At my 12 week scan we didn't get a proper "nub shot." However, my ultrasound tech asked if I wanted her to guess and I said sure. She pointed at what to me just looked like a line (and it didn't even look like it was where a nub should be), and said, "I'm guessing boy, because that's not the umbilical cord." 4 weeks later at my private gender scan- sure enough, it's a boy! I agree with what pp said, if they are willing to guess- then I think they're pretty certain.