The nonstress test. Criteria for the duration of fetal heart rate acceleration.

Although the most common definition of a reactive nonstress test is an acceleration in the fetal heart rate of 15 beats per minute (bpm) for 15 seconds, some investigators require that the fetal heart rate acceleration be maintained at 15 bpm above the baseline for the entire 15 seconds (long criterion), and others require simply that the acceleration be 15 seconds from the beginning until the return to the baseline (short criterion). In 1,241 nonstress tests (NSTs) there was a statistically significant difference in the number of reactive tests between the short and long criteria (1,108 [89%] vs. 985 [79%], P less than .001). This difference in percentage of reactivity may be important when comparing studies performed by various investigators. In comparing cesarean section deliveries for fetal distress we were unable to find a difference in the ability to predict a poor outcome between use of the short and long criteria. However, a type II error is possible since the number of poor outcomes was small. We now use the short criterion for NST interpretation.