Polytocous
Pug Puppies, by De'Nick'Nise
Now let's point out some details about ovulation, which we know occurs during estrus.
How many oocytes are ovulated? Here again we see tremendous variation, with smaller breeds releasing fewer oocytes and therefore having smaller litter sizes than larger breeds. Example ranges are shown here.
The fact that dogs ovulate multiple oocytes and bear multiple offspring, or litters, classifies them as polytocous.
Another unique aspect of dog reproduction is that unlike many species, the oocytes of a dog are not yet ready to be fertilized when they are ovulated. They will need 2-3 days to mature before fertilization can occur.