Afferent venous perfusion of fetal liver: umbilical and portal blood-flow volumes in fetuses born small-for-gestational age

Objective: To quantify the dynamic changes in the afferent venous flow volume of the liver in low-risk pregnancies with fetuses born small-for-gestational age. Methods: This was a prospective study of low-risk singleton pregnancies with estimated fetal weight (EFW) and birth weight ≤ 10^th centile attending for a routine second- or third-trimester ultrasound examination. Their umbilical and portal blood-flow volumes were compared with those of a control group of fetuses born appropriate-for-gestational age from which normal reference ranges were constructed. Absolute and Z-score differences between the groups were assessed. Results: In total, 133 fetuses were included in the study group and 362 in the control group. The mean umbilical blood-flow volume in the study group, both absolute and normalized per kg of EFW, was below that of the appropriate-for-gestational-age fetuses for most of the period of pregnancy studied (overall mean Z-score, –0.82 and –0.84, respectively). In contrast, the mean portal blood-flow volume, per kg of EFW, showed the opposite trend (overall mean Z-score, +0.86), reaching its maximum level (+1.43) in the late third trimester. This resulted in a steep decrease in the mean placental-to-portal-blood-flow volume ratio, from 14.4 at 24 weeks of gestation (above the 60^th centile) to 4.7 at 38 weeks of gestation (15^th centile), corresponding to Z-scores of +0.4 and –1.02, respectively. Conclusion: In fetuses born small-for-gestational age, the ratio of blood-flow volume in the umbilical vein to that in the portal vein decreases consistently during pregnancy, and to a greater extent compared with those born appropriate-for-gestational age, reaching a lower nadir in the third trimester. This additional redistribution of liver perfusion affects negatively fetal growth even in low-risk pregnancy, and should be taken into account when planning delivery. We suggest considering liver venous perfusion as an ancillary tool for monitoring small-for-gestational-age pregnancies.