Uteroplacental blood flow velocity waveforms in the second and third trimesters of normal pregnancy as related to the placenta location.

The systolic/diastolic ratios (A/B) of uteroplacental blood flow velocity waveforms as related to the placenta location were investigated by continuous-wave Doppler ultrasound in 67 normal pregnancies at 18-21 weeks, in 111 at 31-33 weeks, and in 34 at 40-41 weeks of gestation. Using real-time ultrasound, the placenta was located laterally in 48%, posteriorly in 30% and anteriorly in 22% of the pregnancies. A/B values decreased slightly with advancing gestational age. With lateral placenta location, statistically significant higher A/B values were found in vessels on the nonplacental side at 31-33 weeks of gestation (2.07 +/- 0.43 versus 1.74 +/- 0.27; p less than 0.05). In the other gestational age groups there were no statistically significant differences between A/B values in the vessels on the placental and nonplacental sides. There were also no differences in A/B values between left and right uteroplacental vessels with anterior or posterior placenta locations in any of the gestational age groups.