
Go by the number of DAYS since the first mating. I never let my girls go over by more than 2 days without involving my vet. Mine often went into secondary inertia. Bassets aren't as stoic as some might think and at the first feeling of pain, it's ouch mum .... you 'put them in there' now you get them out!! Hopefully you'll have a free whelping bitch - unfortunately mine were not because of the 'ouch mum'. More often than not I opted for a Section to get it over and done with.
And it's highly possible that a Basset will keep their pups up under their ribcage - I never bothered with scanning, again preferring to go by the number of days involved.
Labour isn't scraping up - you may, if you are observant, see some minor rippling which is the start, but this scraping nests can go on for some time.
If you used a known stud dog, with somebody experienced in the breed, SPEAK TO THEM. She is older than I'd put in whelp for a first litter btw. Most of mine were retired by 5, and spayed!!
A false pregnancy, the symptoms, don't start prior to the whelping date, but a few weeks later. Bottom line, if she's more than 2 days over her due date, involve your vet. Don't just sit and wait.