Some mothers do tend to be less conscious of their puppies, and have been known to lie on and in some cases kill their puppies. Most often this is a danger when the puppy is caught between the mother and the sides of the whelping box. Puppies caught directly under the mother tend to cause her to shift and release the puppy.
While the swimming pool used as a whelping box has no mechanism to prevent mothers from lying on their pups or suffocating them against the sides, the premanufactured whelping boxes do have guard rails, sometimes called pig rails, that keep the mother three inches away from the sides of the box. These rails allow the puppies a safe clearance space from their mother on all four sides. So while these manufactured boxes seem expensive, they are relatively inexpensive, if they save the life of even one puppy.
Another concern in the whelping box is puppies getting caught up in the sheets or towels in the nest, that are intended to help keep the area sanitary. Puppies can become tangled up in long sheets or heavy towels, preventing them from eating when necessary (every two hours) and possibly allowing them to chill, as they are not able to gather warmth from their mother or littermates. A chilled pup is not likely to survive long without intervention.
Many breeders use un-printed on "newsprint" to line the whelping box, or whelping mats that are not easy lifted from the bottom, to prevent puppies from tunneling under the mat.