I had one at 33 weeks, dont expect to see your baby, if that makes sense, they`ll be measuring various parts of him, legs, head, belly, fluid etc, so you might not get to see the whole picture, again, I hope that makes sense.
They are supposed to be inaccurate as others have said, I was told 4 weeks ago that mine is measuring too small, in that at 33 weeks he looked like he weighed 3lb 8oz, so I have to go back on Wednesday for another one, and to see a consultant, but Im not worried, Ive got a really active one, and all his bits are there and doing what theyre supposed to do.
They may tell me he has to come out sometime this week or next, or they may tell me he`s had a growth spurt, although not too much of one, mummy doesnt want to squeeze out a 10lber thank you very much 
Try not to worry and do update us on Tuesday