Over the yeras we have given up on scanning it causes more trouble than its worth scanning is done from 28 -32 days and to be honest by 35 days you should be able to tell by looking at her. I would advise scanning at 35 days to make sure there isnt a small litter or one pup if your girl doesnt look in pup of course the breed of dog may make a difference in this case . some breedrers are lucky enough to have found a skilled scanner operator who they can put complete faith in sadly we havent!! so prefer to leave things to nature