Cats reach sexual maturity between 6 and 18 months of age, and they mate naturally. Inbreeding is the breeding of closely related cats, it is perfectly acceptable to breed-related cats and register their offspring. The ability to produce a litter of kittens fathered by more than one tom cat is called superfecundation. Breeders often use inbreeding to predict how their kittens will look, particularly pedigree breeds.