Behavioral changes such as becoming clingy, overly affectionate, grumpy or more distant than normal are all signs a female dog is in heat. While this is broadly true of most female dogs, some show no behavioral changes while in heat.
Although the heat cycle varies from breed to breed and in individual dogs, the average time a female dog is attractive to male dogs (or in heat) is 18 days. Most females are attracted to males for less time than that.
Female dogs typically go into heat every six months, with the first heat occurring between the ages of 6 and 24 months. However, these intervals vary with different breeds and individual dogs.
The symptoms of a dog's first heat are the same as any other heat and include swelling of the vulva and bloody vaginal discharge. Female dogs may urinate more frequently when in heat or develop other behavioral changes.
Unspayed female dogs tend to experience estrus, or go into heat, twice a year. Some breeds go into heat more often than others, and spayed dogs do not go into heat.
When a female dog is in heat, an intact and sexually mature male dog in the area can act anxious and aggressive towards other male dogs. He loses interest in anything except mating. A neutered male dog usually acts normally around a female dog in heat.
When a dog is in heat, she may be seem more alert, nervous and distracted than normal, and she may urinate more frequently. Her vulva swells, and she is likely to have a bloody vaginal discharge at first, but it becomes straw-colored when she is ready to breed.
The length of a dog's heat varies, but on average, it lasts between 12 and 21 days. Some dogs may have a heat cycle that lasts for as little as two days, while others may be in heat for up to four weeks.
A dog that is not spayed first enters heat when she is between 6 months old and 24 months old. If she is spayed, she normally experiences heat twice a year. Smaller dog breeds typically experience heat earlier than larger breeds. Heat lasts about 18 days.
Early signs that a dog may be in heat are a red discharge from the vulva, licking that is excessive, changes in behavior or appetite and an increase in temperature. The beginning phase of a dog’s heat cycle is called proestrus.