For tracking litters and puppies, create two tables, one for litters and one for dogs. Use a relationship to link each dog record to a specific litter record to show the litter into which it was born.
Ultimately, this is a many to many self join relationship with the Litters table serving as the join between two occurrences of the dogs table. A dog can have many litters and a litter can list many puppies.
Start with these relationships:
Dogs::__pkDogID = Litters::_fkParentID
Puppies::__pkDogID = Litters::_fkPuppyID
You can place a portal to Litters on the Dogs layout to list and select Puppies records for each given Dogs record. Fields from Puppies can be included in the Portal to show additional info about each selected Puppies record and the _fkPuppyID field can be set up with a value list for selecting Puppies records by their ID field.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
You have been extremely helpful. Thank you:)