The whelping box, known by many other names such as a nesting box, birthing box, whelping pen etc, is designed to help protect puppies at birth (whelping) and the early days of life by keeping them safely contained inside the whelping box, it helps protect them from the cold, and safe from the danger of crushing or smothering by the mother.
Whelping boxes vary from disposable cardboard boxes to purpose built equipment, typically made of wood or plastic. Desirable features include ease of cleaning and sanitation, and toughness, portability is also desirable. This is generally achieved through light weight designs with emphasis on simple setup and take down. Commercially available whelping boxes often feature areas to store accessories such as spare heat pads, absorbent pads etc.
The sides of the box are designed to be high enough to contain the puppies, yet low enough to allow the mother to get in and out normally with a lower section as the main in out, with consideration given to her protruding mammary glands. The bottom of the box may be lined with specially designed layer or whelping pads and fabric bedding such as pet fleece to provide insulation from cold floors and to absorb fluids.
Most boxes will include 3 low railings generally referred to as rails, pig rails, poles, or roll bars fixed to the inside edges of the box. This is to help stop puppies from being crushed or smothered by the mother should she roll over during birthing or while asleep. This is considered especially important with larger dog breeds.