All puppies go through the same stages of development in their first 3 months. Some individuals develop slightly slower or faster than others of a same breed or even of a same litter. As a general rule, smaller breeds are quicker to develop than larger breeds. By the time a puppy is 3 months old, he should be well socialized.
For the first week of their lives, puppies sleep about 90 percent of the time and suckle the remaining 10 percent. Newborn pups are dependent on their mothers for warmth. Puppies are born blind and deaf. They have touch reflexes in their heads, along with scent and heat sensors in their noses, so they can locate their mothers and crawl to them. Their mothers will stimulate urination and defecation by licking the pups' genitals, and then ingest the waste, until the puppies are 3 to 4 weeks old. At three weeks of age the puppies milk teeth start coming through, and she'll begin to wean them. At about 10 to 14 days the puppies' eyelids open but their sight is poor. By 12 to 14 days the external ear canals open and puppies hear for the first time. The puppies' vision, hearing and balance are fully developed by about 4 weeks. At 2 weeks, puppies can sit up as touch reflexes develop in their front legs; a week later they can stand as touch reflexes develop in the hind legs. It takes a few more days for the mind to coordinate all four legs into walking. Newborn puppies can feel pain, but it takes several seconds for it to register in the brain. Around 3 weeks, pups' brains and nerves register pain as quickly as adults'.
Five to 12 Weeks
Puppies' facial muscles develop at about 5 weeks, when facial expressions, important in canine communication, can be seen. Reflexes, coordination and balance all continue to develop, along with the puppies' confidence as they explore their environments more and play with their littermates. Mother dogs will spend less time with the puppies, having started weaning. They'll continue to occasionally nurse until about 8 weeks.
Between ages 3 weeks to 12 weeks, puppies usually easily accept new experiences. Up to about 7 weeks, it's usually the breeders' responsibility to provide plenty of appropriate experiences, exposing puppies to a variety of sounds, floor surfaces, objects and people. Puppies start going to their new homes generally no sooner than age 8 weeks. That's when house-training can begin. From this point on, it's important they continue to be introduced to lots of people and animals, places and situations they are likely to encounter as adult dogs.
3 Months to Adulthood
Puppies' adult teeth start coming through at 4 to 6 months; they will need plenty of appropriate things to chew on. This age is when they start to become more independent and test out what is allowed. Between 6 and 12 months, puppies reach the adolescent phase and reach sexual maturity. Females can have their first estrus, and males will experience changes in their hormones and exhibit behavior such as cocking a leg to urinate, marking territory, interest in females and squabbling with other males. By a year to 18 months, puppies reach physical maturity, but it will be up to 3 years before some dogs are socially mature and their characters fully developed.