Yep, my DS1 was named Phoebe after the 20 weeks scan, then I was scanned weekly due to my health although I didn't ask again until 30 weeks when he quickly became Jack! The sonographer said that it's easier to make the mistake of saying girl when it's actually a boy at 20 weeks because of bits being tucked away but they almost never get it wrong the other way.
I do recall calling my (now ex) DP and saying "er, your daughter has grown something she shouldn't have... A pair of nuts" 
Now I had my 20 weeks scan done in 2 parts this time by 2 different people and they both said girl but I'm still not convinced after having 3 boys, we have a name picked out for both just incase.
I've taken to sleeping slumped forward over a stack of sofa cushions atm, I cannot wait until the 14th to get this over and done with, then I never have to be pregnant again! however much DP protests!