rely on natural fetal behavior. Although the mechanisms controlling sleep and activity cycles in the fetus are not well understood, knowledge of these behaviors is imperative to appropriately interpret FHR monitoring and fetal biophysical profile (BPP) activities.

In the near-term fetus, there are four behavioral states (occurring repeatedly and stable over time) that have been described: quiet sleep, active sleep, quiet awake, and active awake.1 Quiet sleep is characterized by limited eye or breathing movements, infrequent startle-type body movements, reduced FHR variability, and no accelerations. Active sleep is characterized by frequent gross body movements, rapid eye movements, breathing, normal FHR variability, and accelerations. The fetus predominantly spends its time in either a quiet or an active sleep state.2

The efficacy of the various antenatal fetal surveillance tests depends on the underlying pathophysiologic condition.3 No test is ideal for all high-risk fetuses. Therefore, multiparameter assessment or a
combination of different tests may often be the optimal strategy to determine the cause of suspected anomalies and make informed decisions regarding pregnancy management. Table 21.2 shows the various pathophysiologic processes, examples of maternal/fetal conditions, and the specific surveillance tests that may be the most appropriate.










Fetal Movement Monitoring

This method of surveillance (also known as “fetal kick counts”) by the mother is simple, inexpensive, noninvasive, and understandable to patients. In general, the presence of good fetal movement is a sign of fetal well-being and an indirect measure of normal fetal acid-base status. However, a decrease in fetal movements often (but not invariably) precedes fetal death, in some cases by several days.4 Around 16 to 18 weeks’ gestation, most women become cognizant of fetal activity, and this perception appears to be at its maximum by 28 to 32 weeks.5 However, awareness of fetal movements will vary from patient to patient and is also affected by other maternal, fetal, and uterine factors (Table 21.3). In general, patients perceive about 80% of ultrasonographically visualized fetal movements.6


Technique

Although several protocols have been utilized, neither the ideal duration for counting movements nor the optimal number of movements has been defined. Accordingly, there are many reported techniques that are acceptable, all of which rely on maternal compliance. A popular approach is to have the patient lie on her left side and count distinct fetal movements.7 Counting 10 movements in a period of up to 2 hours is felt to be reassuring. In another approach, women were advised to count fetal movements for 1 hour, three times per week.8 If the count is nonreassuring or decreased, further assessment is recommended (such as nonstress test [NST] with amniotic fluid volume [AFV] assessment or BPP), and the physician should be contacted immediately. More recently, women are using a variety of mobile applications during pregnancy to address fetal movement concerns. The content of these applications can influence maternal knowledge and education on fetal movements and encourage women to seek medical advice.



Possible Results

Multiple studies have demonstrated an association between decreased fetal movement and adverse perinatal outcomes such as stillbirth, low birth weight, oligohydramnios, preterm birth, neonatal death, perinatal brain injury, umbilical cord complications, and placental insufficiency.8,9,10,11,12,13,14,15,16 Leader et al15 found that, among high-risk pregnancies, 15% perceived an inactive fetus; among these, 46% had a poor perinatal outcome (stillbirths or poor neonatal condition at birth). Rayburn16 examined high-risk patients and found that patients with inactive fetuses were more likely to have a stillborn or do poorly during labor and the immediate neonatal period (abnormal labor FHR patterns, cesarean deliveries for fetal distress, 5-minutes Apgar scores ≤ 6). In addition, in the inactive group, the incidence of severe fetal growth restriction was almost 10 times higher than that of the active group, and the overall risk of stillbirth was 35% (vs 0.6% in the active group).While fetal movement monitoring is beneficial in high-risk pregnancies, it may also be useful in low-risk populations in
reducing fetal mortality. In one large prospective study, during a 7-month control period (patients were not advised in formal fetal movement assessment), the fetal mortality rate was 8.7/1000 births.7 However, in a subsequent study period, when a fetal movement screening program was instituted, this rate dropped to 2.1/1000 births. It should be noted that there was a 13% increase in the number of NSTs performed for “decreased fetal movement,” and the intervention rate for fetal distress was 2.6 times higher (vs control subjects). A large retrospective study, including 2393 women who presented to obstetric triage in the third trimester with reduced fetal movements, showed that neonatal intensive care unit (NICU) admissions, low Apgar scores, cesarean deliveries, and conditions requiring further hospitalization were significantly higher in women with persistent decreased fetal movements, compared to women who had a transient episode of decreased fetal movement and normal antenatal testing.17 However, studies have failed to show that the fetal movement counting actually reduces perinatal mortality.18,19









