I think it's as close to 13 weeks as you can get. At 12 weeks it's 75% accurate, 95% at 13 weeks... and after that it can apparently be difficult to nub-guess for some reason.
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I don't believe that, because I have always had dreams of both. The only dream I have had so far this time is one where the baby kicked it's way out of my stomach and I was holding it, trying to figure out how to stuff it back in. I couldn't see if it was a girl or a boy, but looked at the feet, which looked like mine, and decided it was probably a boy because both my daughters have my DH's feet. But then I stuffed the baby back in before I saw whether I was right. So I guess that dream was inconclusive!
Good luck with your scan pinkin!!
I've only ever had one dream of having a baby girl! :p