Rabbits may have a reputation for fast breeding, but cats should share some of that fame. A female cat that's not spayed can usually begin to breed as soon as their heat cycles begin. Cat's can have two or three litters every year if not spayed. With each litter being between one and eight kittens that can add up to a lot of kittens very quickly if you're not careful! Learning about your cat's reproductive cycle can help you plan for breeding purposes or just to ensure that you don't have more pets than you bargained for. The quick cycles are why many vets recommend spaying pet cats unless they are specifically being used to breed.
Heat, Oestrus, Estrus
All of the above terms are correct in describing female cats' periods of receptiveness to mating, but we will refer to them as the more often-used heat cycles.
Breeding Season
The breeding season in cats is practically year-round, running as early as February, and as late as December, but in the western hemisphere, March through September is generally regarded as the breeding season.
Rescue people dread Kitten Season because it means that in kill shelters, the new crop of kittens will eliminate any chances of adoption for older cats, including last year's kittens. If for no other reason, this alone is ample justification for the spaying and neutering of pet cats.
Heat Cycles
Cats are referred to as polyestrous, which means that they will go into heat cycles periodically during their fertile years. These heat cycles may start as early as the fourth or fifth month of a kitten's life and will continue until she is either bred or spayed. Many veterinarians now practice early spay and neuter for this reason. Heat cycles in cats last from several days to two weeks or longer and repeat every two to three weeks. You can see then, how a female cat may almost always seem to be in heat.
No one can say with any accuracy that heat cycles are painful to cats; however, from the calling (loud yowling) and other symptoms they exhibit, it would appear that they are very uncomfortable. A cat in heat is usually pretty noticeable. The main goal of a female cat is to get attention after all, and with all the calling, it can be hard to miss.