[The duration of the outpatient nonstress test (NST) and the diagnosis of fetal reactivity with noncomputerized cardiotocography, A critical review of 1160 tracings].

STUDY DESIGN

Our purpose was to verify the times of appearance and the relative percentage of fetal reactivity during a nonstress test. The minimum time for each control has been fixed at 30 minutes. To identify fetal reactivity we have observed the conventional patterns of fetal heart rate.

MATERIALS AND METHODS

We have examined 1160 cardiotocograms of 483 patients with obstetric low risk controlled by ambulatory NST in the morning and by appointment. We have divided the tracings according to three different periods of pregnancy (< 37, between 37 and 40 and over 40 weeks) and each tracing in three periods of 10 minutes. In each period we have checked the mean values and standard deviations of accelerations, decelerations, uterine contractions and fetal movements. For each patient the total numbers of NST and the type of birth (spontaneous or caesarean section for fetal distress) has been considered. At statistic analysis we have employed "t"-Student test.

RESULTS

We have observed a significative difference (p < 0.001) between the presence of fetal reactivity after 20 and 30 minutes and the number of doubtful cases after 30 minutes for the different periods of pregnancy. With minimum time of 30 minutes of NST, an incidence of doubtful cases of 4.48% and an incidence of caesarean section for fetal distress for the same group of 19.27% has been observed, while for all the cases with reactive NST this incidence intrapartum has been of 9.73%.

CONCLUSION

Our experience confirms that the optimal duration of NST has been of 30 minutes. We think that this time must be suggested also from prudence, since the noncomputerized cardiotocography is often characterized by false positiveness and low predictability.