I had the same thing with my u/s too...They had to do an internal coz they couldn't get a clear picture. They saw the h/b fine, but put my dates back a week. By my calculations though, there is no way I could have conceived when the u/s dates say I did - I was in Sydney with a girlfriend, and no boys allowed!!

One of my best friends had it happen to her as well. She knew her dates exactly coz she fell pregnant the one and only time she had sex! Yet the sonographer consistently told her she was 2-3 weeks behind her dates!

Go figure