The average length of time between a cat's last estrus, or heat, cycle and the birth of her kittens is between 60 and 65 days. It could, however, be as short as 58 days or as long as 71 days.
Interestingly, the reasons for the variability in the length of gestation and the process that begins labor remain unknown. Some factors may include hormonal balance, the size of the uterus, and the size and weight of the kittens. In fact, it is not uncommon for owners to go to sleep with a pregnant cat and wake the next morning to a litter of kittens.