A female dog carries her young for about nine weeks before they are born. The number of puppies in a litter is determined mainly by the size of the mother. Small breeds usually have 1 to 4 puppies, medium breeds have 4 to 8, and large breeds have 6 to 10.

Puppies are helpless at birth. They cannot see until their eyelids open about ten to fifteen days later. Their ear canals also remain sealed for two to three weeks. A newborn pup cannot empty its bladder or bowels unless its mother licks the area under the base of its tail. Puppies normally begin walking just after two weeks and become steady on their feet when they are three weeks old. At that age, puppies also begin to bark and wag their tails.

Like other mammals, puppies live solely on their mother's milk at first. Most pups begin to eat solid food at about three weeks of age and stop nursing by six weeks.

It is important for puppies to have contact with human beings if they are to become good pets. Newborn puppies should be handled gently for a few minutes each day. Between four and ten weeks of age, pups need even more human contact or they will never relate well to human beings. The best time to adopt a new puppy is when it is six to eight weeks old. Contact with other dogs is also important for puppies, or the dog may grow up to be too aggressive or too shy with other dogs.

Many puppies reach their adult height by six months of age but may continue to gain weight until one or two years old. Dogs often become sexually mature before they are fully grown. For small breeds such as the chihuahua, this may be as early as five to six months old. For giant breeds, it may be as late as one and one half years.

The length of a dog's life depends on its breed and on the care it receives. In general, small breeds live longer than large ones. It is not unusual for a toy poodle or pug to live fifteen to sixteen years. Giant breeds, such as the Great Dane and St. Bernard, are considered old at eight or nine. Dogs that eat a healthy diet, get regular exercise, and receive good medical care usually live longest. The longest life span ever recorded for a dog was twenty-nine years and five months, which is extraordinary for an Australian Cattle Dog. Because dogs mature at different rates, it is difficult to compare their ages directly to those of human beings.