Bringing a puppy home is one of life’s great joys — and one of its great responsibilities. Understanding what’s happening in your puppy’s brain and body during each developmental stage helps you provide the right care at the right time. Miss a critical window and you may spend years working around the consequences. Get it right and you set the foundation for a confident, healthy, well-adjusted dog.

Week 1–2: Neonatal Period

Puppies are born with closed eyes and ear canals. They are entirely dependent on their mother for thermoregulation, feeding, and urination/defecation stimulation. Ninety percent of their time is spent sleeping; the remaining 10% feeding. Despite appearing inert, important neurological development is occurring. Gentle tactile stimulation during this period — briefly holding puppies, exposing them to mild temperature changes — has been shown in research to produce calmer, more trainable adult dogs (the US Military’s “Bio Sensor” program demonstrated this in the 1970s).

Weight should double in the first week. Use our Puppy Weight Tracker to log weekly weights and confirm normal growth trajectory.

Week 3–4: Transitional Period

Eyes and ears open between 2–3 weeks, and the puppy’s world suddenly expands enormously. The first teeth emerge. Coordination improves rapidly — puppies begin walking, rather than crawling. Social behaviour towards littermates begins. This is when the first vocalisation (barking) appears. Weaning typically begins at 3–4 weeks, with solid food introduced gradually alongside continued nursing.

Week 4–12: Primary Socialisation Period — The Most Critical Window

This is the most important developmental period in a dog’s life. Between 4 and 12 weeks, puppies form their foundational understanding of what is “normal” in the world. Exposure to a wide variety of people, animals, surfaces, sounds, environments, and experiences during this window dramatically reduces fear responses and aggression in adult life.

The “fear imprint period” peaks at approximately 8 weeks — which is, coincidentally, when most puppies go to their new homes. Negative experiences during this window (pain, fear, trauma) can have lasting effects. Vaccination gaps make full public socialisation challenging, but the risk of inadequate socialisation is greater than the risk of controlled exposure to low-risk environments.

Key socialisation targets before 12 weeks: 100 different people (including children, elderly, people in hats/uniforms/beards), 50 different sounds (thunder, traffic, vacuum cleaners, fireworks), 25 different surfaces (grass, gravel, metal, stairs), other dogs and animals, car travel, being handled everywhere on their body.

Week 12–16: Secondary Socialisation Period

Socialisation ability begins to decline after 12 weeks, though meaningful learning continues until about 16 weeks. The primary vaccination schedule is typically complete around 16 weeks, allowing wider social exposure. Training responsiveness is high during this period — basic commands (sit, down, come, stay) can be reliably established with consistent positive reinforcement.

Weeks 16–24: Juvenile Period

The “teenage phase” begins. Puppies may appear to forget training they previously knew — this is normal neurological reorganisation, not deliberate defiance. Consistency during this period pays dividends. Growth continues rapidly; nutrition requirements are high. Use our Puppy Calorie Calculator to ensure adequate caloric intake for growth without promoting obesity.

The First Fear Phase (8–10 Weeks) and Second Fear Phase (6–14 Months)

Two distinct fear phases occur during development. The first coincides with the primary socialisation period at 8–10 weeks. The second, less predictable fear phase occurs anywhere between 6 and 14 months, often associated with adolescence. During fear phases, avoid flooding (forced exposure to feared stimuli) — counterconditioning and desensitisation are far more effective and prevent fear from becoming entrenched.

Growth Rate by Breed Size

Growth trajectories differ dramatically by adult size. Small breeds reach 50% of adult weight by approximately 16 weeks and full adult weight by 9–10 months. Giant breeds don’t reach full adult weight until 18–24 months. Importantly, the skeletal growth plates of large and giant breeds don’t close until 18–24 months — meaning excessive high-impact exercise before this point can cause permanent joint damage.

Nutrition During Puppyhood

Puppies require approximately twice the caloric density of adult dogs relative to body weight. Large and giant breed puppies specifically require controlled calcium and phosphorus intake — paradoxically, excess calcium during rapid growth causes developmental orthopaedic disease in large breeds. Large-breed specific puppy food is formulated for this requirement. Use our Puppy Food Calculator to calculate accurate daily portions by breed and age.

Vaccination Schedule

Core vaccines (distemper, hepatitis, parvovirus) are typically given at 8, 12, and 16 weeks, with a 12-month booster and then every 1–3 years depending on the product. Rabies (where required), leptospirosis, kennel cough, and other non-core vaccines are given based on lifestyle risk. Your vet will tailor the protocol to your puppy’s specific circumstances.