During the first stage of labour the cervix begins to dilate and uterine contractions begin. These contractions are painful and perplexing to the dog. She will appear quite uncomfortable and restless – pacing, shivering and panting. During the second stage of labour, uterine contractions begin in force. As this stage progresses the placental water sacks break and a straw-colour fluid is passed. When your dog is about to give birth, her body temperate will drop a few degrees.
How do I Know When my Dog is Ready to Give Birth? – http://answers.ask.com/Science/Nature/how_do_i_know_when_my_dog_is_ready_to_give_birth