The primary goal of antenatal testing is to identify fetuses at risk for intrauterine injury or death, so that these adverse outcomes can be prevented.
Since the development of technologies for electronic fetal heart rate monitoring in the 1970s, and with increasing sophistication of ultrasound and Doppler imaging, an array of techniques for antenatal assessment of fetal well being have been introduced into clinical practice. The ultimate goal of antepartum fetal monitoring is to improve perinatal outcome, specifically by decreasing stillbirth and longer-term neurologic impairments such as injury to the fetal central nervous system. Injury to the fetal central nervous system (CNS) is expressed after delivery in a number of clinical entities and syndromes with cerebral palsy the most common.
Fetal hypoxia and acidosis represent the final common pathway to fetal injury and death in many high-risk pregnancies. The basis for antepartum testing relies on the premise that the fetus whose oxygenation in utero is challenged will respond with a series of detectable physiologic adaptive or decompensatory signs as hypoxemia or frank metabolic acidemia develop. A number of investigators have described sequences of measurable changes in fetal blood flow and biophysical parameters that occur as placental insufficiency worsens and fetal hypoxemia and acidemia develop.
The American College of Obstetricians and Gynecologists mentions following clinical considerations and recommendations in its practice bulletin (July 2014);
How reassuring is a normal antepartum fetal surveillance result?
In most cases, a normal antepartum fetal test result is highly reassuring, as reflected in the low false-negative rate of antepartum fetal surveillance, defined as the incidence of stillbirth occurring within 1 week of a normal test result. The stillbirth rate, corrected for lethal congenital anomalies and unpredictable causes of fetal demise, was 1.9 per 1,000 in the largest series of NSTs (5,861) versus 0.3 per 1,000 in 12,656 CSTs, 0.8 per 1,000 in 44,828 BPPs, and 0.8 per 1,000 in 54,617 modified BPPs. Based on these data, the negative predictive value is 99.8% for the NST (Nonstress Test) and is greater than 99.9% for the CST (Contraction Stress Test), BPP (Biophysical Profile), and modified BPP. Although similar data from a large series are not available for umbilical artery Doppler velocimetry, in one randomized clinical trial among women with pregnancies complicated by fetal growth restriction, no stillbirths occurred in 214 pregnancies in which umbilical artery Doppler velocimetry was the primary means of antepartum fetal surveillance (negative predictive value of 100%). The low false-negative rate of these tests depends on an appropriate response to any significant deterioration in the maternal clinical status, including retesting of the fetal condition. As previously mentioned, these tests generally do not predict stillbirths related to acute changes in maternal–fetal status, such as those that occur with abruptio placentae or an umbilical cord accident. Moreover, recent normal antepartum fetal test results should not preclude the use of intrapartum fetal monitoring.
Is there evidence that antepartum fetal surveillance decreases the risk of fetal demise or otherwise improves perinatal outcomes?
Evidence for the value of antepartum fetal surveillance is circumstantial and rests principally on the observation that antepartum fetal surveillance has been consistently associated with rates of fetal death that are substantially lower than the rates of fetal death in both untested (and presumably lower-risk) contemporaneous pregnancies from the same institutions and pregnancies with similar complicating factors that were managed before the advent of currently used techniques of antepartum fetal surveillance (historic controls). There is a lack of high-quality evidence from RCTs that antepartum fetal surveillance decreases the risk of fetal death. A definitive evaluation of antepartum fetal surveillance in RCTs (which would require the random allocation of pregnant patients to prenatal care that included antepartum fetal surveillance versus prenatal care that did not include antepartum fetal surveillance) is unlikely to be conducted in a setting that can be generalized to current U.S. obstetric practice. In spite of its unproven value, antepartum fetal surveillance is widely integrated into clinical practice in the developed world.
What are the indications for antepartum fetal surveillance?
Because antepartum fetal surveillance results have not been definitively demonstrated to improve perinatal outcome, all indications for antepartum testing must be considered somewhat relative. In general, antepartum fetal surveillance has been used in pregnancies in which the risk of antepartum fetal demise is increased. Accordingly, some of the conditions for which testing may be indicated include, but are not limited to, those listed in Box 1 .
Box 1. Indications for Antepartum Fetal Surveillance Testing
• Pregestational diabetes mellitus
• Systemic lupus erythematosus
• Chronic renal disease
• Antiphospholipid syndrome
• Hyperthyroidism (poorly controlled)
• Hemoglobinopathies (sickle cell, sickle cell– hemoglobin C, or sickle cell–thalassemia disease)
• Cyanotic heart disease
• Gestational hypertension
• Decreased fetal movement
• Gestational diabetes mellitus (poorly controlled or medically treated)
• Fetal growth restriction
• Late term or postterm pregnancy
• Previous fetal demise (unexplained or recurrent risk)
• Monochorionic multiple gestation (with significant growth discrepancy)
When during gestation should antepartum fetal surveillance be initiated?
Choosing the appropriate point in gestation to begin antepartum fetal testing depends on several considerations, including the prognosis for neonatal survival, the risk of fetal death, the severity of maternal disease, and the potential for iatrogenic prematurity complications resulting from false-positive test results. The importance of the last consideration is illustrated by the experience of one large center, in which 60% of infants delivered because of an abnormal antepartum test result had no evidence of short-term or long-term fetal compromise. Both theoretic models and large clinical studies suggest that initiating antepartum fetal testing no earlier than 32 0/7 weeks of gestation is appropriate for most at-risk patients. However, in pregnancies with multiple or particularly worrisome high-risk conditions (e.g., chronic hypertension with suspected fetal growth restriction), testing might begin at a gestational age when delivery would be considered for perinatal benefit.
What is the recommended frequency of testing?
There are no large clinical trials to guide the frequency of testing, and thus, the optimal frequency remains unknown; it depends on several factors and should be individualized and based on clinical judgment. If the indication for testing is not persistent (e.g., a single episode of decreased fetal movement followed by reassuring testing in an otherwise uncomplicated pregnancy), testing need not be repeated. When the clinical condition that prompted testing persists, the test should be repeated periodically to monitor for continued fetal well-being until delivery. If the maternal medical condition is stable and test results are reassuring, tests of fetal well being (NST, BPP, modified BPP, or CST) are typically repeated at weekly intervals; however, in the presence of certain high-risk conditions, some investigators have performed more frequent testing, although the optimal regimen has not been established.
In pregnancies complicated by fetal growth restriction, the optimal interval for fetal growth assessment and the optimal surveillance regimen have not been established. Most growth-restricted fetuses can be adequately evaluated with serial ultrasonography every 3–4 weeks; ultrasonographic assessment of growth should not be performed more frequently than every 2 weeks because the inherent error associated with ultrasonographic measurements can preclude an accurate assessment of interval growth. Any significant change in maternal or fetal status requires further reevaluation.
What is the recommended management of an abnormal antepartum fetal test result?
An abnormal antepartum fetal test result should always be considered in the context of the overall clinical picture. Certain acute maternal conditions (e.g., diabetic ketoacidosis or pneumonia with hypoxemia) can result in abnormal test results, which generally will normalize as the maternal condition improves. In these circumstances, correcting the maternal condition and retesting the fetus may be appropriate.
In cases in which an abnormal test result is not associated with any clinical evidence of acute and potentially reversible worsening in the maternal status, a stepwise approach to the investigation of the fetal condition should be undertaken. Because antepartum fetal surveillance tests have high false-positive rates and low positive predictive values, abnormal test results are usually followed by another test or delivery based on consideration of test results, maternal and fetal condition, and gestational age. Such an approach takes advantage of the high negative predictive value generally exhibited by all commonly used antepartum tests and minimizes the potential for unnecessary delivery based on a single false-positive (i.e., false-abnormal) test result. Therefore, the response to an abnormal test result should be tailored to the clinical situation.
Maternal reports of decreased fetal movement should be evaluated by an NST (Nonstress Test), CST (Contraction Stress Test), BPP (Biophysical Profile), or modified BPP. Abnormal results from an NST or from a modified BPP generally should be followed by additional testing with either a CST or a BPP. A BPP score of 6 out of 10 is considered equivocal and should prompt further evaluation or delivery based on gestational age. In a fetus at or beyond 37 0/7 weeks of gestation, this score generally should prompt further evaluation and consideration of delivery, whereas in the fetus at less than 37 0/7 weeks of gestation, it should result in a repeat BPP in 24 hours. A BPP score of 4 usually indicates that delivery is warranted, although in pregnancies at less than 32 0/7 weeks of gestation, management should be individualized, and extended monitoring may be appropriate. In most circumstances, a BPP score of less than 4 should result in delivery. If delivery is not planned (e.g., given early gestational age), then antenatal surveillance should not be performed because the results will not inform management.
There are no definitive randomized clinical trials to guide the timing of delivery of the growth-restricted fetus on the basis of umbilical artery Doppler velocimetry. Guidelines from the Society for Maternal-Fetal Medicine suggest that with absent end-diastolic flow, delivery should be considered at or beyond 34 0/7 weeks of gestation, and with reversed end-diastolic flow, delivery should be considered at or beyond 32 0/7 weeks of gestation (after corticosteroid administration, if the maternal and fetal condition permit). When the S/D ratio is elevated (i.e., greater than the 95th percentile) but diastolic flow is still present, delivery should be considered at or beyond 37 0/7 weeks of gestation. In the absence of obstetric contraindications, delivery of the fetus with an abnormal test result often may be attempted by induction of labor, with continuous intrapartum monitoring of the FHR and uterine contractions.
How should a finding of oligohydramnios affect the decision for delivery?
Amniotic fluid volume is estimated using ultrasonography. Commonly used definitions of oligohydramnios include a single deepest vertical pocket of amniotic fluid of 2 cm or less (not containing umbilical cord or fetal extremities) and an amniotic fluid index of 5 cm or less. However, the use of a percentile of amniotic fluid should not be used in management decisions. The available data from RCTs indicate that the use of the deepest vertical pocket measurement, as opposed to the amniotic fluid index, to diagnose oligohydramnios is associated with a reduction in unnecessary interventions without an increase in adverse perinatal outcomes.
