What is a nesting box?
The nesting box is a box where your pregnant cat (known as a queen) will give birth in and care for her litter of kittens in the first few weeks of life.
It should be set up around 2 weeks before the litter is due so the queen can settle in. Don't force her to stay in the box at first, just slowly introduce it to her, making it as comfortable as possible. Place her food/water bowls and litter tray in the room.
The box should be placed in a quiet room, a home office, bathroom or spare room are ideal. It should be at a comfortable temperature for the queen and her kittens. It should be away from other pets and children.
Sometimes, despite your best efforts, the queen will refuse to use her nesting box, opting for another location (such as the bottom of your wardrobe, in a drawer, under dressers). If this happens, close off close doors to other parts of the house to prevent her switching locations. If she seemingly shows a preference for one room, as long as it is suitable, consider moving the box. Some queens like to still be near their owners, others would rather be on their own in the early weeks of motherhood.
The nesting box:
The box should be large enough to accommodate the queen so that she stand up and turn around and accommodate her litter (once they arrive). A sturdy cardboard box is perfect, the box should be enclosed on all sides (and above), with a hole cut out at the front for the queen to climb in and out of. When cutting out the hole, make it raised (about 9 inches) so that the newborn kittens can't roll out. Another option for a nesting box is a new enclosed litter tray, with the clear plastic "door" removed. The advantage of this is that it can be cleaned out if necessary. Don't use an old litter tray as it will harbour residual odours (along with possible fecal contamination).
Line the bottom of the box with a 1-inch layer of newspapers, on top of that, place an absorbent puppy training pad and cover with soft, clean blankets or towels. Soiled bedding should be removed after the birth and disposed or laundered. Regularly replace with clean/fresh bedding.
If the litter is due in cooler months, a heating pad can be added to the nesting box. Make sure it doesn't cover the entire bottom surface, so they can move away from the heat if necessary. Check that the temperature isn't too hot.