Contraction Stress Test

Historically, a contraction stress test (CST) was the first antepartum fetal heart test used to detect uteroplacental insufficiency. This test is based on the response of the FHR to uterine contractions and relies on the premise that fetal oxygenation will be transiently worsened by contractions, which primarily occurs intrapartum. Therefore, in a suboptimally oxygenated fetus, the resultant intermittent worsening in oxygenation will, in turn, lead
to the FHR pattern of late decelerations.20 This test is rarely used today because it is cumbersome, is costly, has a high false-positive rate, and has been replaced by noninvasive tools such as the NST and Doppler velocimetry. However, the CST may be of particular value in preterm fetuses who demonstrate intermittent, “spontaneous” heart rate decelerations to assess the potential for uteroplacental insufficiency.








Although no longer standard practice, a brief mention of CST techniques and possible test results bears mention here.









Technique

Lying in a lateral recumbent position, the patient has an external fetal monitor recording both the FHR and uterine contractions simultaneously for a 20- to 30-minutes interval. If the patient is spontaneously contracting, the frequency is ≥3 contractions/10 minutes, and the duration of each contraction is ≥ 45 seconds, then uterine stimulation is not required. However, if these criteria are not met, then either nipple stimulation21 or exogenous oxytocin can be used to elicit contractions. Once adequate contractions are achieved, stimulation is discontinued.



Possible Results

Freeman criteria (Table 21.4) are used by most to interpret CST results.22 The most common result is a negative CST, which indicates adequate fetal oxygenation in the presence of contractions22,23,24 and has been consistently associated with a good fetal outcomes. One study reported that the incidence of antepartum fetal death (within 1 week of a negative CST) was as low as 0.2% to 0.7%.24 In another study, Lagrew and colleagues evaluated this antepartum test in 614 women with diabetes.25 Only one patient had an intrauterine demise within 1 week of a negative CST. Thus, the literature suggests that there is a low incidence (<1%) of antepartum fetal death within 1 week of negative testing. However, this test may not predict fetal compromise related to causes other than placental insufficiency.

Interpretation of CST results should also take into consideration FHR reactivity. Fetal outcome is controversial when a negative, nonreactive CST is
seen. Druzin et al have reported an increased likelihood of fetal death.26 Other studies failed to show an increase in perinatal mortality or in low Apgar scores in this group of fetuses.27 Another study of Merrill et al used BPP to further evaluate preterm fetuses with a positive nonreactive CST; when the BPP was normal, they were able to safely prolong such pregnancies for up to 13 days.28 Investigation into possible causes of nonreactivity (ie, prematurity, medications, and congenital anomalies) should be performed along with repeat testing within 24 hours.23

Positive CST implies uteroplacental insufficiency and has been associated with adverse perinatal outcomes and an increased incidence of intrauterine demise.22 However, there is a high incidence of false-positive rates, reported to be >50% depending on the perinatal outcome defined, which can lead to unnecessary interventions, especially in preterm cases.29

When the CST is positive and nonreactive and the FHR is nonreassuring, the corrected perinatal mortality rate has been noted to be as high as 17% and up to 25% of the cases are associated with fetal growth restriction.22,30 Thus, this type of CST result usually necessitates delivery, and cesarean delivery should be considered.

