Dogs are polytocous, meaning they have more than two young ones each time they give birth. They have an average litter size ranging from three to seven, depending on the breed. But sometimes, it is seen that only a single or two puppies are present in a litter. This is quite rare and is called Single Puppy Syndrome.
Though it may seem unlikely, a single puppy litter can cause a multitude of problems during the actual time of whelping. Whelping or birth starts once the foetus gets too big to be nourished and maintained in the uterus by the placenta.
Due to the lack of adequate oxygen and nutrients, the foetus releases cortisol (a stress hormone). This hormone acts on the placenta and the uterus, causing a cascade of hormonal and physical changes and finally ending in the birth of the puppies.
However, when a single puppy is present in a litter, the amount of cortisol released by the lone puppy isn’t enough to initiate birth.
This spells death for the foetus due to inadequate nourishment. Since the environment of the uterus is sterile, the foetus doesn’t rot after death, but rather dries up and becomes ‘mummified’. Sometimes the birth process might initiate incompletely and cause the cervix to open up partially.
This allows microorganisms to enter and due to these microorganisms, the foetus starts to decay. If left long enough, this might permanently damage the uterus. In addition, a single foetus might grow abnormally large and get stuck on its way out during whelping.