Most umbilical cords have two small arteries (conveying pulses of blood from the left ventricle of the heart to the placenta) and one large vein (that returns a constant stream of oxygenated blood from the placenta through the liver to the right atrium of the heart). The arteries usually spiral around the vein, giving the cord the toughness of a cable. These are color coded 3D Doppler images showing blood flow through the cord, arteries in red and vein in blue.

We pay special attention to the arrangement of vessels in the cord, because segments with the vein exposed (as in the right view above) can be compressed leading to the potential for fetal injury, especially when the cord loops around the neck or a limb. Some elegant experimental studies have shown that intermittent, partial compressions of the umbilical vein can cause a range of serious injuries of the brain, the organ that is most sensitive to blood oxygen level.

We also look for those cases in which the vessels of the cord start to branch and spread outside the placenta. This is called a Velamentous Insertion, and the worries are that these thin and delicate vessels cannot support all of the blood flow needed late in pregnancy and that they are weak.   An important variant of this is the Vasa Previa in which the the cord or branched cord vessels get trapped between the head and the cervix during early labor. Delicate vessles will rupture with the force of contractions with sudden and massive fetal hemorrhage. Recognizing vasa previa before delivery averts a catastrophy. For more information about visit the Vasa Previa Foundation.