Another possible occurrence is the positive, reactive CST. Devoe et al demonstrated that, cases with positive CST with presence of accelerations were associated with lower rates of perinatal mortality, intrapartum fetal distress, low 5-minutes Apgar scores, primary cesarean deliveries, and neonatal morbidity compared to cases with positive, nonreactive CSTs.31 Another group found that 70% of patients with a positive CST and accelerations could tolerate labor without distress and30 do not necessarily require cesarean delivery; labor induction is acceptable. In addition, if the fetus is preterm, further alternative testing is a reasonable option.23

The outcomes associated with suspicious CST are controversial.32,33 It is recommended that patients with a suspicious CST should have a repeat CST within 24 hours or be evaluated with another form of antepartum testing.

Unsatisfactory CST results, due to uterine hyperstimulation or an inability to obtain a satisfactory FHR tracing, should be followed up in a similar fashion to suspicious results. The etiology of equivocal tests includes obesity, excessive fetal activity, and polyhydramnios. Nowadays, given the high false-positive rates, the CST has been mostly replaced by other noninvasive modalities, such as NST, BPP, and Doppler studies.34,35 However, the CST may be of particular value in preterm fetuses who demonstrate intermittent, “spontaneous” heart rate decelerations to assess the potential for utero-placental insufficiency.


Nonstress Test

The purpose of the NST is to identify healthy fetuses and prolong preterm pregnancies, as well as identify those in jeopardy so that timely intervention can improve outcomes. This testing modality is based on the premise that the heart rate of the fetus that is not acidemic or neurologically depressed will temporarily accelerate with fetal movement. FHR reactivity is speculated to be a good indicator of normal fetal autonomic function and well-being; it relies on normal brainstem neurological development and normal integration of the central nervous system (CNS) control of FHR. In contrast, loss of reactivity is most commonly associated with a sleep cycle, but can result from any cause of CNS depression, including fetal acidemia, infection, or anemia. Compared with the CST, the NST is faster, is easier to interpret, lacks contraindications, and is most commonly used in contemporary practice.36


Technique

FHR is monitored using a Doppler ultrasound transducer, while a tocodynamometer may be used to record any uterine contractions. Fetal activity is also recorded on the strip; however, the patient does not need to document fetal movement for the test to be interpreted. Less than 1% of NSTs provide unsatisfactory results owing to inadequate recording of the FHR tracing.37 Technical difficulties that may be encountered include obesity, fetal hiccups, excessive fetal activity, and polyhydramnios.29



Possible Results

The FHR tracing can be categorized as reactive or nonreactive. Although various definitions of reactivity have been used, the most common is ≥2 FHR accelerations (which peak, but do not necessarily remain, at least 15 beats per minute [bpm] in amplitude above the baseline, and last 15 seconds from baseline to baseline) within a 10- or 20-minutes period, with or without fetal movement.38 In preterm fetuses <32 weeks, FHR acceleration is defined as 10 bpm above baseline for at least 10 seconds.
It may be necessary to continue the NST for 40 minutes to account for variations in the fetal sleep-wake cycle because it may take longer for a healthy term fetus to display two FHR accelerations.39 If, after 40 minutes, the criteria are still not met, fetal movement may be induced by acoustic stimulation (see below). If acceleration criteria are not met, the test is considered nonreactive. On initial testing, almost 85% of high-risk patients show a reactive NST, and the remaining 15% are nonreactive.37 Other factors besides fetal compromise that can lead to a nonreactive NST include depressants (narcotics, phenobarbital), beta-blockers (propranolol), and smoking.40,41,42 It is also important to note that mild variable decelerations can be observed in up to 50% of NSTs.43 If these decelerations are not repetitive and brief (<30 seconds), they are likely not due to fetal compromise and may not necessitate obstetric intervention. However, repetitive variable decelerations (at least three in 20 minutes), even if mild, have been associated with an increased risk of cesarean delivery for a nonreassuring intrapartum FHR pattern.44

