A nonstress test (NST) is a screening test used in
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ca ...
to assess fetal status by means of the fetal heart rate and its responsiveness. A
cardiotocograph
Cardiotocography (CTG) is a technique used to monitor the fetal heartbeat and the uterine contractions during pregnancy and labour. The machine used to perform the monitoring is called a cardiotocograph.
Fetal heart sounds was described as ea ...
is used to monitor the
fetal
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...
heart rate and presence or absence of
uterine contractions
Uterine contractions are muscle contractions of the uterine smooth muscle that occur during the menstrual cycle and labour. Uterine contractions occur throughout the menstrual cycle in the non-pregnant state and throughout gestation.
Throughout ...
. 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 (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal deve ...
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 rate ...
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 (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
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
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by th ...
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
A contraction stress test (CST) is performed near the end of pregnancy (34 weeks' gestation) to determine how well the fetus will cope with the contractions of childbirth. The aim is to induce contractions and monitor the fetus to check for heart ...
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. .
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.
*
Cardiotocography
Cardiotocography (CTG) is a technique used to monitor the fetal heartbeat and the uterine contractions during pregnancy and labour. The machine used to perform the monitoring is called a cardiotocograph.
Fetal heart sounds was described as early ...
*
Contraction stress test
A contraction stress test (CST) is performed near the end of pregnancy (34 weeks' gestation) to determine how well the fetus will cope with the contractions of childbirth. The aim is to induce contractions and monitor the fetus to check for heart ...
References
{{Pregnancy
Tests during pregnancy