Cats most often go into heat around 5 or 6 months of age, but some cats go into heat as early as 4 months and other cats as late as 10 months. The time of heat will depend on many factors, including the breed of the cat and whether the cat is living outdoors or indoors.
Outdoor cats and feral cats tend to enter heat much earlier than indoor cats. Additionally, breeds such as the Siamese often enter heat early around 4 or 5 months, while breeds such as the Persians enter heat later around 10 months.
As soon as a cat goes into heat, she can become pregnant. This can happen even if the cat is just barely past the kitten age. It is not healthy for the cat when this happens because she has not been given enough time to develop the emotions and instincts needed to care for a litter of kittens properly. Reputable cat breeders will wait until their female cats are at least 1 year old before they attempt to breed them.
The cat usually has heat cycles that last between 14 to 21 days. Each cat will have her own rhythm for her heat cycle. Cats tend to be restless, want to eat more, have extra affection for their owners, have a slightly enlarged vulva and mark with urine during heat.