True regarding vessels of the umbilical cord:

Umbilical cord: The umbilical cord consist of two aeries and one vein. The aeries carries deoxygenated blood from the fetus to the placenta and the vein carries oxygenated blood from the placenta to the fetus.Umbilical Cord: Initially, there are four vessels two aeries and two veins.Of the two umbilical veins, the right vein disappears by the 4th month, leaving behind one vein (left) which carries oxygenated blood from the placenta to the fetus. (Remember : Left vein is left behind). The aeries carry the venous blood from the fetus to the placenta. Abnormalities:Single umbilical aery is present in about 1% of cases. This finding is sometimesIt is referred to as a 2-vessel cord. The loss of 1 umbilical aery likely occurs secondary to thrombotic atrophy of a previously normal aery. Less likely, the loss may result from primary agenesis of the aery. It is the left umbilical aery that is more commonly absent. It is more common in twins and in babies born of diabetic mothers. It is frequently associated with congenital malformation of the fetus. Ref: 1) Ultrasound in Obstetrics and Gynecology: Obstetrics. Volume 1, edited by Eberhard Merz, Page 1042) Textbook of obstetrics, By Dutta, 4th Edition, Page 40