I wouldn't worry either although I know a due date would be handy!
I do know this though, ultrasounds performed early in the pregnancy give the most accurate due date. Your 12 week one should give you almost the same date also.
As you head into the second trimester babies will all grow at different rates so it can be difficult to get an exact due date of your baby then.
I wouldn't believe what the web site says either. Is it an American site? I think they calculate pregnancy different to us over here is Australia anyway.
Good luck to you!
<a href="http://lilypie.com/"><img src="">http://lbd