Deciduous teeth or teeth that fall out are common for mammals including dogs. These primary teeth are formed temporarily while your puppy's jaws are still growing. Baby teeth will start growing at around the same time puppies start to open their eyes, about 2 to 4 weeks from birth. At week 5 or 6, they will have a full set of teeth and should be able to eat soft and moist dog food. Your puppy will have 28 sharp and needle-like teeth which will start falling out at around 12 weeks from birth and then be replaced by 42 permanent teeth. Your 6-month old puppy should have a full set of permanent teeth.
If you notice your puppy retains some puppy teeth, you should consult your veterinarian and have them removed. Malocclusion, or retained puppy teeth, can lead to serious dental problems especially when food gets trapped between teeth and gums.