A cardiotocograph is used to monitor the fetal heart rate and presence or absence of uterine contractions.
[1] The premise of the NST is that a well-oxygenated, non-acidemic fetus will spontaneously have temporary increases, termed "accelerations," in the fetal heart rate (FHR).
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.
[1][4][5] "Non-reactive" is defined as fewer than two adequate accelerations during a prolonged period, which may be over an hour.
[1] If the nonstress test is reactive, the rate of fetal demise in the following week is approximately 1.9 per 1000.