Weeks incorrect? Boy/Girl theories?

Kiah
I went for my 12 week scan yesterday but the lady said I'm measuring 10weeks 4days.. 
Has anyone else had different dates to what they thought?
I calculate it from my last period but she's adamant I'm 10 weeks 4?
Could little flump have a growth spurt and measure up? I'll be going back in 2 weeks!
Plus... what are gender guesses (early I know)