A nonstress test (NST) is an assessment of fetal heart rate, as measured through the mother's abdomen using a Doppler ultrasound. A tocometer is also used to measure uterine contractions.
It is indicated if uteroplacental insufficiency or fetal distress is suspected.
The mother is placed in a lateral tilt position to prevent supine hypotension.
A normal (reactive) response is two accelerations of >15 bpm over baseline, sustained for at least 15 seconds, over a 20-minute period.
An abnormal (nonreactive) response is less than two accelerations over 20 minutes, warranting a biophysical profile.
A lack of FHR accelerations can occur with a sleeping fetus, GA<32 weeks, fetal CSN abnormalities, or sedative/narcotic medications. The fetus can be stimulated using fundal pressure or acoustic or vibratory stimulation.