I had a similar experience with DS2... by dates I was 6 weeks, however the u/s showed nothing and doc said I was < 4 weeks (HGC agreed with doc). I was super confused as I'd had a BFP almost a week beforehand so couldn't work it out.
At 6 weeks I had another scan and everything was ticking along nicely, dates gelled with the docs estimates so I just gave up and went by their timing.
At my 12 week scan I was given another date somewhere between my LMP due date and the docs estimate.
At 20 weeks they agreed with the "middle ground" due date
Had lots of scans for various reasons, but started measuring a bit on the "larger" side
Induced on his due date, he was over 1kg bigger than DS1 and had the bright red 'peely' skin that overdue babies often have. In the back of my mind I get the feeling my original dates may have been more accurate, but I guess everybody bakes their babies at different rates and spot on 40 weeks isn't "normal" for most people.
Wow... that was a novel! I guess my point is... don't give up hope just yet, it's hard waiting, but you may find in a couple of weeks you'll be seeing a little hearbeat on the screen! ... good luck!