How about the other signs of calving,
How is the bag, the teats are they full looking or still sloppy?
Has she fallen?...around the tail bone, is it soft. My husband says the cord is normal when it is tight like a fiddle string...I have a hard time with this unless there is a visual difference.
I tell by the bag, the attitude, wandering off by herself, the crook in the tail, tail off to the side, (it does not cover the poop hole, hangs beside it), and crawling the fence line.