Modulation of middle cerebral artery flow velocity waveforms by breathing movements in the normal term fetus.

Breathing dependency of fetal middle cerebral artery flow velocities was studied in 10 normal pregnancies at term. Peak systolic, end-diastolic and time-averaged flow velocities and pulsatility index during fetal breathing were not significantly different from those observed during apnoea. However, the standard deviation for each flow velocity parameter was significantly increased during fetal breathing activity, indicating marked modulation of middle cerebral artery velocities.