Whelping box
A whelping box/den, also known as a nesting box, whelping den, or whelping pen, is designed to protect puppies during birth (whelping) and early life by keeping them safely contained, protected from cold, and safe from the danger of crushing or smothering by the mother. Whelping boxes vary from improvised cardboard boxes, durable corrugated plastic to purpose built equipment, typically made of plywood or plastic. Desirable features include ease of cleaning and sanitation, extendibility, and toughness. However, there can sometimes not be enough traction, and with the added factor of an unnaturally flat floor, often puppies can  develop, and suffer from what is known as hip dysplasia. A good solution is actually a "whelping den", a new form of whelping that has struck the market for breeders that virtually erases the risk of hip dysplasia for usually less than the cost of losing a single pup. Whelping Dens are available in open, enclosed and enclosed sheds that have four or six built in dens, ready for your runs. Unlike garden boxes, whelping boxes include a floor to protect the carpets and hard floors from staining and soiling. In some breeding environments portability is also desirable. This is  typically achieved through light weight design and emphasis on simple setup and takedown. Commercially available whelping boxes often feature accommodations for accessories such as heat control devices and IP camera attachment. The sides of the box are designed to be high enough to safely contain the puppies, yet low enough to allow the mother to enter and leave comfortably, with consideration given to her protruding mammary glands. Alternatively, the box is provisioned with a doorway with adjustable height for the same purpose. The bottom of the box may or may not have a floor and may be lined with specially  designed "whelping pads," or layers of newspaper (or fabric) to provide insulation from cold floors and to absorb fluids. Fabric covers or partial covers are often used on top to provide a "den-like" environment for the mother. Full covers may be used to prevent puppies from escaping. Most boxes will include a low railing (termed rails, pig rails, or roll-bars) fixed to the inside perimeter of the box. This is to protect 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.