Fetal and maternal Doppler flow velocity waveforms in normal sheep pregnancy.

Abnormalities in placental vascular resistance have been shown to have an important association with intrauterine growth retardation in both human and sheep pregnancies. In this study we have assessed impedance to blood flow through the ovine placental circulation by Doppler flow velocity waveform analysis. As gestation advanced from 66 to 136 days, a significant decrease in impedance to flow was observed in both umbilical and maternal placental arteries. The greatest decrease in impedance was observed between 66 and 109 days' gestation in umbilical arteries and between 66 and 80 days' gestation in maternal placental arteries. In these normal pregnancies, no significant correlations could be found between Doppler flow velocity waveform analyses and fetal or placental growth. We conclude that Doppler flow velocity waveform analysis is a valuable and non-invasive method to assess impedance to blood flow through the placental circulation in pregnant sheep.