Appreciate this is a bit of a specific issue, but I had my 20 week scan and they couldn't find the uterine arteries to take a reading of the blood flow. The sonographer said "you can never not find them" and even called a colleague in for a second opinion and help.
I was in for a good long while but eventually she had to give up, and I've gone by default on the extra scans pathway to check for growth.
Anyone else come across this scenario? I can't find anything online, it does appear that you can never not see them!