In some cases, however, the barrier may not be completely effective, and sperm from a second male may be able to fertilize some of the eggs. If this happens, the female dog may give birth to puppies of both breeds. This phenomenon is known as superfecundation, and it is relatively rare in dogs.
If you are concerned that your female dog may have gotten pregnant from two different males, you can have her tested for paternity. A paternity test can be performed using DNA from the puppies and the suspected fathers. The test will determine which male is the father of each puppy.