The premise of the NST is that a well-oxygenated, non-acidemicfetus will spontaneously have temporary increases in the fetal heart rate (FHR).
Reactive (normal)- presence of two or more fetal heart rate accelerations within a 20-minute period, with or without fetal movement discernible by the woman. Accelerations are defined as 15 bpm above baselines for at least 15 seconds if beyond 32 weeks gestation, or 10 bpm for at least 10 seconds if at or below 32 weeks.
Nonreactive- presence of less than two fetal heart rate accelerations within a 20-minute period over a 40-minute testing period.
Vibroacoustic stimulation can wake the fetus, and is sometimes used to speed up the test or to facilitate further evaluation of a nonreactive nonstress test.
^Smith, C. V.; Nguyen, H. N.; Phelan, J. P.; Paul, R. H. (1986). "Intrapartum assessment of fetal well-being: a comparison of fetal acoustic stimulation with acid-base determinations". Am J Obstet Gynecol155: 726–728.