Umbilical vein to umbilical artery ratio with single umbilical artery

Schematic drawing shows the compensatory increase in size of the single umbilical artery. In the normal three-vessel umbilical cord, the circulation is theoretically distributed equally in two arteries (left), resulting in an umbilical vein to umbilical artery ratio (UV/UA) greater than 2. In fetuses with single umbilical artery (SUA), the size increase of the artery results in a UV/UA ratio lower than 2.