The effect of maternal supine position on umbilical and cerebral blood flow indices

The brain sparing phenomenon in the fetus is a protective mechanism aimed at maintaining sufficient blood flow towards the brain during chronic or acute fetal stress, such as that caused by hypoxemia or utero-placental insufficiency. In this study we investigated whether the brain sparing effect can also be elicited by a physiological stress associated with maternal posture. Study design Twenty-three low-risk pregnant women participated in the study. Between 36 and 40 weeks' gestation, Doppler flow velocity waveforms were obtained from the fetal middle cerebral and the umbilical artery in the supine and the left lateral decubitus positions. Pulsatility index, systolic/diastolic index, and peak systolic velocities were measured and comparison was made between the left lateral and supine positions.

The pulsatility index in the middle cerebral artery decreased from 1.78±0.27 in the left lateral decubitus position to 1.29±0.16 in supine position (p<0.0001). Peak systolic velocity decreased from 46.05±7.85cm/s to 39.43±7.95cm/s, respectively (p=0.001). The pulsatility index in the umbilical artery decreased from 0.89±0.13 in the left lateral position to 0.74±0.11 in the supine position (p<0.0001).

This study demonstrates that the supine position in late pregnancy, causing aortic and venacaval compression, leads to brain auto-regulation that activates the brain sparing effect in the fetus. This protective mechanism, shown here for the first time to be linked to a physiological stress, may provide the basis for a novel approach in the assessment of fetal wellbeing.