Hello and WELCOME
!
A "good" breeder of tiny dogs will always keep puppies until at least 12wks, and often longer. My tiny Marcel was kept until 16wks, and I sure am grateful to my breeder that he was! The tiny ones are incredibly fragile and vulnerable, trust me.
As far as weight - one can't even begin to guess until at least 8wks of age - and even then, you can still be 100% wrong in that guesstimate, lol
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Can't wait to see pics of your little one - post them when you get some
!