So, first, let's start with a clip that explains the important signs to look out for.
How do I know when my dog is going to give birth or whelping?
The thing is, most people don't know the litter size of your pregnant dogs and start showing their concern about number of puppies remaining inside mother during labor.
I always advise for such owners to not to go whelping by themselves. Even if they are experienced, they should get ultrasound of their dog after 50 days of pregnancy to know possible litter size and birth defects.
As a general rule, Dogs shake and strain during labor and then delivery of first puppy takes place followed by a rest of 30-45 minutes. This resting time can be of maximum 2 hours.
After this rest, she again strains to deliver and second puppy is born and cycle is repeated again until all litter is born. If your dog is not straining for more than 2 hours then it means that she has finished and no more in labor.