Effects of coiling index and vessel diameter on umbilical artery flow: A computational fluid dynamics study

The hemodynamics of the umbilical artery (UA) are critical to fetal development, with the umbilical coiling index (UCI) and vessel diameter identified as key geometric factors influencing blood flow patterns and related pregnancy outcomes. In this study, a three-dimensional computational fluid dynamics (CFD) model based on the Carreau non-Newtonian viscosity of blood is applied, and validated by particle image velocimetry experiments, to investigate the independent and combined effects of UCI and vessel diameter on UA hemodynamics. The results reveal that the UCI increase promotes the uniformity of velocity distribution, significantly enhances inlet pressure (up to twice that of a normal UA) and peak wall shear stress (WSS), with the extension of high Q-criterion region. It potentially induces endothelial stress and impairs placental perfusion, thus increasing the risk of fetal growth restriction (FGR). However, the UCI decrease can accelerate blood flow, also destabilizing flow and possibly disrupting placental function. The vessel diameter reduction has a pronounced impact that remarkably increases flow velocity, pressure, high Q-criterion region size, and WSS (up to fivefold), which may exacerbate flow irregularities and elevate the risk of FGR. Enlarged diameters reduce velocity and may raise blood viscosity, with limited effects on pressure and WSS, potentially increasing the risk of thrombosis. When UCI and diameter vary simultaneously, the vessel diameter predominantly determines hemodynamic behavior, while the UCI only has a secondary influence. The secondary flow structures remain spatially consistent, showing robustness to geometric variations. This study quantitatively reveals how UCI and vessel diameter affect UA flow, offering insights into placental dysfunction and adverse outcomes.