When your cat is in heat: What to expect
Most female cats will go into heat by 6 months of age. But certain environmental factors can cause it to happen more quickly. And certain breeds—like Siamese—may go into heat as early as 4 months, while for other breeds 10 months old is a more typical age for going into heat.
When a cat goes into heat, her meow will sound more like a howl and she will be more affectionate toward you, other cats and even the dining room chairs, rubbing against nearly anything and purring loudly. She'll hold her tail straight up and raise her rear in response to the slightest provocation. She may also spray urine and make a run for the outdoors—in an effort to find a mate—any chance she gets.
During adolescence, your cat will experience a surge in estrogen that will cause her to go into heat for about a week. This cycle will repeat every two to three weeks. During this time she may have a small amount of vaginal discharge, which will appear bloody.
Cats tend to breed in the spring and summer and then give birth about 65 days after conception. A cat can go back into heat one week to one month after delivering kittens.