This is an excerpt of Dr. Ellen Lindell’s article: Developmental Stages of Puppies
Dr. Lindell is a certified behaviorist from the American College of Veterinary Behaviorists teaches us that there are 4 stages of development in puppies and shows us how to achieve proper socialization in every one of them.
A newborn puppy needs gentle handling.
Dr. Lindell says that the first stage of puppy development is the neonatal stage, which goes from birth to 7 days of age. At this stage, puppies are blind and deaf, and they explore their surroundings by olfaction and feel. It was discovered that puppies that receive more maternal care during this stage, have better social skills as adults. It’s also been studied that puppies that are gently handled by humans by day 3 of age are calmer at 8 weeks of age, thus it is advised to foster families to softly handle puppies at an early age.
The next stage is called Transitional Stage and lasts around 7 days, eyes and ears start to work while muscle coordination improves. This stage is very important because it’s when social communications start to develop. Puppies start to move around and this is the perfect moment to introduce elimination substrate that will help later to teach them to poop and pee in the correct area of the house.
Newborn 6 weeks 
Juvenile Adult
The third stage is called Socialization Stage and is the more important stage for developing social skills, which includes interacting with humans and other animals and react properly to different situations. This stage begins at 3 weeks of age and lasts until 12 to 14 weeks of age.
If puppies are not properly socialized during this stage, they will become fearful dogs towards humans and strange situations. Is in this stage where the introduction of puppies to new people and situations is fundamental, but it is also risky as they will be exposed to diseases to which they are not protected, however, small and short exposition to different humans and controlled environments will be very beneficial and will mean very low risk for puppies.
It is very important that new experiences for puppies are positive, as negative experiences will have a long-lasting effect on them, separating them from their littermates or their mother, can have a very strong effect on separation anxiety issues later in life, thus removing a puppy from its littermates and mother, must be done very carefully and never before 6 weeks of age.
The last stage is called Juvenile Stage and extends from the end of the socialization period until sexual maturity – which in some dogs can be around 6 months of age, but in large breeds, it can extend well past 1 year of age.
Developing a socialization program
Puppies should be exposed to different environments as soon and often as possible, but it’s also never too late to begin either. It’s a good idea to take them out twice a week avoiding areas where unvaccinated dogs can wander around. It’s also important that puppies meet different kinds of people.
These sessions must be pleasant for the puppy, so toys or treats can be used, and the puppy must be let at its own pace. If the puppy is too frightened, then it should be stopped. We do not want to overwhelm the puppy.