I have never bothered with scans as even with the most docile b itch, to me there is no point subjecting her to the stress of going to a vet (and yes, most do find a vet visit stressful - it doesn't have to mean they have a poor temperament) where there is also the risk of picking up an infection. By 7 weeks you should see puppies moving around. For most, if you know your animals, you may notice slight thickening of the flanks by 4 - 5 weeks. and with mine, the vulva doesn't go back to how it would post-season with a successful mating. Many of mine would come out of season sooner than normal, regardless of making sure they are not got at for the 3+ weeks of a normal season. Scans only show a litter, or not, at the time of the scan too - that doesn't mean the b itch will carry to term.
I'm afraid you have to wait. Frustrating as it can be, and make your plans assuming there is a litter there. With mine, I don't let them go much over 2 days from the due date without seeing my vet. At that point he may do an x-ray to confirm the situation.
Just to add, it's good not to let a pregnant b itch get 'fat'. Just as with humans, being fat isn't helpful in terms of whelping.
I am sad that too many here jump to conclusions that may or may not be the case.