About 1% of all babies will have a two vessel umbilical cord, which is a single artery (Single Umbilical Artery) rather than two. When this is found, you should be referred for a more detailed ultrasound. The reason that another ultrasound is done is to double check the findings and to ensure that your baby does not have any abnormalities. About 20% of babies who only have one artery in the umbilical cord will have malformations.
Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.