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I have to say my vet says I am very good also,the earliest would be 26 days for me,by 34 days the usterus is filling with fluid and is hard to feel them unless you have a singleton.Some bitches carry them high up to the backbone almost and some way down low in the abdomen,some are way back back by their butts and some are way up in the ribs. Hard to find them sometimes and again it depends on where they are implanted and where your bitch carries her pups.By 26 days they are marble size maybe a tinch smaller but by 30-32 days they are grape size like Pat said.(unless you have a singleton and then they are about quarter size by 30-32 days.)
It is easier for me if they are standing and she has to be relaxed. The fatter the bitch the harder to palpate the skinner the bitch easier it is and a empty stomach.
My vet showed me how to do it about 20 years ago.Can always ask your vet to show you how.