The left portal vein as an indicator of watershed in the fetal circulation: development during the second half of pregnancy and a suggested method of evaluation.

Abstract

The left portal vein represents a watershed area in the fetal circulation and the change of flow direction is a suggested marker of circulatory compromise. Our aim was to study the left portal vein of uncompromised human pregnancies and establish reference ranges. One hundred and sixty low-risk pregnant women were recruited to a longitudinal study that included four to five Doppler measurements at monthly intervals during the second half of pregnancy. Based on 554 Doppler recordings we found time-averaged maximum flow velocities increasing from a mean of 10.6 cm/s to 14.2 cm/s during 21-31 weeks of gestation, remaining stable until 37 weeks and decreasing towards term. Intermittent flow reversal occurred during fetal respiratory movements, and continuous flow reversal was seen in 2/160 fetuses close to delivery. We propose time-averaged maximum blood velocity measurement for quantification of the watershed phenomenon in the left portal branch, and have provided reference ranges for single observations and serial measurements.