A nonstress test (NST) is a screening test used in
pregnancy
Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins.
Conception (biology), Conception usually occurs ...
to assess fetal status by means of the fetal heart rate and its responsiveness. A
cardiotocograph is used to monitor the
fetal
A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...
heart rate and presence or absence of
uterine contractions. The test is typically termed "reactive" (also "reassuring") or "nonreactive" (also "nonreassuring").
Premise
The premise of the NST is that a well-oxygenated,
non-acidemic fetus
A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...
will spontaneously have temporary increases, termed "accelerations," in the fetal heart rate (FHR).
Vibroacoustic stimulation
Vibroacoustic stimulation (VAS), sometimes referred to as fetal vibroacoustic stimulation or fetal acoustic stimulation test (FAST), is the application of a vibratory sound stimulus to the abdomen of a pregnant woman to induce FHR (fetal heart rat ...
can wake the fetus, and is sometimes used to speed up the test or to facilitate further evaluation of a non-reactive nonstress test.
Interpretation
A nonstress test can be classified as normal, atypical, or abnormal. A normal nonstress test will show a baseline fetal
heart rate
Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...
between 110 and 160 beats per minute with moderate variability (5- to 25-interbeat variability) and 2 qualifying accelerations in 20 minutes with no decelerations.
"Reactive" is defined as the presence of two or more fetal heart rate
accelerations within a 20-minute period. Each acceleration must increase the heart rate 15 beats per minute above the baseline rate, and last for at least 15 seconds when the fetus is above 32 weeks' gestation, or 10 beats per minute over 10 seconds when the fetus is at or below 32 weeks' gestation.
"Non-reactive" is defined as fewer than two adequate accelerations during a prolonged period, which may be over an hour.
Confounding factors
An NST may be non-reactive for reasons unrelated to fetal oxygenation status. These include
fetal sleep, exposure to
central nervous system depressants, exposure to
beta-blockers, or chronic smoking during the pregnancy.
Utility
If the nonstress test is reactive, the rate of fetal demise in the following week is approximately 1.9 per 1000. A nonreactive nonstress test has a false positive rate of 75%-90%, and therefore is followed by more definitive testing, which may include a
contraction stress test or a
biophysical profile
A biophysical profile (BPP) is a prenatal ultrasound evaluation of fetal well-being involving a scoring system, with the score being termed Manning's score. It is often done when a non-stress test (NST) is non reactive, or for other obstetrical ...
.
See also
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Biophysical profile
A biophysical profile (BPP) is a prenatal ultrasound evaluation of fetal well-being involving a scoring system, with the score being termed Manning's score. It is often done when a non-stress test (NST) is non reactive, or for other obstetrical ...
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Cardiotocography
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Contraction stress test
References
{{Pregnancy
Tests during pregnancy