Should I be concerned that my baby's cord has an extra vessel?

See below. While there are some congenital abnormalities that are associated with extra vessels in the umbilical cord, these are usually able to be detected on ultrasound. Talk to your OB doctor and/or ultrasound doctor to ask their opinions.
Yes. This is a very rare event. The baby could be entirely normal but may have birth defects. You should see a high risk doctor and get a thorough ultrasound. You may consider getting an amniocentesis to check the baby's chromosomes.