The normal umbilical cord contains two arteries and a single vein. A single umbilical artery (SUA) is the most common umbilical pathology in humans and occurs in approximately 0.2-1% of pregnancies. The SUA usually occurs as an isolated defect; however; a relationship has been confirmed between this abnormality and various fetal malformations, such as abnormalities concerning the musculoskeletal, urogenital, alimentary and cardiovascular systems. In addition, concomitant chromosomal abnormalities and intrauterine growth retardation (IUGR) are common in fetuses with SUA. SUA is an independent risk factor of perinatal mortality, and its high percentage (up to 25%) is associated with prematurity. Ultraso- nography is the principal diagnostic technique for identifying the structural abnormalities of the umbilical cord. In recent years, the detectibility of the SUA has increased, together with the development of ultrasonographic techniques. Color Doppler Imaging is extremely useful in the assessment of the structure of the umbilical cord, and should be an inseparable element of ultrasonographic imaging performed between weeks 18-22 of pregnancy. The diagnosis of the SUA should be associated with deepened diagnostics and intensified monitoring of the normal development and wellbeing of the fetus. After delivery, a comprehensive assessment of a newborn should be performed by a neonatologist. The study presents current knowledge concerning a two-vessel umbilical cord, the diagnostic methods available, and the effect of this abnormality on the course of pregnancy.