Determining when to intervene for oligohydramnios depends on several factors, including gestational age, maternal condition, and fetal clinical condition as determined by other indices of fetal well-being. Because rupture of the fetal membranes can cause diminished amniotic fluid volume, an evaluation for membrane rupture in the setting of oligohydramnios may be appropriate; correspondingly, if membrane rupture is documented, a low amniotic fluid measurement can no longer be considered valid for prediction of diminished placental function. Based on expert opinion, in the setting of otherwise uncomplicated isolated and persistent oligohydramnios (deepest vertical pocket measurement less than 2 cm), delivery at 36–37 weeks of gestation is recommended. In pregnancies at less than 36 0/7 weeks of gestation with intact membranes and oligohydramnios, the decision to proceed with expectant management or delivery should be individualized based on gestational age and the maternal and fetal condition. If delivery is not undertaken, follow-up amniotic fluid volume measurements, NSTs, and fetal growth assessments are indicated. If the oligohydramnios results from fetal membrane rupture, follow-up amniotic fluid volume assessment often may be safely omitted.
What is the role of umbilical artery and other Doppler velocimetry studies?
In growth-restricted fetuses, umbilical artery Doppler velocimetry used in conjunction with standard fetal surveillance, such as NSTs or BPPs, or both, is associated with improved outcomes. Umbilical artery Doppler velocimetry has not been shown to be predictive of outcomes in fetuses without growth restriction. Investigation of other fetal blood vessels with umbilical artery Doppler velocimetry, including assessments of the middle cerebral artery and the precordial venous system, has been explored in the setting of fetal growth restriction. However, these flow measurements have not been shown to improve perinatal outcome, and the role of these measures in clinical practice remains uncertain. (see the American College of Obstetricians and Gynecologists Practice Bulletin Number 134, Fetal Growth Restriction).
Should all women perform daily fetal movement assessment?
Multiple studies have demonstrated that women who report decreased fetal movement are at an increased risk of adverse perinatal outcomes. Although fetal kick counting is an inexpensive test of fetal well-being, the effectiveness in preventing stillbirth is uncertain (see Practice Bulletin Number 102, Management of Stillbirth). Consistent evidence that a formal program of fetal movement assessment in low-risk women will result in a reduction in fetal deaths is lacking. Moreover, whether fetal movement assessment adds benefit to an established program of regular fetal surveillance has not been evaluated. Formal fetal movement assessment may increase, by a small degree, the number of antepartum visits and fetal evaluations. In RCTs, however, this increased surveillance did not result in a higher rate of intervention. Although not all women need to perform a daily fetal movement assessment, if a woman notices a decrease in fetal activity, she should be encouraged to contact her health care provider, and further assessment should be performed.
The American College of Obstetricians and Gynecologists mentions following conclusions and recommendations in practice bulletin (July 2014);
The following conclusions are based on good and consistent scientific evidence (Level A):
- The use of the deepest vertical pocket measurement, as opposed to the amniotic fluid index, to diagnose oligohydramnios is associated with a reduction in unnecessary interventions without an increase in adverse perinatal outcomes.
- In growth-restricted fetuses, umbilical artery Doppler velocimetry used in conjunction with standard fetal surveillance, such as NSTs, or BPPs, or both, is associated with improved outcomes.
The following recommendation is based on limited or inconsistent scientific evidence (Level B):
- Abnormal results from an NST or from a modified BPP generally should be followed by additional testing with either a CST or a BPP.
The following recommendations are based primarily on consensus and expert opinion (Level C):
- Initiating antepartum fetal testing no earlier than 32 0/7 weeks of gestation is appropriate for most at risk patients. However, in pregnancies with multiple or particularly worrisome high-risk conditions (e.g., chronic hypertension with suspected fetal growth restriction), testing might begin at a gestational age when delivery would be considered for perinatal benefit.
- When the clinical condition that prompted testing persists, the test should be repeated periodically to monitor for continued fetal well-being until delivery. If the maternal medical condition is stable and test results are reassuring, tests of fetal well-being (NST, BPP, modified BPP, or CST) are typically repeated at weekly intervals; however, in the presence of certain high-risk conditions, some investigators have performed more frequent testing, although the optimal regimen has not been established.
- In the absence of obstetric contraindications, delivery of the fetus with an abnormal test result often may be attempted by induction of labor, with continuous intrapartum monitoring of the FHR (Fetal Heart Rate) and uterine contractions.
- Based on expert opinion, in the setting of otherwise uncomplicated isolated and persistent oligohydramnios (deepest vertical pocket measurement less than 2 cm), delivery at 36–37 weeks of gestation is recommended. In pregnancies at less than 36 0/7 weeks of gestation with intact membranes and oligohydramnios, the decision to proceed with expectant management or delivery should be individualized based on gestational age and the maternal and fetal condition.