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First off MOm most likely will have a hard time with delivery being that small.
Eclampsia pops right up also with a 3lb girl and puppies and her being a pup herself the poor gal will be drained.
Not being a good mommie for she is still just a baby herself.
Not having healthy pups for she is growing her self and trying to support the growth of puppies.
If section is needed then you have the risk of loosing her with that and pups also (and a big hefty bill to go along with it).
My vet would have a fit if I did something like that!!
How do they know she will be big enough???(did they do a rectal on her,how does he know????)
Just because mom had small pups does not mean she will.
Read all you can ( you can never know enough) and do you have a mentor or someone to help you with her?
Count 63 days after the first tie ,that will be your EDD.
Debbie Jensen has a wonderful site on whelping etc.....
Good luck to you and your little mamma.
I would get another vet.