Estrus (“heat”) is the mating period of female animals. When estrus occurs, animals are said to be “in heat” or “in season”. Cats normally have their first estrous cycle between 5 and 10 months of age, with the average age around 6 months.
The female cat has 2-4 estrous periods every year, each lasting 15-22 days. If she is bred, estrus seldom lasts more than 4 days.
If successful mating does not occur, estrus may last for 7-10 days and recur at 15– to 21-day intervals. It is possible for an unmated female to cycle every 3-4 weeks indefinitely.
Cats also have an estrous period 1-6 weeks after giving birth, so a female may be nursing one little while pregnant with another.
Since there is usually no obvious vaginal discharge or swelling of the genitals during estrus in cats, as is seen in dogs, behavioral changes are the only obvious signs that your cat is in estrus.
A cat in estrus carries her tail to one side, keeps her hindquarters elevated, exhibits “treading” movements of the hind legs, and seems unusually affectionate. She spends a good deal of time rolling on the floor and seems much more restless than usual. The cat’s voice seems more piercing than usually and she may “call” for 1-2 days before she accepts the male.