From inside the womb, you can see that the embryos of a golden retriever are about the size of a pea when they're 20 days old. By day 30, the embryos are the size of a grape and the puppies have beating hearts. As the puppies develop, their intestines grow outside the body because there's not enough room inside. As the puppies get bigger, the intestines slowly move into their abdomens.
With less than a month to go, the fetuses are about the size of a golf ball. The puppies must constantly move in utero to keep their joints from freezing. With only a week to go, the puppies have grown a full coat of fur.
After 63 days, the tiny golden retrievers are born blind and unable to walk. Moments after the birth, the mother instinctively tears open the birth sac so her babies can breathe. Then, she eats the placenta to ingest chemicals that stimulate milk flow.
To survive, the puppies use their noses to find their mother's nipples. They will bark for the first time about 18 days later, and at just six months old, these little pups can start having puppies of their own!