In the Womb
A puppy’s life begins 63 days before he is born. While in the womb, he develops physically. His sense of smell is active even before he is born. Puppies can learn to distinguish scents before they are born as well as in their first 3 weeks of life. It is their first and most important sense in learning about the world. Events that occur while a puppy is still in eutero can affect a dog’s brain and behavior. If the mother is stressed, cortisol is released and the pups will be timid and fearful as adults, especially if stress is not mitigated early in puppyhood. 

The sex of other pups in the womb also affect them due to presence or absence of testosterone in the amniotic fluid they share. A female pup in an all male litter will act more male (more active, leg lifting and territorial). Males in all male litters also have exaggerated male traits. A male in an all female litter will act more female (exhibits reduced male behaviors). 

Brain Development
At birth, a pup’s brain is only about 10% (10cc) of the volume it will be as an adult (100cc). At just a few days old, the brain activity is very low, “…the fibers that connect the neurons have not yet developed from the fatty white sheath (called myelin) that speeds communications between locations in the brain and electrically insulate each nerve cell…” from the neighboring cells. Development continues out of the womb and the pups are dependent on their mother for everything, including being stimulated to pee and poop, and even the heat from her body as they cannot regulate their own body heat until about 4 weeks of age. It is at this age they start moving around a little more.

Puppy brains grow at a rapid pace from birth to 7 weeks. By about 7 weeks, the brain’s volume is about 50% of an adult and the brain wave activity cannot be told apart from an adult dog. At this point, the pup’s temperament and intelligence is not predetermined or unchangeable. The more brain activity, the more neural connections are made, the more adaptable the pup will be as an adult. 
 What you see at 7 weeks is not what you see as an adult as the environment has such a huge influence. Dogs are “born with such an immature, incomplete nervous system that, by controlling its experiences we can even shape the structure of the brain.” Coren. p. 124 

Developing Resiliency
Once they are born, studies show that mild physical and psychological stressors, rather than quiet lives, actually benefit the pups and develop resiliency and intelligence. Pups that have been mildly stressed show stronger adrenal glands, stronger heart beat, more tolerance to the physical effects of stress and residence to disease. Adult animals were better able to withstand new and more severe stressors, responded with less emotion, and lower levels of excitement. This is why the Early Neurological Program is done from day 3 to 16, once a day. The opposite is also true. Dogs that are deprived of stress (isolated from stressors), are able to cope with stressors as adults. Even as an adult, if a dog is exposed to environmental enrichment, their brains continue to grow. They showed gains of as much as 5% compared to dogs from unenriched environments. Adult dogs actually grew new neurons, an event that wasn’t believed possible not long ago. This growth affects memory, learning, intelligence and emotional responses. The dogs raised in enriched environments were more stable, less easily frightened, and better able to recover from stressful events. This points to the importance of continued enrichment for dogs as they mature. A dog that gets the wrong start in life can grow into an overly fearful or anxious dog. Some dogs have great resiliency despite the lack of early stimulation, but since we don’t know which ones will have it, why take a risk with your future service dog? 

Senses
Puppies can taste, smell and feel at birth. They are born with their eyes closed, and ear canals closed. Their eyes open at 10 days or later and they begin relationships with litter mates and mother.  They can hear at about 2 weeks old.  

Immunity to Disease
While nursing, pups are given immunity through the mother’s milk. This is why it is so important they be allowed to nurse until the mother starts naturally weaning them at 4 to 4.5 weeks of age. Like children, a pup that is nursed has a better natural immunity and will have a stronger immune system as an adult than one that is bottle fed on milk substitute.   

Feeding
Their teeth start coming through at about 3 weeks and they test mushy food. By 4 weeks they are starting to eat more softer foods as their intestinal tract can is starting to grow the good bacteria needed to process it. By 6 weeks all puppy teeth are in and they start to control their need to potty. In one litter I saw, puppies stopped mid-play and mid-training and went to the door to ask to go out! The mother is becoming increasingly uncomfortable with shape teeth grabbing at her teats. By 8 weeks, they can eat softer solid food 3 times a day and the feeding schedule helps to regulate their potty habits.

Learning
Pups can learn from a few days old. In one study, pups were exposed to the scent of anise (licorice) that was painted on the mother’s teat. When the scent alone was presented, they turned and moved towards it. Other puppies who had not been exposed to it turned away and tried to escape. This is early conditioning at it’s finest. In another study, puppies were fed on a textured surface and preferred to feed there. They would crawl onto that surface to feed when given a choice. The surface texture became conditioned to mean food for the pup. 

Both of those are great examples that conditioning can be started early. This is why it is a good idea to take or send an article of clothing with your scent on it with the litter. The puppies smell it and it becomes part of their familiar environment. They will already be familiar with your scent when they are taken to your home which will make the transition away from mom and litter easier for them.
 Puppies can begin learning from humans as early as 4 weeks using food and gentle stroking as a reinforcer. They can learn to sit before eating, sit before being picked up, come when encouraged etc. Their little brains are sponges. 

Size:
Puppy size in the litter has no relation to adult size. The smallest pup may be the largest adult dog. A tiny pup may grow to be average size. A large pup may be average. Nutrition, age of spay or neuter and environment play a huge factor.