What is the earliest your girls have given birth and was the litter okay ....

I have had pups 5 days before due date for first mating, also 4 days and most of my breed whelp before 63 days (usually day 61 or 62), very rarely go over.

I know people whose bitches have had pups a full week early in other breeds with no problems

55 days -just, labour started day 54 from first mating, were born overnight. All pups were large and perfectly healthy and it was a fairly big litter for the breed. My current litter, big for that breed, 3 were born on day 57 and 9 on day 58 as again labour went overnight. All fine except we did have a thirteenth that was stillborn.

I have a breed that regularly whelps early.  Day 56 being their preferred date!  Perfectly formed pups, no bald ones.  I once had a bitch whelp 10 days early, and it turned out to be just too early.  The lungs had not finished developing properly.