Umbilical artery and uteroplacental blood flow velocity waveforms in normal pregnancy--a cross-sectional study.

Umbilical and arcuate artery blood flow velocity waveforms (FVW) were recorded in 125 normal singleton pregnancies from 20 to 42 weeks of gestation. The FVW were analysed for pulsatility index (PI), peak systolic velocity/minimum diastolic velocity ratio (S/D ratio), rising slope (RS) and descending slope (DS). Both in the umbilical and arcuate arteries, values for all variables declined with advancing gestation, indicating decreasing placental vascular resistance. The umbilical artery PI was unaffected by the fetal heart rate, but the arcuate artery PI was negatively correlated to the maternal heart rate (r = -0.40). The arcuate artery PI decreased by 0.00394 with each beat per minute increase in maternal heart rate. Normal limits (mean +/- 2 SD) were established for umbilical artery PI corrected for gestational age, and arcuate artery PI corrected for gestational age and maternal heart rate.