One of the telltale signs of your dog's impending delivery is the rupture of the chorioallantoic sac, or the water breaking. This typically comes late in the first stage of labor. Watery discharge that marks the beginning of the second stage of delivery, during which she begins to actually deliver the puppies.
As your dog experiences contractions and strains, the chorioallantoic sac breaks, expelling a watery discharge. This discharge indicates that the puppies are about to be delivered, and it should come within 30 minutes or so of the beginning of contractions. If your dog experiences contractions for longer than 30 minutes and you do not notice that her water has broken or she has delivered a puppy, contact your vet, as it could be a sign of complications. Keep in mind, however, that the time between the water breaking and the first puppy emerging can be significantly longer -- as long as four hours or so.