Effect of maternal posture on the umbilical artery flow velocity waveform.

The effect of maternal postural changes on the umbilical artery flow velocity waveform, fetal heart rate and maternal blood pressure was studied in 27 normal singleton pregnancies between 23 and 36 weeks of gestation. A statistically significant change in umbilical artery Pulsatility Index (PI) was established for both maternal standing to supine position (rise) and supine to standing position (drop). These PI changes were not related to gestational age. A statistically significant drop in maternal systolic blood pressure was observed from standing to supine position. The rise in umbilical PI resistance when changing from standing to supine position may be caused by the sluice flow mechanism.