Has there been any change in her behaviour? Are her teats enlarged? How many times was she mated? Did you do anything to know if it was the 'right time' for her to be mated? I believe there is an ovulation test 
When Jo Jo was mated, i was advised to check from about day 7 onwards to see if the bleed had gone a pinky/yellowy colour, which basically just means dabbing some white kitchen roll in her ladies area. Oh, just thought of another thing, is the stud dog proven? sorry for all the questions, just thought it may help.
I am certainly no expert but i would have thought that 31 days is a bit too early for a scan 
It still could be possible that she is pregnant.
I scan my girls at 21 days and if pregnant can always pick up pups, so I would say at 31 days if there were pups there you would definitely see them.
Are there different or better scanners than others?
Or is it a case of the more experienced the user, the more accurate the scan?
Its just that i have heard of vet scans being negative, or being told there are say 3 puppies, but the dog going onto have many more puppies than the scan revealed. This must happen, Eve mentioned in her post about the lab who was scanned negative and then went onto to have 'surprise' puppies.
