Knowledge of the development, normal sonographic appearance, and potential abnormalities of the umbilical cord is important in fetal assessment. The umbilical cord can be visualized throughout most of gestation and is detectable sonographically soon after visualization of the fetal pole. The normal umbilical cord is 50-60 cm long and may coil as many as 40 times, usually to the left. Abnormalities in umbilical cord size, degree of coiling, attachment, and position can have important implications for the outcome of the pregnancy. Structural abnormalities of the umbilical cord such as single umbilical artery, knots, cysts, and tumors may be associated with fetal distress or malformations. Color Doppler ultrasound (US) is useful in the identification and evaluation of structural abnormalities of the cord. By allowing measurement of blood flow velocity in the umbilical artery, duplex Doppler US may provide additional information in the evaluation of intrauterine growth retardation and twin-twin transfusion syndrome.