A pregnant dog's body temperature typically drops 24 hours or so before she enters the first stage of labor, so check her with a rectal thermometer once or twice a day. When you notice her temperature drop by a few degrees, delivery is coming soon.
A dog's body temperature is usually between 100 and 102.5 degrees Fahrenheit. When labor is imminent, the mother's body temperature usually drops by a couple degrees. When your dog's body temperature falls to 99 degrees Fahrenheit or lower, start keeping a close eye on her, because she'll likely enter labor within 12 to 24 hours. Have your whelping supplies ready and a phone nearby -- when she goes into labor, you may need to assist with cleaning off the newborn pups, snipping umbilical cords and general cleanup duties to make your dog more comfortable. Usually, though, no assistance is necessary.