Nonstress Test

Most often the nonstress test (NST) is used as the primary tool in antepartum fetal surveillance. It has been used to document second-trimester and third-trimester fetal well-being for the past 40 years. It serves as a surrogate measure of the developing fetal autonomic nervous system and the adequacy of the uteroplacental function (Myrick and Harper, 1996).

The NST is more specific than sensitive and thus a better indicator of fetal health than fetal illness. The test itself is read as reactive or nonreactive and may be repeated at intervals as a screen for high-risk maternal conditions. A reactive or reassuring NST is defined as one with at least two accelerations above baseline fetal heart rate of 15 beats/min for 15 seconds, in a 20-minute period. If a reactive pattern is not present at the end of the first 20 minutes, attempts may be made to arouse the fetus. Fetal rest periods, reportedly 30 to 40 minutes in duration, must be excluded for the fetus to demonstrate a reactive NST. Because fetuses can have normal sleep cycles lasting up to 40 minutes, NST might require over an hour to complete

Figure 21-6 Three common twin presentations: A, cephalic-cephalic; B, cephalic-breech; C, cephalic-transverse.

Figure 21-6 Three common twin presentations: A, cephalic-cephalic; B, cephalic-breech; C, cephalic-transverse.

if it is initially nonreactive. It is important to differentiate whether a nonreactive tracing truly represents a compromised fetus versus a temporary behavioral state (Knuppel et al., 1982).

The absence of fetal accelerations with the exclusion of a fetal sleep state denotes a nonreactive test. There are no contraindications to the NST as a primary screening tool, and it is easily reproducible, relatively inexpensive, and acceptable to most patients. Maternal narcotics, extreme prematurity, and fetal cardiac or CNS anomalies may also be responsible for a nonreactive NST. A nonreactive NST does not indicate fetal jeopardy but should be viewed as an indication for further evaluation. This evaluation may take the form of a biophysical profile or contraction stress test.

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