Short-term variability of fetal heart rate during pregnancies with normal and insufficient placental function.

The differential index (DI) describing the short-term component of the fetal heart rate (FHR) was measured serially in 65 fetuses during normal pregnancies, in 68 fetuses with suspected intrauterine growth retardation (IUGR), and in 84 fetuses with suspected postmaturity. The analyses of FHR variability were made by a microprocessor-based "on-line" method with the use of the abdominal fetal electrocardiogram (aFECG) in the statistical calculation of the interval differences. The analysis of FHR variability was successful 436 of 581 times (80%) in the reference group, 73 of 89 times (82%) in the group with suspected IUGR, and 121 of 127 times (95%) in the group with suspected postmaturity. There was no fetal distress in the reference group, but fetal distress did occur in 24 (35%) of the IUGR fetuses and in 13 (15%) of the postmature fetuses. The first percentile limit of DIs in the normal group was used as the normal limit of DI for fetuses with IUGR. In the postmature group the cutoff level of ID was set at 3.5. The sensitivity of DI in predicting fetal distress was 64% in the group with suspected IUGR and 54% in the group with suspected postmaturity. The predictive values of DI were 80% and 58%, respectively. The risk ratios of abnormal and normal DIs were 3.6 in suspected IUGR and 6.8 in suspected postmaturity. Hence, DI seems to be a sensitive and reliable indicator of developing fetal distress in suspected IUGR and postmaturity. The fairly high failure rate of aFECG at the onset of the third trimester of pregnancy limits the practical value of FHR analysis.