
It's not always possible to see anything by week 5, especially if she's a lengthy bitch with a smal litter, carried up under the ribs. My biggest clue was the fact that the vulva usually stayed 'up'/more obvious if there was a pregnancy, than if not. I never bothered with scanning - by week 7 you usually see puppies moving around and 2 weeks is plenty enough time, if you haven't already, to make plans, put up the whelping box etc. before the puppies arrive.
For me, and I was lucky enough not to be working once we were into a breeding programme, it was just a question of waiting. And even if a scan had told me empty, I'd STILL have been hoping up to the 2 days I left mine before involving my vet (palpate/x-ray to confirm what was going on).