The preterm nonstress test: effects of gestational age and length of study.

The application of the nonstress test between 24 and 32 weeks' gestation has been limited by high rates of "false" nonreactivity in normal fetuses, by use of term criteria, and the lack of age-appropriate interpretative standards. To establish such standards, we studied 30 normal fetuses undergoing 90-minute fetal heart rate recordings at 2-week intervals from 24 to 32 weeks' gestational age. Using a specially programmed computer we quantified (1) baseline fetal heart rate, (2) incidence of 10- and 15-beat accelerations, and (3) incidence of fetal heart rate decelerations. With a criterion of three 15-beat accelerations per 30 minutes 91% of tests were reactive within 90 minutes. A criterion of three 10-beat accelerations per 30 minutes was associated with 100% reactivity within 60 minutes. Suitable interpretative criteria may be established for nonstress tests before 32 weeks' gestation by extending the testing time or by decreasing the minimum amplitude required of fetal heart rate accelerations.