Population differences of fetal biophysical and behavioral characteristics

Population differences in nonstress test reactivity have been reported with a threefold increase in the likelihood of nonreactive nonstress tests observed in black fetuses as compared with white fetuses. We analyzed fetal behavioral states and fetal heart rates in 14 black and 15 white fetuses at term to explain this observed difference in nonstress test reactivity. Two-hour Doppler and real-time ultrasonographic examination of each patient revealed no differences in percent time spent in the four behavioral states between the two populations. A 9.5 beats/min difference between black and white fetuses was found. The higher baseline heart rate of the black fetuses persisted in each behavioral state and may affect nonstress test reactivity because of rate-dependent decreases in short-term variability and rate-dependent limitations of maximal acceleration amplitude.