I have been Googling about this for weeks and not sure why this just came up for me now. This is the best, short, comprehensive (AND reassuring) piece I have found about isolated SUA (SUA with no other markers). I might send this to my parents. Just thought someone else may want it, too.
https://www.obfocus.com/high-risk/birthdefects/single%20umbilical%20artery.htm
Re: Single Umbilical Artery Info--for anyone who has it and wants a resource to share