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A whelping box is one in which the dog gives birth. It should also keep the puppies warm and protect them from being squashed if their mother lies on them. The box should have 4 sides and a base. Choose a length and width that allows the mother to lie down with her head and legs extended. Add half her height again to the width of the box, which will be used to create a space for the puppies. Make sure the sides are tall enough that puppies will stay in but the mother can jump out without difficulty. You can buy a whelping box at most pet stores. You can also use a cardboard box, or you can make a box out of hardboard or plywood. Acquire 2 large, stiff boxes, such as television or appliance boxes. Cut one end out of each box and push them together to make one longer box. The puppies will need a safe haven in the box where the mother can't lie on them (which could suffocate them). Mark the additional width in the box, and install a sturdy wooden rail raised about 4-6 inches from the bottom of the box. A broom handle works well as a rail in the box. This is especially important when the puppies are over 2 weeks old and are more mobile. Line the floor with plenty of newspaper and some thick towels. Alternatively, use a vetbed, which is a polyester fleece that wicks dampness away from the bitch and puppies. After you've constructed the puppy area, place a heat mat underneath the paper in this area. After the puppies are born, you'll turn this heat mat on to a low setting. This helps the puppies to keep warm while they are away from the mother. An alternative to a heat mat is a heat lamp, angled towards one corner of the box to provide a warm spot. However, a heat lamp provides dry heat, which can dry out the puppy's skin. If you have to use a lamp, be sure to check the puppies regularly for flaky or red skin. Remove the lamp if this happens. Use a hot water bottle wrapped in a towel to provide temporary warmth. During the whelping, the dog may want to feel like she is in a den. This helps her feel secure, which also allows labor to continue more readily. Place a large towel or blanket over part of the box to give her some covering. The bitch shouldn't be disturbed while she is giving birth, so choose a quiet room to place the whelping box. Make it easy for your dog to eat and drink by making sure food and water is available nearby. You can still keep food and water in your usual spot, but making sure your dog knows there is food and water near the whelping box will help make her feel more at ease here.

Summary:
Choose a box that is a comfortable size for your dog. Create a space for the puppies. Line the floor of the whelping box. Place a heat mat in the puppy area. Provide a covering for the box's opening. Place the box in a quiet room inside. Provide food and water near the box.