The blood flow velocity waveform in the fetal internal carotid and umbilical artery; its relation to fetal behavioural states in normal pregnancy at 37-38 weeks.

The relation between the blood flow velocity waveform in the fetal internal carotid artery (n = 12) and umbilical artery (n = 15) and fetal behavioural states at 37-38 weeks of gestation was studied. In the fetal internal carotid artery, under standardized fetal heart rate conditions, the pulsatility index (PI), as a measure of peripheral vascular resistance, was significantly lower during behavioural state 2F (active sleep) compared with state 1F (quiet sleep) according to the classification by Nijhuis et al. (1982), suggesting increased fetal cerebral blood flow during state 2F. In the umbilical artery, no significant difference in PI between the two behavioural states could be established. This suggests a fetal origin of the state dependency observed in fetal blood flow velocity waveforms.