Nonstress test: dimensions of normal reactivity.

Despite the widespread use of the nonstress test, there has been no consensus on the criteria for normal reactivity. The authors studied 495 nonstress tests of 230 normal term fetuses, using a standardized protocol. The frequency distribution of movement-associated fetal heart rate (FHR) accelerations exceeding 15 beats per minute and 15 seconds' duration was examined in windows, ranging from ten to 40 minutes. Mean, median, and modal frequencies of movement-associated FHR accelerations were determined in 1910, 1418, 895, and 473 windows of ten, 20, 30, and 40 minutes, respectively. No significant difference in movement-associated FHR acceleration frequency was observed for any sequential ten-minute window, but movement-associated FHR accelerations were absent in 19.5, 8.8, 5.2, and 5.0% of 10-, 20-, 30-, and 40-minute windows, respectively. The broad frequency distributions of movement-associated FHR accelerations, produced by normal fetuses, precludes discrete diagnostic cutoffs in short time windows, unless one is willing to accept significant negative predictive errors.