NST interpretation should take gestational age into account; this is an important consideration, as preterm fetuses are less likely to have FHR accelerations in association with fetal movements. Navot et al45 found a linear increase in the incidence of FHR accelerations in association with fetal movements, from 20% (25 weeks) to 65% (40 weeks). The amplitude of accelerations also appears to be related to gestational age, as accelerations of >15 bpm are responsible for only 20% of the total number of accelerations at early gestations (24-26 weeks).46 Gagnon et al described the normal maturation of the FHR pattern from 26 weeks to term: decrease in basal FHR, increase in amplitude of accelerations, and increase in long-term variability.47 All these changes evolved by 30 weeks, and no further significant changes developed later until term. As the gestational age increases, a higher percentage of reactive NSTs is found.48 For instance, a study of Smith et al showed that the percentage of reactive NSTs were 16.7%, 65.6%, 90.5%, and 94.4% at 23 to 27, 28 to 32, 33 to 37, and 38 to 42 weeks, respectively.48 This also implies that the lower the gestational age, the higher the percentage of nonreactive NSTs.

Studies also have evaluated the predictive value of NSTs in pregnancies <32 weeks of gestation based on a lower threshold for accelerations (ie, at least 10 bpm above the baseline and at least 10 seconds from baseline-to-baseline) and have shown that there is no appreciable difference between the 10-beat criteria and 15-beat criteria in predicting outcomes.49,50 This concept should be kept in mind, as many fetuses may undergo antepartum surveillance at <32 weeks. Preterm fetuses may also normally exhibit decelerations between 20 and 30 weeks.51 They become less common as gestation advances and are more frequent at <30 weeks. However, although gestational age should always be considered when interpreting NSTs, nonreassuring FHR patterns in the preterm fetus should not be automatically—and perhaps improperly—attributed to prematurity.

Lavery reviewed perinatal mortality among patients in whom the NST was the main method of fetal assessment.52 In nine separate studies (7759 patients), a gross perinatal mortality of 12.5/1000 was found. The predictive value of a negative NST (normal outcome associated with a reactive NST) was noted to be very high. The reactive NST predicts good perinatal outcome in about 95% of cases.53 Accordingly, false-negative NSTs are infrequent. Within 1 week of a reactive NST, the perinatal mortality rate is about 3 to 5/1000. In addition, Devoe found that, compared with nonreactive tests, patients with reactive NSTs were less likely (5% vs 22%) to experience perinatal morbidity (intrapartum fetal distress, low Apgar scores, neonatal complications, intrauterine growth restriction).54 Barss et al found that the antepartum stillbirth rate (within 7 days of a reactive test) was 2.7/1000 and 2.8/1000 for the general high-risk population and postdate pregnancies, respectively.55 For the two groups, the mean interval between a reactive test and stillbirth was 3.8 and 3.5 days. Thus, some have recommended that increasing the testing frequency to twice per week could prevent fetal mortality. Boehm et al found a reduction in the stillbirth rate in a high-risk population (6.1/1000-1.9/1000) when the NST frequency was increased from weekly to twice a week.56 Similarly, a study that reviewed 1000 patients with diabetes or fetal growth restriction suggested that weekly testing was not effective, and that twice-weekly testing should be established.57 The American College of Obstetricians and Gynecologists (ACOG) states that NSTs and antenatal testing are typically repeated if the clinical condition that prompted testing persists. Even though the ideal interval has
not been established, they are typically repeated at weekly intervals or in more frequent intervals in high-risk conditions.20

Whereas a reactive NST has excellent specificity, the predictive value of a positive test has been reported to be <40%.58 Concomitantly, the false-positive rate of a nonreactive NST has been found to be 57% to 100% for perinatal mortality, and 44% to 92% for perinatal morbidity.29 Therefore, when an NST is nonreactive, either prolongation of the time of the NST or use of other forms of testing (such as the BPP and/or Doppler velocimetry) is recommended. Even after prolongation of monitoring, a nonreactive NST may still be consistent with good fetal outcomes. However, persistent absence of reactivity without an identified underlying cause, such as medication, prematurity, or congenital anomalies, may be associated with fetal compromise.59 Those fetuses that remained nonreactive after 90 minutes had 67% and 93.3% perinatal mortality and morbidity rates, respectively.

