There are plenty of myths, yes, but none of them will tell you the sex of your baby!
It's not a gender scan btw, it's an anomaly scan to check for abnormalities in the baby's brain, heart, lungs, stomach etc. But they can usually tell what sex the baby probably is. Not all hospitals will tell you, is that why you are not going to find out there?
there is no way to tell 100% until birth BUT a private gender scan is prob most reliable (99%) with it being private they have the time to have a good look and if babys not in the correct possition most will give you another day to come back free since they want to keep there reputaion they will make sure before they tell you. Having said that in my 20 week scan i asked for confirmation he's a boy and she showed me on the scan and there was NO mistaking he has boy bits could very clearly see his willy n balls so if it's a girl she's got extra bits!