Can a cat delay it's labour?

My daughter's little cat is imminently due with her first litter of kittens and has showed signs of being in labour (shallow breathing, being quiet, finding a dark spot etc.) since Sunday and it's now Wednesday. Sometimes she behaves and breathes quite normally, and at other times her back is rippling, and there seems to be some labour symptons. They haven't come to anything and I'm wondering if she's delaying matters. We know nothing about such things but want to be able to recognise the stages. We have books that we are referring to but none say about the kind of behaviour that our cat is displaying. We are a bit worried and phoned the vet but the advice they give is if she's not in distress don't worry. She was a stray that adopted my daughter so she's sure going to be spayed after this !!