In summary, while a reactive NST is usually associated with good outcomes, absence of accelerations is not necessarily linked to fetal compromise. We believe that when the NST is used for fetal surveillance, once per week testing may not always be adequate; one may consider the use of biweekly testing, especially in women at high risk, who have multiple comorbidities such as poorly controlled diabetes mellitus or preeclampsia. As will be discussed later, indication-specific testing with a more individualized approach may be more appropriate.


Vibroacoustic Stimulation

Because the majority of nonreactive NSTs occur in healthy fetuses secondary to a physiologically normal sleep state, fetal stimulation has been used as a way to distinguish normal fetal sleep from fetal compromise. Vibroacoustic stimulation (VAS) is an effective technique to improve the efficiency of antepartum FHR testing because it safely reduces the time needed to perform an NST without compromising detection of the sick fetus.60


Technique

To perform a VAS, an artificial larynx is positioned on the maternal abdomen over the fetal vertex with a stimulus of 1 to 2 seconds. This procedure may be repeated up to three times (at 1-minute intervals) for progressively longer durations of up to 3 seconds to elicit FHR accelerations. The increases in intrauterine sound decibels created by VAS are thought to be safe and harmless to the fetus.



Possible Results

The normal fetal response to VAS includes not only FHR accelerations, but also increases in variability and gross body movements. Utilizing this approach on a normal fetus may elicit accelerations that appear to be suggestive of fetal well-being. Studies have demonstrated the advantages of using VAS in conjunction with the NST. Trudinger and Boylan reported that using VAS with NST versus NST alone had higher sensitivity in detecting abnormal fetal outcomes (66% vs 39%).61 Those fetuses with an abnormal response, who remained nonreactive after VAS, demonstrated increased rates of intrapartum fetal distress, fetal growth restriction, and lower Apgar scores.62 A 2013 systematic review, including 12 trials with a total of 6822 participants, showed that VAS decreased the incidence of nonreactive NST, in comparison to nonacoustic stimulation, and reduced the false-positive but not the false-negative rate of the NST.63 As with the NST, gestational age appears to affect the FHR response to VAS, with a maturational response as gestation advances.61,62 Unusual FHR patterns, including prolonged tachycardia, were seen after VAS in some preterm fetuses.


Biophysical Profile

The BPP is unique in that it assesses both acute (ie, FHR reactivity, fetal breathing movements, fetal movements, fetal tone) and chronic (ie, AFV) markers of fetal condition. It is a noninvasive modality with a high negative predictive value for adverse perinatal outcomes.34,64 When used in conjunction with other surveillance and testing measures, the BPP helps to assess fetal hypoxia and acidemia, presence of infection in patients with premature rupture of membranes (PROM), placental dysfunction, and stillbirth.

The BPP is based on the premise that the biophysical activities developed last in utero are the first to become abnormal in the presence of fetal acidemia or infection.65 This theory, the gradual hypoxia concept, was proposed by Vintzileos et al in 1983. At about 7.5 weeks, the CNS center controlling fetal tone is the first to develop, followed by development of body movement at 8.5 to 9.5 weeks. The center controlling regular breathing movements develops after 20 to 21 weeks, and the
center controlling FHR reactivity functions by the end of the second/beginning of the third trimester. Therefore, in accordance with the gradual hypoxia concept, early stages of compromise are revealed by abnormalities in FHR reactivity and breathing, whereas movement and tone are not abolished until much later stages of compromise.


Technique

The BPP is performed using real-time ultrasonography to assess multiple fetal biophysical activities as well as AFV. Sonographic observation is continued until either normal activity is seen or after 30 consecutive minutes of scanning have elapsed. The interval of BPP testing frequency (1-2 per week) is arbitrary; however, it is often a matter of individual clinical judgment, training, preferences, and experience. An advantage of the BPP over the NST is that observations of fetal movement and breathing (or their absence) are unequivocal, and usually there are no interobserver discrepancies in interpretation.

