18 week scan, boy or girl? Who's knows, and who is finding out?

[Meluchja]

We are hopefully going to find out the sex of your new bubba this afternoon. Just wanting to know who has already and who plans to find out.

I was told, that if they say it's a boy they are almost definate and it's safe to say it will be for obvious reasons, but if they say it's a girl, that it may still be a boy as it's possible the little penis just got missed. Has anyone else heard this before? Do you think it's true? It sort of makes sense to me.