The average length for a cat's pregnancy is between 63 and 65 days. If they're not spayed, cats can get pregnant as young as four months old.
The total length of the pregnancy depends on multiple factors, such as breeding conditions and the challenge of pinpointing the exact gestation start-point. Female cats need the mating stimulus of a male cat to ovulate and it is common for a cat to need several matings over a few days to result in a successful pregnancy. This is why the pet owner or breeder can't identify the exact mating that led to the pregnancy.