The Unborn Crittur puppies are the unborn litter of Sarah and Danny Crittur. Some hacks have been known to cause problems with the pregnancy.
The litter will usually be made up of three puppies, generally two of one gender and one of the opposite. Interestingly, if there are two or more female puppies then the whole litter tends to inherit Sarah's coat markings; if two or more of the puppies are male, all of them tend to resemble Danny.
The unborn puppies have incomplete character data, and may cause neighborhood corruption when the pregnancy comes to term. The player can avoid this by aborting the pregnancy using a mod such as the InSimenator. This bug was corrected in the Seasons patch, but it will only fix neighborhoods that were created after the patch was installed.