Apparently baby girls have rounder skulls and baby boys have flatter, longer skulls. There isn't any scientific theory behind it though supposedly.
Nub theory is that you can see a nub between babies legs in some scans, and if it is horizontal/in line with the spine then it's a girl but if it's at an angle, then it's a boy, for obvious reasons I guess lol. I wish there was a way to know as soon as they start growing their bits lol xxx