
A lot depends on the expertise of the operator. Mosdt like to see them at 30 days post mating. I did have one scanned at day 25 by a vet who specialises in scanning for diagnosing all sorts of things (Cancer etc)not just preganancy. I was almost certain she couldn't be, as this was my picky bitch who would not eat well throughout her season and usually a few weeks after that. She was very thin and he thought she would have 5 or moer pups (ended up with 9).
Since that time my own vet is very good at palpating so have usually had them in at day 28 for him to feel, and only used the scanner if he was uncertain.
Unfortunately he is on holiday and by Monday it will be a bit late to reliably palpate so will have to rely on his fledgling scanning skills on day 31.
His stand in did feel yesterday, as she is quite good with small breeds. She rather thought she was in whelp (hope she is right) but wanted me to go back and have her scanned as she wasn't as sure as she would like to be, as this is the largest bitch she has ever tried to palpate.