Umbilical cord

Has anyone else been told they only have 1 artery in their umbilical cord instead of 2? I was told that today at the 20 week scan. The Drs don't seem too concerned because I've had the available blood tests for chromosomal abnormalities which all came back negative and everything else on the ultra sound looked good. Loved to hear others experience of this. They also say they don't know why it happens.