This is a common abnormality and in most cases the baby is born with no problems - thus, your health care provider's casual response. Before routine ultrasound scans most people didn't learn of the condition until birth when the nurse counted the vessels and noted whether there were two or three.
There is a slightly higher incidence of other anomalies but the fact that your ultrasound did not reveal any other concerns suggests your case will be one of those that does just fine with only one artery. It seems the second one is a bonus; kind of like having two kidneys when one will do the job.
The doctor will be watching closely for any unusual findings because of the potential for associated abnormalities. Sometimes there could be kidney problems so they will watch for fluid problems - either more or less than normal. Growth is the other concern so this will be monitored as your pregnancy proceeds. But, as I said, since your ultrasound was normal, chances are great you will not have any problems with your baby.
The answers in online forums are for information only and are not intended to substitute for medical advice and care. Please see your personal health care provider for further evaluation of your individual case.
Debbie Miller, RN