The modified BPP is composed of the NST (an acute indicator of fetal acid-base status) and AFV (indicator of chronic uteroplacental function). It is used by many centers as a primary mode of surveillance. Using the modified BPP to assess perinatal morbidity/mortality compares favorably with using the BPP alone. Algorithm 21.1 shows our suggested protocol for the modified BPP. By using this protocol in 17,211 tests, we had only four deaths of normal fetuses, with a false-negative rate of 0.02%.66 Another study compared the outcomes of high-risk patients whose last antepartum assessment was a negative CST or negative modified BPP.67 The incidence of adverse perinatal outcome, after reassuring testing, was significantly less in those managed by the modified BPP than in the CST group (5.1% vs 7.0%).








Possible Results

Manning et al introduced the BPP score in 1980.68 It provides an estimate of the risk of fetal death in the immediate future, with the risk being low when a normal score is present. The current scoring systems assign a numeric value (usually 0 or 2) to each of the biophysical components (Table 21.5). A normal modified BPP exists when the NST is reactive and the amniotic fluid index (AFI) is >5 cm.20 An abnormal test occurs if either the NST is nonreactive or the AFI is 5 cm or less.

An understanding of the fetal biophysical response to compromise is essential to interpreting the BPP score. Each BPP parameter reflects a normally functioning area of the fetal CNS that evolves in utero at predictable gestational ages based on fetal neurophysiology. The fetus will respond to central hypoxemia/acidemia, as well infection, by altering its movement, tone, breathing, and heart rate pattern.69 The corollary is also true: in the presence of normal biophysical activities, brainstem CNS tissue is functional and is not significantly hypoxic.

The absence of a particular activity assessed by the BPP may be due to diurnal variation, maternal drugs such as steroids or magnesium, acute fetal asphyxia, or fetal infection.70,71,72 The sequential
loss of BPP variables serves as a marker of the degree of placental dysfunction. A BPP score of 8 or more is considered reassuring. In fact, if all four sonographic components are normal, the NST may be omitted without compromising the validity of the BPP test results.73 When the score is <8, however, analyzing which individual components of the BPP are abnormal can assist in determining true fetal status and minimize false-positive examinations. The BPP score should also be interpreted within the overall clinical context. In general, a score of 6 is considered equivocal, and a score of ≤4 is abnormal. In the mature fetus, a BPP of 6/10 may indicate compromise and may be an indication for delivery; however, in the immature fetus, repeat testing or use of Doppler velocimetry may be in order before intervention is recommended. Fetal hiccups are interpreted as a variant of normal fetal breathing. Presently, the rate and pattern of the breathing movements are not considered clinically significant, except in extreme cases.








Regardless of the total score, in the presence of oligohydramnios (largest vertical pocket of AFV ≤2 cm), further evaluation is warranted.74 Manning believed that oligohydramnios (in the presence of a normal fetus, functioning renal tissue, intact membranes) was always considered an indication for labor induction, despite the presence of normal FHR reactivity, breathing, movement, and tone. This approach was based on an extensive review of the relationship between ultrasound-defined oligohydramnios and perinatal mortality,75 and a subsequent prospective study indicating that intervention for oligohydramnios can improve perinatal outcome.76 However, more recent data advocate the use of Doppler velocimetry (umbilical artery, middle cerebral artery, ductus venosus) to assist in management.

