A female dog is called a bitch - even as a puppy. This term distinguishes female dogs from male dogs, the latter of which are called dogs, studs or sires. When a female dog is pregnant with a litter of puppies she is called a dam.
In dams, pregnancy lasts for approximately 63 days and it is during the gestation period that a bitch is referred to as a dam. Once the dam whelps, or gives birth, to a litter of puppies, the term bitch applies again.
Male dogs are referred to as studs or sires after they have fathered a litter of puppies.