When can you breed a dog?

At home, no - this would be illegal and you would risk killing your dog. If your female dog has recently come out of heat and was bred by a male dog, your veterinarian can administer a "mis-mating" shot (basically high-dose progesterone, or Birth Control) that should keep her from carrying any puppies. However, if your female is already confirmed pregnant, the most a veterinarian can do is spay your female, during which the fetal puppies are terminated. If you absolutely need a single litter aborted, but the female left intact, you will need to contact a veterinary theriogenologist for assistance - this is going to be an expensive and risky surgery.