The data strongly suggest that the application of BPP to the high-risk pregnant population results in a dramatic improvement in perinatal mortality rates.77,78 A normal BPP conveys a low risk of stillbirth; however, the false-negative rate of the BPP (fetal death within 1 week of a last normal test result) ranges from 0.645 to 7 per 1000.75,77,79 Figure 21.1 shows an inverse and exponential relationship between last BPP score and perinatal mortality. There is also a strong relationship between the last BPP score and perinatal morbidity variables.74,78 Figure 21.2 shows a highly significant inverse linear correlation between last BPP score and perinatal morbidity. Combinations of these variables (fetal distress, admission to neonatal intensive care unit, intrauterine growth restriction, 5-minutes Apgar score ≤ 7, and cord pH < 7.20) also showed the same highly significant inverse linear correlation with BPP score. However, a 2008 meta-analysis failed to show significant differences in perinatal deaths, neonatal Apgar scores, or cesarean delivery rates when comparing BPP or other antepartum tools of surveillance.80







Vintzileos et al examined the relationship between BPP components and cord pH in 124 patients undergoing cesarean birth (prior to labor onset).81 An umbilical arterial pH of < 7.20 was used to define fetal acidemia. At pH < 7.20, inhibition of fetal breathing and FHR nonreactivity occurred; however, a pH < 7.10 was needed before movements and tone were abolished. Importantly, none of the fetuses with a reactive NST, or breathing, or both, had a pH < 7.20. By analyzing the individual components of the BPP (vs score), the sensitivity, specificity, positive, and negative predictive values in predicting fetal acidemia were 100%, 92%, 71%, and 100%, respectively. Vintzileos and colleagues, in a later study, examined the relationship between the absence of fetal biophysical activities and umbilical artery blood gas values (pH, PO2, PCO2, HCO3, and base excess levels), thus confirming the gradual hypoxia concept.82

The BPP has also been useful as a method of assessing fetal well-being and predicting the development of infectious complications in patients with PROM. Vintzileos et al examined the effect of PROM on the biophysical components.83 In patients with PROM and without intrauterine infection, there is an increased likelihood of FHR reactivity and oligohydramnios throughout gestation, adequate fetal breathing is decreased, and the other BPP components are not affected.

A strong correlation has been reported between abnormal BPP assessment and the development of infectious maternal or neonatal morbidity in patients with PROM.84 The authors concluded that correlation between an abnormal BPP and infectious outcome is time dependent. The relationship between an abnormal BPP and infection is established only if the BPP is performed within 24 hours of delivery. In contrast, no correlation exists between an abnormal BPP and infectious outcome if the test is performed >24 hours before delivery. It is also vital to note that the development of maternal clinical chorioamnionitis without neonatal infection or infection of the intra-amniotic cavity with Mycoplasma species are two conditions that are not necessarily associated with an abnormal BPP. Overall, frequent NSTs or BPPs in patients with preterm PROM are helpful in distinguishing the healthy fetuses that can safely remain in utero from those that are either already infected or at high risk of developing neonatal infection.







Studies have also validated the use of BPP in the intrapartum period and during active labor. Tongpraset and colleagues showed that a rapid BPP, including fetal movements and AFI index, can be helpful in assessing fetal well-being in the early intrapartum period in low-to-middle-income countries.85 Another study evaluating the use of BPP in 100 women in active labor showed that a BPP score of 6/10 or less in labor was associated with a significantly higher risk for cesarean delivery, and that cessation of any ultrasound component of BPP significantly increased the risk of cesarean delivery and admission to the NICU.86


Amniotic Fluid Volume Assessment

Amniotic fluid (AF) is essential to pregnancy, providing an environment for normal development, growth, and movement of the fetus. AFV is a chronic marker of fetal well-being, and a normal AFV also protects the fetus from cord compression during fetal activity or uterine contractions. This volume changes during pregnancy: at 22 weeks, the average AFV is 630 mL, and this increases to 770 mL at 28 weeks87 (Figure 21.3). Between 29 and 37 weeks, there is little change in volume, which averages 800 mL. Beyond 39 weeks, AFV decreases sharply (average 515 mL at 41 weeks). At postdates, there is a 33% decline in AFV per week, consistent with clinical observations of an increased incidence of oligohydramnios in postterm gestations.88 Surveillance of the AFV is crucial, as fluid disorders may not only represent but also lead to fetal disease states.

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Jun 19, 2022 | Posted by in OBSTETRICS | Comments Off on Antepartum and Intrapartum Surveillance of the Fetus and the Amniotic Fluid

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