
I have never found mine scratching up their beds until less than 24 hours before whelping, and usually not until first stage labour starts as it isn't so much nesting as reaction to the pain.
full term pups may be born as much as a week before the due date, and most of mine have whelped from 4 days before to a day before, rarely going to date or over, but then my girls tend not to mate until after ovulation, usually when the eggs have ripened, and not at or just before ovulation as many bitches will.
To explain, the times I have had my bitches premate tested they have ovulated around day 12, but not been mated until two or three days later.
the later in the fertile period they are mated generally the shorter the pregnancy will seem.