When My Ds was born the nurse did not note down the number of blood vessels in the umbilical cord, this was 30 years ago btw, so I assume that midwives routinely check the cord as the two vessels mean two kidneys
No it doesn’t.
Due to the way the umbilical cord and kidneys develop, there is a slightly increased risk of a kidney malformation if there is an umbilical one.
Most 20 week scans will pick up any issues including how many umbilical vessels as cord blood flow is one of the major things to check. So probably no need to count vessels any more.