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Your girl should have her first pup within two-four hours when labor starts. However if you notice she is pushing or having stomach contractions and no pup is born within a half hour then you will need to seek vet care. After the first pup is born the others should follow every 1/2 hour to three hours, any longer than than three hours and you'll want to call the vet. You should call them when labor starts anyway so they can be prepared if there is an emergency. I am going to direct you to a good site on whelping. It will tell you what tools you need to have ready, how to handle cutting the cord and the placenta.
Keep your vets number and an emergency vets number handy as they can instruct you over the phone if necessary.
Scroll down to preparing a whelping kit and whelping.