Problems during gestation
Sometimes a bacterial infection in the uterus will affect the puppies and they die. The breeder may have no way of knowing, depending on the stage of pregnancy. At some stage the uterine content, if not expelled will begin to decompose. Sometimes the vulva will have a foul-smelling dark discharge, and a high temperature, the vet will have to do a Caesarian, to find dead puppies that have been slowly dissolving into "jelly".
This is a disease of the reproductive tract. It causes sterility and absorbed or aborted litters. Males and females can carry it. Dogs coming into contact with vaginal secretions from affected dogs spread it. You can have your stock tested for this before including them in your breeding program.
ABSORPTION OF A LITTER
There are reported cases where a female will be pregnant but, for some reason, will absorb the entire litter or part of a litter into her system. A female will often whelp one to two live puppies and have one or more dead ones in various stages of decomposition.