The umbilical cord typically contains two arteries and one vein. Single umbilical artery (SUA) refers to a variation of umbilical cord structure in which there is only one umbilical artery. The incidence ranges from 0.5 to 6 percent of pregnancies, depending on the population studied.
SUA is an isolated finding in approximately 65 percent of affected fetuses . In the remainder, aneuploidy with or without structural malformations and intrauterine growth restriction are present and congenital anomalies.
Between three and five weeks of gestation, the umbilical cord is formed by the fusion of the yolk sac and the body stalk . During this process, the two umbilical veins fuse to form a single vein. The umbilical vein carries oxygenated blood from the placenta to the fetus. Concurrently, the umbilical arteries arise from the allantois, which is a portion of the yolk sac. The umbilical arteries carry deoxygenated blood from the fetus to the placenta.
Three mechanisms have been proposed to explain the embryogenesis of SUA: primary agenesis of one of the umbilical arteries, persistence of the original single allantoic artery of the body stalk, and secondary atresia or atrophy of a previously normal umbilical artery . The latter mechanism is probably the most common, and is strongly supported by the detection of muscular remnants in approximately 40 percent of umbilical cords from cases of SUA undergoing microscopic examination .
SUA may be classified into four types based upon the likely developmental etiology [5,6]: