Sorry - misread your post. I thought you were saying she had been scanned by the vet, and had then returned.
Most of the scanning equipment used by vets on site isn't sophisticated enough to tell how many, how far, etc. I'd go back to the breeder and check on his dates - take the date he thought she was served and then work out the dates of subsequent cycles. That should give you a better idea of when to look for signs of impending farrowing.