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

ABSTRACT 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. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.