Antepartum Fetal Testing

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Chapter 184 Antepartum Fetal Testing

THE CHALLENGE

To reduce the risk of fetal demise in those at high risk through the use of noninvasive tests that have acceptably low false-positive and false-negative results.

Scope of the Problem: There are roughly 15 fetal or neonatal deaths for every 1000 births in the United States.

Objectives of Management: To identify those fetuses at high risk and those whose status is deteriorating or nonreassuring so that intervention can take place to prevent mortality. Ultimately, the reduction of fetal morbidity and the improvement of neurologic outcome would be ideal as well, but objective studies to support the effectiveness of antepartum fetal testing are lacking.

TACTICS

Relevant Pathophysiology: The nonstress test (NST) and contraction stress test (CST) are based on the premise that when fetal oxygenation is only marginally adequate, the fetus will not possess the normal ability to modulate heart rate in response to fetal movement or to tolerate the stress of placental ischemia induced by uterine contractions. A normal (reactive) NST has two or more accelerations (15 beats/min for 15 seconds) in a 20-minute period. Acoustic stimulation may also be used to startle the fetus and induce a heart rate increase. In the CST, the occurrence of late decelerations occurring with 50% or more contractions (regardless of frequency) is “positive” and suggests fetal risk. The biophysical profile (BPP) is based on the NST, augmented by measures of fetal breathing movements, fetal activity and tone, and quantitation of amniotic fluid volume rated on a 10-point scale (normal: 8–10/10, equivocal: 6/10, abnormal: ≤5/10). The pulsatile character of fetal blood flow in the umbilical cord or the middle cerebral artery may be used to assess the health of high-risk pregnancies, but these tests require special expertise to both perform and interpret.

Strategies: The most commonly used antepartum fetal tests are the NST, CST, BPP, and movement assessment (kick count and others). These may be used individually, in sequence, or in any combination as the individual case demands. Each has advantages and disadvantages; no single test can be said to be definitive. Some possible indications for the use of antenatal fetal testing are shown.

Patient Education: American College of Obstetricians and Gynecologists Patient Education Pamphlet AP098 (Special Tests for Monitoring Fetal Health), AP015 (Fetal Heart Rate Monitoring During Labor).

IMPLEMENTATION

Special Considerations: The NST is more easily performed than the CST, but it has the highest falsepositive rate (up to 90% of positive tests) and the highest risk of a false-negative test result (1.4 of 1000). The CST requires the induction of contractions by intravenous oxytocin or nipple stimulation. There must be ≥3 contractions in a 10-minute test period and no late decelerations. The CST has a lower false-positive rate (50%) and a lower risk of a false-negative result (0.4 of 1000). The BPP has the lowest false-positive and false-negative rates (0 to 0.6 of 1000) but is the most expensive and requires the most expertise and equipment. All testing must be viewed in the context of the clinical picture. The choice of timing and test must be made on clinical grounds, degree of risk, and the availability and expertise of those who will perform and interpret the test. Normal test results generally warrant further testing in a few days to a week. Positive or nonreassuring test results may suggest the need for a more invasive test (e.g., NST to CST, CST to BPP) or more direct intervention in the course of the pregnancy (delivery). Despite the extent of study that has accompanied these technologies, all studies must always be interpreted in light of all available clinical factors.

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